File: rfc1247.txt

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Network Working Group                                             J. Moy
Request for Comments: 1247                                 Proteon, Inc.
Obsoletes: RFC 1131                                            July 1991


                             OSPF Version 2



Status of this Memo

This RFC specifies an IAB standards track protocol for the Internet
community, and requests discussion and suggestions for improvements.
Please refer to the current edition of the ``IAB Official Protocol
Standards'' for the standardization state and status of this protocol.
Distribution of this memo is unlimited.


Abstract

This memo documents version 2 of the OSPF protocol.  OSPF is a link-
state based routing protocol.  It is designed to be run internal to a
single Autonomous System.  Each OSPF router maintains an identical
database describing the Autonomous System's topology.  From this
database, a routing table is calculated by constructing a shortest-path
tree.

OSPF recalculates routes quickly in the face of topological changes,
utilizing a minimum of routing protocol traffic.  OSPF provides support
for equal-cost multipath.  Separate routes can be calculated for each IP
type of service.  An area routing capability is provided, enabling an
additional level of routing protection and a reduction in routing
protocol traffic.  In addition, all OSPF routing protocol exchanges are
authenticated.

Version 1 of the OSPF protocol was documented in RFC 1131.  The
differences between the two versions are explained in Appendix F.

Please send comments to ospf@trantor.umd.edu.


1. Introduction

This document is a specification of the Open Shortest Path First (OSPF)
internet routing protocol.  OSPF is classified as an Internal Gateway
Protocol (IGP).  This means that it distributes routing information
between routers belonging to a single Autonomous System.  The OSPF
protocol is based on SPF or link-state technology.  This is a departure



[Moy]                                                           [Page 1]

RFC 1247                     OSPF Version 2                    July 1991


from the Bellman-Ford base used by traditional internet routing
protocols.

The OSPF protocol was developed by the OSPF working group of the
Internet Engineering Task Force.  It has been designed expressly for the
internet environment, including explicit support for IP subnetting,
TOS-based routing and the tagging of externally-derived routing
information.  OSPF also provides for the authentication of routing
updates, and utilizes IP multicast when sending/receiving the updates.
In addition, much work has been done to produce a protocol that responds
quickly to topology changes, yet involves small amounts of routing
protocol traffic.

The author would like to thank Rob Coltun, Milo Medin, Mike Petry and
the rest of the OSPF working group for the ideas and support they have
given to this project.


1.1 Protocol overview

OSPF routes IP packets based solely on the destination IP address and IP
Type of Service found in the IP packet header.  IP packets are routed
"as is" -- they are not encapsulated in any further protocol headers as
they transit the Autonomous System.  OSPF is a dynamic routing protocol.
It quickly detects topological changes in the AS (such as router
interface failures) and calculates new loop-free routes after a period
of convergence.  This period of convergence is short and involves a
minimum of routing traffic.

In an SPF-based routing protocol, each router maintains a database
describing the Autonomous System's topology.  Each participating router
has an identical database.  Each individual piece of this database is a
particular router's local state (e.g., the router's usable interfaces
and reachable neighbors).  The router distributes its local state
throughout the Autonomous System by flooding.

All routers run the exact same algorithm, in parallel.  From the
topological database, each router constructs a tree of shortest paths
with itself as root.  This shortest-path tree gives the route to each
destination in the Autonomous System.  Externally derived routing
information appears on the tree as leaves.

OSPF calculates separate routes for each Type of Service (TOS).  When
several equal-cost routes to a destination exist, traffic is distributed
equally among them.  The cost of a route is described by a single
dimensionless metric.

OSPF allows sets of networks to be grouped together.  Such a grouping is



[Moy]                                                           [Page 2]

RFC 1247                     OSPF Version 2                    July 1991


called an area.  The topology of an area is hidden from the rest of the
Autonomous System.  This information hiding enables a significant
reduction in routing traffic.  Also, routing within the area is
determined only by the area's own topology, lending the area protection
from bad routing data.  An area is a generalization of an IP subnetted
network.

OSPF enables the flexible configuration of IP subnets.  Each route
distributed by OSPF has a destination and mask.  Two different subnets
of the same IP network number may have different sizes (i.e., different
masks).  This is commonly referred to as variable length subnets.  A
packet is routed to the best (i.e., longest or most specific) match.
Host routes are considered to be subnets whose masks are "all ones"
(0xffffffff).

All OSPF protocol exchanges are authenticated.  This means that only
trusted routers can participate in the Autonomous System's routing.  A
variety of authentication schemes can be used; a single authentication
scheme is configured for each area.  This enables some areas to use much
stricter authentication than others.

Externally derived routing data (e.g., routes learned from the Exterior
Gateway Protocol (EGP)) is passed transparently throughout the
Autonomous System.  This externally derived data is kept separate from
the OSPF protocol's link state data.  Each external route can also be
tagged by the advertising router, enabling the passing of additional
information between routers on the boundaries of the Autonomous System.


1.2 Definitions of commonly used terms

Here is a collection of definitions for terms that have a specific
meaning to the protocol and that are used throughout the text.  The
reader unfamiliar with the Internet Protocol Suite is referred to [RS-
85-153] for an introduction to IP.


Router
    A level three Internet Protocol packet switch.  Formerly called a
    gateway in much of the IP literature.

Autonomous System
    A group of routers exchanging routing information via a common
    routing protocol.  Abbreviated as AS.

Internal Gateway Protocol
    The routing protocol spoken by the routers belonging to an
    Autonomous system.  Abbreviated as IGP.  Each Autonomous System has



[Moy]                                                           [Page 3]

RFC 1247                     OSPF Version 2                    July 1991


    a single IGP.  Different Autonomous Systems may be running different
    IGPs.

Router ID
    A 32-bit number assigned to each router running the OSPF protocol.
    This number uniquely identifies the router within an Autonomous
    System.

Network
    In this paper, an IP network or subnet.  It is possible for one
    physical network to be assigned multiple IP network/subnet numbers.
    We consider these to be separate networks.  Point-to-point physical
    networks are an exception - they are considered a single network no
    matter how many (if any at all) IP network/subnet numbers are
    assigned to them.

Network mask
    A 32-bit number indicating the range of IP addresses residing on a
    single IP network/subnet.  This specification displays network masks
    as hexadecimal numbers.  For example, the network mask for a class C
    IP network is displayed as 0xffffff00.  Such a mask is often
    displayed elsewhere in the literature as 255.255.255.0.

Multi-access networks
    Those physical networks that support the attachment of multiple
    (more than two) routers.  Each pair of routers on such a network is
    assumed to be able to communicate directly (e.g., multi-drop
    networks are excluded).

Interface
    The connection between a router and one of its attached networks.
    An interface has state information associated with it, which is
    obtained from the underlying lower level protocols and the routing
    protocol itself.  An interface to a network has associated with it a
    single IP address and mask (unless the network is an unnumbered
    point-to-point network).  An interface is sometimes also referred to
    as a link.

Neighboring routers
    Two routers that have interfaces to a common network.  On multi-
    access networks, neighbors are dynamically discovered by OSPF's
    Hello Protocol.

Adjacency
    A relationship formed between selected neighboring routers for the
    purpose of exchanging routing information.  Not every pair of
    neighboring routers become adjacent.




[Moy]                                                           [Page 4]

RFC 1247                     OSPF Version 2                    July 1991


Link state advertisement
    Describes to the local state of a router or network.  This includes
    the state of the router's interfaces and adjacencies.  Each link
    state advertisement is flooded throughout the routing domain.  The
    collected link state advertisements of all routers and networks
    forms the protocol's topological database.

Hello protocol
    The part of the OSPF protocol used to establish and maintain
    neighbor relationships.  On multi-access networks the Hello protocol
    can also dynamically discover neighboring routers.

Designated Router
    Each multi-access network that has at least two attached routers has
    a Designated Router.  The Designated Router generates a link state
    advertisement for the multi-access network and has other special
    responsibilities in the running of the protocol.  The Designated
    Router is elected by the Hello Protocol.

    The Designated Router concept enables a reduction in the number of
    adjacencies required on a multi-access network.  This in turn
    reduces the amount of routing protocol traffic and the size of the
    topological database.

Lower-level protocols
    The underlying network access protocols that provide services to the
    Internet Protocol and in turn the OSPF protocol.  Examples of these
    are the X.25 packet and frame levels for PDNs, and the ethernet data
    link layer for ethernets.


1.3 Brief history of SPF-based routing technology

OSPF is an SPF-based routing protocol.  Such protocols are also referred
to in the literature as link-state or distributed-database protocols.
This section gives a brief description of the developments in SPF-based
technology that have influenced the OSPF protocol.

The first SPF-based routing protocol was developed for use in the
ARPANET packet switching network.  This protocol is described in
[McQuillan].  It has formed the starting point for all other SPF-based
protocols.  The homogeneous Arpanet environment, i.e., single-vendor
packet switches connected by synchronous serial lines, simplified the
design and implementation of the original protocol.

Modifications to this protocol were proposed in [Perlman].  These
modifications dealt with increasing the fault tolerance of the routing
protocol through, among other things, adding a checksum to the link



[Moy]                                                           [Page 5]

RFC 1247                     OSPF Version 2                    July 1991


state advertisements (thereby detecting database corruption).  The paper
also included means for reducing the routing traffic overhead in an
SPF-based protocol.  This was accomplished by introducing mechanisms
which enabled the interval between link state advertisements to be
increased by an order of magnitude.

An SPF-based algorithm has also been proposed for use as an ISO IS-IS
routing protocol.  This protocol is described in [DEC].  The protocol
includes methods for data and routing traffic reduction when operating
over broadcast networks.  This is accomplished by election of a
Designated Router for each broadcast network, which then originates a
link state advertisement for the network.

The OSPF subcommittee of the IETF has extended this work in developing
the OSPF protocol.  The Designated Router concept has been greatly
enhanced to further reduce the amount of routing traffic required.
Multicast capabilities are utilized for additional routing bandwidth
reduction.  An area routing scheme has been developed enabling
information hiding/protection/reduction.  Finally, the algorithm has
been modified for efficient operation in the internet environment.


1.4 Organization of this document

The first three sections of this specification give a general overview
of the protocol's capabilities and functions.  Sections 4-16 explain the
protocol's mechanisms in detail.  Packet formats, protocol constants,
configuration items and required management statistics are specified in
the appendices.

Labels such as HelloInterval encountered in the text refer to protocol
constants.  They may or may not be configurable.  The architectural
constants are explained in Appendix B.  The configurable constants are
explained in Appendix C.

The detailed specification of the protocol is presented in terms of data
structures.  This is done in order to make the explanation more precise.
Implementations of the protocol are required to support the
functionality described, but need not use the precise data structures
that appear in this paper.


2. The Topological Database

The database of the Autonomous System's topology describes a directed
graph.  The vertices of the graph consist of routers and networks.  A
graph edge connects two routers when they are attached via a physical
point-to-point network.  An edge connecting a router to a network



[Moy]                                                           [Page 6]

RFC 1247                     OSPF Version 2                    July 1991


indicates that the router has an interface on the network.

The vertices of the graph can be further typed according to function.
Only some of these types carry transit data traffic; that is, traffic
that is neither locally originated nor locally destined.  Vertices that
can carry transit traffic are indicated on the graph by having both
incoming and outgoing edges.



                   Vertex type   Vertex name    Transit?
                   _____________________________________
                   1             Router         yes
                   2             Network        yes
                   3             Stub network   no


                        Table 1: OSPF vertex types.


OSPF supports the following types of physical networks:


Point-to-point networks
    A network that joins a single pair of routers.  A 56Kb serial line
    is an example of a point-to-point network.

Broadcast networks
    Networks supporting many (more than two) attached routers, together
    with the capability to address a single physical message to all of
    the attached routers (broadcast).  Neighboring routers are
    discovered dynamically on these nets using OSPF's Hello Protocol.
    The Hello Protocol itself takes advantage of the broadcast
    capability.  The protocol makes further use of multicast
    capabilities, if they exist.  An ethernet is an example of a
    broadcast network.

Non-broadcast networks
    Networks supporting many (more than two) routers, but having no
    broadcast capability.  Neighboring routers are also discovered on
    these nets using OSPF's Hello Protocol.  However, due to the lack of
    broadcast capability, some configuration information is necessary
    for the correct operation of the Hello Protocol.  On these networks,
    OSPF protocol packets that are normally multicast need to be sent to
    each neighboring router, in turn.  An X.25 Public Data Network (PDN)
    is an example of a non-broadcast network.





[Moy]                                                           [Page 7]

RFC 1247                     OSPF Version 2                    July 1991


The neighborhood of each network node in the graph depends on whether
the network has multi-access capabilities (either broadcast or non-
broadcast) and, if so, the number of routers having an interface to the
network.  The three cases are depicted in Figure 1.  Rectangles indicate
routers.  Circles and oblongs indicate multi-access networks.  Router
names are prefixed with the letters RT and network names with N.  Router
interface names are prefixed by I.  Lines between routers indicate
point-to-point networks.  The left side of the figure shows a network
with its connected routers, with the resulting graph shown on the right.


Two routers joined by a point-to-point network are represented in the
directed graph as being directly connected by a pair of edges, one in
each direction.  Interfaces to physical point-to-point networks need not
be assigned IP addresses.  Such a point-to-point network is called
unnumbered.  The graphical representation of point-to-point networks is
designed so that unnumbered networks can be supported naturally.  When
interface addresses exist, they are modelled as stub routes.  Note that
each router would then have a stub connection to the other router's
interface address (see Figure 1).

When multiple routers are attached to a multi-access network, the
directed graph shows all routers bidirectionally connected to the
network vertex (again, see Figure 1).  If only a single router is
attached to a multi-access network, the network will appear in the
directed graph as a stub connection.

Each network (stub or transit) in the graph has an IP address and
associated network mask.  The mask indicates the number of nodes on the
network.  Hosts attached directly to routers (referred to as host
routes) appear on the graph as stub networks.  The network mask for a
host route is always 0xffffffff, which indicates the presence of a
single node.

Figure 2 shows a sample map of an Autonomous System.  The rectangle
labelled H1 indicates a host, which has a SLIP connection to router
RT12.  Router RT12 is therefore advertising a host route.  Lines between


                 ______________________________________

                 (Figure not included in text version.)

                    Figure 1: Network map components
                 ______________________________________






[Moy]                                                           [Page 8]

RFC 1247                     OSPF Version 2                    July 1991


routers indicate physical point-to-point networks.  The only point-to-
point network that has been assigned interface addresses is the one
joining routers RT6 and RT10.  Routers RT5 and RT7 have EGP connections
to other Autonomous Systems.  A set of EGP-learned routes have been
displayed for both of these routers.


A cost is associated with the output side of each router interface.
This cost is configurable by the system administrator.  The lower the
cost, the more likely the interface is to be used to forward data
traffic.  Costs are also associated with the externally derived routing
data (e.g., the EGP-learned routes).

The directed graph resulting from the map in Figure 2 is depicted in
Figure 3.  Arcs are labelled with the cost of the corresponding router
output interface.  Arcs having no labelled cost have a cost of 0.  Note
that arcs leading from networks to routers always have cost 0; they are
significant nonetheless.  Note also that the externally derived routing
data appears on the graph as stubs.


The topological database (or what has been referred to above as the
directed graph) is pieced together from link state advertisements
generated by the routers.  The neighborhood of each transit vertex is
represented in a single, separate link state advertisement.  Figure 4
shows graphically the link state representation of the two kinds of
transit vertices: routers and multi-access networks.  Router RT12 has an


                 ______________________________________

                 (Figure not included in text version.)

                  Figure 2: A sample Autonomous System
                 ______________________________________



               __________________________________________

                (Figures not included in text version.)

                 Figure 3: The resulting directed graph
               Figure 4: Individual link state components
               __________________________________________






[Moy]                                                           [Page 9]

RFC 1247                     OSPF Version 2                    July 1991


interface to two broadcast networks and a SLIP line to a host.  Network
N6 is a broadcast network with three attached routers.  The cost of all
links from network N6 to its attached routers is 0.  Note that the link
state advertisement for network N6 is actually generated by one of the
attached routers: the router that has been elected Designated Router for
the network.


2.1 The shortest-path tree

When no OSPF areas are configured, each router in the Autonomous System
has an identical topological database, leading to an identical graphical
representation.  A router generates its routing table from this graph by
calculating a tree of shortest paths with the router itself as root.
Obviously, the shortest-path tree depends on the router doing the
calculation.  The shortest-path tree for router RT6 in our example is
depicted in Figure 5.


The tree gives the entire route to any destination network or host.
However, only the next hop to the destination is used in the forwarding
process.  Note also that the best route to any router has also been
calculated.  For the processing of external data, we note the next hop
and distance to any router advertising external routes.  The resulting
routing table for router RT6 is pictured in Table 2.  Note that there is
a separate route for each end of a numbered serial line (in this case,
the serial line between routers RT6 and RT10).


Routes to networks belonging to other AS'es (such as N12) appear as
dashed lines on the shortest path tree in Figure 5.  Use of this
externally derived routing information is considered in the next
section.






                 ______________________________________

                 (Figure not included in text version.)

                 Figure 5: The SPF tree for router RT6
                 ______________________________________






[Moy]                                                          [Page 10]

RFC 1247                     OSPF Version 2                    July 1991




                   Destination   Next  Hop   Distance
                   __________________________________
                   N1            RT3         10
                   N2            RT3         10
                   N3            RT3         7
                   N4            RT3         8
                   Ib            *           7
                   Ia            RT10        12
                   N6            RT10        8
                   N7            RT10        12
                   N8            RT10        10
                   N9            RT10        11
                   N10           RT10        13
                   N11           RT10        14
                   H1            RT10        21
                   __________________________________
                   RT5           RT5         6
                   RT7           RT10        8


    Table 2: The portion of router RT6's routing table listing local
                             destinations.

2.2 Use of external routing information

After the tree is created the external routing information is examined.
This external routing information may originate from another routing
protocol such as EGP, or be statically configured (static routes).
Default routes can also be included as part of the Autonomous System's
external routing information.

External routing information is flooded unaltered throughout the AS.  In
our example, all the routers in the Autonomous System know that router
RT7 has two external routes, with metrics 2 and 9.

OSPF supports two types of external metrics.  Type 1 external metrics
are equivalent to the link state metric.  Type 2 external metrics are
greater than the cost of any path internal to the AS.  Use of Type 2
external metrics assumes that routing between AS'es is the major cost of
routing a packet, and eliminates the need for conversion of external
costs to internal link state metrics.

Here is an example of Type 1 external metric processing.  Suppose that
the routers RT7 and RT5 in Figure 2 are advertising Type 1 external
metrics.  For each external route, the distance from Router RT6 is
calculated as the sum of the external route's cost and the distance from



[Moy]                                                          [Page 11]

RFC 1247                     OSPF Version 2                    July 1991


Router RT6 to the advertising router.  For every external destination,
the router advertising the shortest route is discovered, and the next
hop to the advertising router becomes the next hop to the destination.

Both Router RT5 and RT7 are advertising an external route to destination
network N12.  Router RT7 is preferred since it is advertising N12 at a
distance of 10 (8+2) to Router RT6, which is better than router RT5's 14
(6+8).  Table 3 shows the entries that are added to the routing table
when external routes are examined:



                     Destination   Next  Hop   Distance
                     __________________________________
                     N12           RT10        10
                     N13           RT5         14
                     N14           RT5         14
                     N15           RT10        17


    Table 3: The portion of router RT6's routing table listing external
                               destinations.


Processing of Type 2 external metrics is simpler.  The AS boundary
router advertising the smallest external metric is chosen, regardless of
the internal distance to the AS boundary router.  Suppose in our example
both router RT5 and router RT7 were advertising Type 2 external routes.
Then all traffic destined for network N12 would be forwarded to router
RT7, since 2 < 8.  When several equal-cost Type 2 routes exist, the
internal distance to the advertising routers is used to break the tie.

Both Type 1 and Type 2 external metrics can be present in the AS at the
same time.  In that event, Type 1 external metrics always take
precedence.

This section has assumed that packets destined for external destinations
are always routed through the advertising AS boundary router.  This is
not always desirable.  For example, suppose in Figure 2 there is an
additional router attached to network N6, called Router RTX.  Suppose
further that RTX does not participate in OSPF routing, but does exchange
EGP information with the AS boundary router RT7.  Then, router RT7 would
end up advertising OSPF external routes for all destinations that should
be routed to RTX.  An extra hop will sometimes be introduced if packets
for these destinations need always be routed first to router RT7 (the
advertising router).

To deal with this situation, the OSPF protocol allows an AS boundary



[Moy]                                                          [Page 12]

RFC 1247                     OSPF Version 2                    July 1991


router to specify a "forwarding address" in its external advertisements.
In the above example, Router RT7 would specify RTX's IP address as the
"forwarding address" for all those destinations whose packets should be
routed directly to RTX.

The "forwarding address" has one other application.  It enables routers
in the Autonomous System's interior to function as "route servers".  For
example, in Figure 2 the router RT6 could become a route server, gaining
external routing information through a combination of static
configuration and external routing protocols.  RT6 would then start
advertising itself as an AS boundary router, and would originate a
collection of OSPF external advertisements.  In each external
advertisement, router RT6 would specify the correct Autonomous System
exit point to use for the destination through appropriate setting of the
advertisement's "forwarding address" field.


2.3 Equal-cost multipath

The above discussion has been simplified by considering only a single
route to any destination.  In reality, if multiple equal-cost routes to
a destination exist, they are all discovered and used.  This requires no
conceptual changes to the algorithm, and its discussion is postponed
until we consider the tree-building process in more detail.

With equal cost multipath, a router potentially has several available
next hops towards any given destination.


2.4 TOS-based routing

OSPF can calculate a separate set of routes for each IP Type of Service.
The IP TOS values are represented in OSPF exactly as they appear in the
IP packet header.  This means that, for any destination, there can
potentially be multiple routing table entries, one for each IP TOS.

Up to this point, all examples shown have assumed that routes do not
vary on TOS.  In order to differentiate routes based on TOS, separate
interface costs can be configured for each TOS.  For example, in Figure
2 there could be multiple costs (one for each TOS) listed for each
interface.  A cost for TOS 0 must always be specified.

When interface costs vary based on TOS, a separate shortest path tree is
calculated for each TOS (see Section 2.1).  In addition, external costs
can vary based on TOS.  For example, in Figure 2 router RT7 could
advertise a separate type 1 external metric for each TOS.  Then, when
calculating the TOS X distance to network N15 the cost of the shortest
TOS X path to RT7 would be added to the TOS X cost advertised by RT7



[Moy]                                                          [Page 13]

RFC 1247                     OSPF Version 2                    July 1991


(see Section 2.2).

All OSPF implementations must be capable of calculating routes based on
TOS.  However, OSPF routers can be configured to route all packets on
the TOS 0 path (see Appendix C), eliminating the need to calculate non-
zero TOS paths.  This can be used to conserve routing table space and
processing resources in the router.  These TOS-0-only routers can be
mixed with routers that do route based on TOS.  TOS-0-only routers will
be avoided as much as possible when forwarding traffic requesting a
non-zero TOS.

It may be the case that no path exists for some non-zero TOS, even if
the router is calculating non-zero TOS paths.  In that case, packets
requesting that non-zero TOS are routed along the TOS 0 path (see
Section 11.1).


3. Splitting the AS into Areas

OSPF allows collections of contiguous networks and hosts to be grouped
together.  Such a group, together with the routers having interfaces to
any one of the included networks, is called an area.  Each area runs a
separate copy of the basic SPF routing algorithm.  This means that each
area has its own topological database and corresponding graph, as
explained in the previous section.

The topology of an area is invisible from the outside of the area.
Conversely, routers internal to a given area know nothing of the
detailed topology external to the area.  This isolation of knowledge
enables the protocol to effect a marked reduction in routing traffic as
compared to treating the entire Autonomous System as a single SPF
domain.

With the introduction of areas, it is no longer true that all routers in
the AS have an identical topological database.  A router actually has a
separate topological database for each area it is connected to.
(Routers connected to multiple areas are called area border routers).
Two routers belonging to the same area have, for that area, identical
area topological databases.

Routing in the Autonomous System takes place on two levels, depending on
whether the source and destination of a packet reside in the same area
(intra-area routing is used) or different areas (inter-area routing is
used).  In intra-area routing, the packet is routed solely on
information obtained within the area; no routing information obtained
from outside the area can be used.  This protects intra-area routing
from the injection of bad routing information.  We discuss inter-area
routing in Section 3.2.



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3.1 The backbone of the Autonomous System

The backbone consists of those networks not contained in any area, their
attached routers, and those routers that belong to multiple areas.  The
backbone must be contiguous.

It is possible to define areas in such a way that the backbone is no
longer contiguous.  In this case the system administrator must restore
backbone connectivity by configuring virtual links.

Virtual links can be configured between any two backbone routers that
have an interface to a common non-backbone area.  Virtual links belong
to the backbone.  The protocol treats two routers joined by a virtual
link as if they were connected by an unnumbered point-to-point network.
On the graph of the backbone, two such routers are joined by arcs whose
costs are the intra-area distances between the two routers.  The routing
protocol traffic that flows along the virtual link uses intra-area
routing only.

The backbone is responsible for distributing routing information between
areas.  The backbone itself has all of the properties of an area.  The
topology of the backbone is invisible to each of the areas, while the
backbone itself knows nothing of the topology of the areas.


3.2 Inter-area routing

When routing a packet between two areas the backbone is used.  The path
that the packet will travel can be broken up into three contiguous
pieces: an intra-area path from the source to an area border router, a
backbone path between the source and destination areas, and then another
intra-area path to the destination.  The algorithm finds the set of such
paths that have the smallest cost.

Looking at this another way, inter-area routing can be pictured as
forcing a star configuration on the Autonomous System, with the backbone
as hub and and each of the areas as spokes.

The topology of the backbone dictates the backbone paths used between
areas.  The topology of the backbone can be enhanced by adding virtual
links.  This gives the system administrator some control over the routes
taken by inter-area traffic.

The correct area border router to use as the packet exits the source
area is chosen in exactly the same way routers advertising external
routes are chosen.  Each area border router in an area summarizes for
the area its cost to all networks external to the area.  After the SPF
tree is calculated for the area, routes to all other networks are



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calculated by examining the summaries of the area border routers.


3.3 Classification of routers

Before the introduction of areas, the only OSPF routers having a
specialized function were those advertising external routing
information, such as router RT5 in Figure 2.  When the AS is split into
OSPF areas, the routers are further divided according to function into
the following four overlapping categories:


Internal routers
    A router with all directly connected networks belonging to the same
    area.  Routers with only backbone interfaces also belong to this
    category.  These routers run a single copy of the basic routing
    algorithm.

Area border routers
    A router that attaches to multiple areas.  Area border routers run
    multiple copies of the basic algorithm, one copy for each attached
    area and an additional copy for the backbone.  Area border routers
    condense the topological information of their attached areas for
    distribution to the backbone.  The backbone in turn distributes the
    information to the other areas.

Backbone routers
    A router that has an interface to the backbone.  This includes all
    routers that interface to more than one area (i.e., area border
    routers).  However, backbone routers do not have to be area border
    routers.  Routers with all interfaces connected to the backbone are
    considered to be internal routers.

AS boundary routers
    A router that exchanges routing information with routers belonging
    to other Autonomous Systems.  Such a router has AS external routes
    that are advertised throughout the Autonomous System.  The path to
    each AS boundary router is known by every router in the AS.  This
    classification is completely independent of the previous
    classifications: AS boundary routers may be internal or area border
    routers, and may or may not participate in the backbone.


3.4 A sample area configuration

Figure 6 shows a sample area configuration.  The first area consists of
networks N1-N4, along with their attached routers RT1-RT4.  The second
area consists of networks N6-N8, along with their attached routers RT7,



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RT8, RT10, RT11.  The third area consists of networks N9-N11 and host
H1, along with their attached routers RT9, RT11, RT12.  The third area
has been configured so that networks N9-N11 and host H1 will all be
grouped into a single route, when advertised external to the area (see
Section 3.5 for more details).


In Figure 6, routers RT1, RT2, RT5, RT6, RT8, RT9 and RT12 are internal
routers.  Routers RT3, RT4, RT7, RT10 and RT11 are area border routers.
Finally as before, routers RT5 and RT7 are AS boundary routers.

Figure 7 shows the resulting topological database for the Area 1.  The
figure completely describes that area's intra-area routing.  It also
shows the complete view of the internet for the two internal routers RT1
and RT2.  It is the job of the area border routers, RT3 and RT4, to
advertise into Area 1 the distances to all destinations external to the
area.  These are indicated in Figure 7 by the dashed stub routes.  Also,
RT3 and RT4 must advertise into Area 1 the location of the AS boundary
routers RT5 and RT7.  Finally, external advertisements from RT5 and RT7
are flooded throughout the entire AS, and in particular throughout Area
1.  These advertisements are included in Area 1's database, and yield
routes to networks N12-N15.

Routers RT3 and RT4 must also summarize Area 1's topology for
distribution to the backbone.  Their backbone advertisements are shown
in Table 4.  These summaries show which networks are contained in Area 1
(i.e., networks N1-N4), and the distance to these networks from the
routers RT3 and RT4 respectively.


The topological database for the backbone is shown in Figure 8.  The set
of routers pictured are the backbone routers.  Router RT11 is a backbone
router because it belongs to two areas.  In order to make the backbone
connected, a virtual link has been configured between routers R10 and
R11.




               __________________________________________

                 (Figure not included in text version.)

               Figure 6: A sample OSPF area configuration
               __________________________________________






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                     Network   RT3 adv.   RT4 adv.
                     _____________________________
                     N1        4          4
                     N2        4          4
                     N3        1          1
                     N4        2          3


  Table 4: Networks advertised to the backbone by routers RT3 and RT4.


                 ______________________________________

                 (Figure not included in text version.)

                      Figure 7: Area 1's Database
                    Figure 8: The backbone database
                 ______________________________________


Again, routers RT3, RT4, RT7, RT10 and RT11 are area border routers.  As
routers RT3 and RT4 did above, they have condensed the routing
information of their attached areas for distribution via the backbone;
these are the dashed stubs that appear in Figure 8.  Remember that the
third area has been configured to condense networks N9-N11 and Host H1
into a single route.  This yields a single dashed line for networks N9-
N11 and Host H1 in Figure 8.  Routers RT5 and RT7 are AS boundary
routers; their externally derived information also appears on the graph
in Figure 8 as stubs.

The backbone enables the exchange of summary information between area
border routers.  Every area border router hears the area summaries from
all other area border routers.  It then forms a picture of the distance
to all networks outside of its area by examining the collected
advertisements, and adding in the backbone distance to each advertising
router.

Again using routers RT3 and RT4 as an example, the procedure goes as
follows: They first calculate the SPF tree for the backbone.  This gives
the distances to all other area border routers.  Also noted are the
distances to networks (Ia and Ib) and AS boundary routers (RT5 and RT7)
that belong to the backbone.  This calculation is shown in Table 5.


Next, by looking at the area summaries from these area border routers,
RT3 and RT4 can determine the distance to all networks outside their



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                 Area  border   dist  from   dist  from
                 router         RT3          RT4
                 ______________________________________
                 to  RT3        *            21
                 to  RT4        22           *
                 to  RT7        20           14
                 to  RT10       15           22
                 to  RT11       18           25
                 ______________________________________
                 to  Ia         20           27
                 to  Ib         15           22
                 ______________________________________
                 to  RT5        14           8
                 to  RT7        20           14


     Table 5: Backbone distances calculated by routers RT3 and RT4.

area.  These distances are then advertised internally to the area by RT3
and RT4.  The advertisements that router RT3 and RT4 will make into Area
1 are shown in Table 6.  Note that Table 6 assumes that an area range
has been configured for the backbone which groups I5 and I6 into a
single advertisement.


The information imported into Area 1 by routers RT3 and RT4 enables an
internal router, such as RT1, to choose an area border router
intelligently.  Router RT1 would use RT4 for traffic to network N6, RT3
for traffic to network N10, and would load share between the two for


                   Destination   RT3 adv.   RT4 adv.
                   _________________________________
                   Ia,Ib         15         22
                   N6            16         15
                   N7            20         19
                   N8            18         18
                   N9-N11,H1     19         26
                   _________________________________
                   RT5           14         8
                   RT7           20         14


  Table 6: Destinations advertised into Area 1 by routers RT3 and RT4.





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traffic to network N8.

Router RT1 can also determine in this manner the shortest path to the AS
boundary routers RT5 and RT7.  Then, by looking at RT5 and RT7's
external advertisements, router RT1 can decide between RT5 or RT7 when
sending to a destination in another Autonomous System (one of the
networks N12-N15).

Note that a failure of the line between routers RT6 and RT10 will cause
the backbone to become disconnected.  Configuring another virtual link
between routers RT7 and RT10 will give the backbone more connectivity
and more resistance to such failures.  Also, a virtual link between RT7
and RT10 would allow a much shorter path between the third area
(containing N9) and the router RT7, which is advertising a good route to
external network N12.


3.5 IP subnetting support

OSPF attaches an IP address mask to each advertised route.  The mask
indicates the range of addresses being described by the particular
route.  For example, a summary advertisement for the destination
128.185.0.0 with a mask of 0xffff0000 actually is describing a single
route to the collection of destinations 128.185.0.0 - 128.185.255.255.
Similarly, host routes are always advertised with a mask of 0xffffffff,
indicating the presence of only a single destination.

Including the mask with each advertised destination enables the
implementation of what is commonly referred to as variable-length subnet
masks.  This means that a single IP class A, B, or C network number can
be broken up into many subnets of various sizes.  For example, the
network 128.185.0.0 could be broken up into 64 variable-sized subnets:
16 subnets of size 4K, 16 subnets of size 256, and 32 subnets of size 8.
Table 7 shows some of the resulting network addresses together with
their masks:



              Network address   IP address mask   Subnet size
              _______________________________________________
              128.185.16.0      0xfffff000        4K
              128.185.1.0       0xffffff00        256
              128.185.0.8       0xfffffff8        8


                     Table 7: Some sample subnet sizes.





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There are many possible ways of dividing up a class A, B, and C network
into variable sized subnets.  The precise procedure for doing so is
beyond the scope of this specification.  The specification however
establishes the following guideline: When an IP packet is forwarded, it
is always forwarded to the network that is the best match for the
packet's destination.  Here best match is synonymous with the longest or
most specific match.  For example, the default route with destination of
0.0.0.0 and mask 0x00000000 is always a match for every IP destination.
Yet it is always less specific than any other match.  Subnet masks must
be assigned so that the best match for any IP destination is
unambiguous.

The OSPF area concept is modelled after an IP subnetted network.  OSPF
areas have been loosely defined to be a collection of networks.  In
actuality, an OSPF area is specified to be a list of address ranges (see
Section C.2 for more details).  Each address range is defined as an
[address,mask] pair.  Many separate networks may then be contained in a
single address range, just as a subnetted network is composed of many
separate subnets.  Area border routers then summarize the area contents
(for distribution to the backbone) by advertising a single route for
each address range.  The cost of the route is the minimum cost to any of
the networks falling in the specified range.

For example, an IP subnetted network can be configured as a single OSPF
area.  In that case, the area would be defined as a single address
range: a class A, B, or C network number along with its natural IP mask.
Inside the area, any number of variable sized subnets could be defined.
External to the area, a single route for the entire subnetted network
would be distributed, hiding even the fact that the network is subnetted
at all.  The cost of this route is the minimum of the set of costs to
the component subnets.


3.6 Supporting stub areas

In some Autonomous Systems, the majority of the topological database may
consist of external advertisements.  An OSPF external advertisement is
usually flooded throughout the entire AS.  However, OSPF allows certain
areas to be configured as "stub areas".  External advertisements are not
flooded into/throughout stub areas; routing to AS external destinations
in these areas is based on a (per-area) default only.  This reduces the
topological database size, and therefore the memory requirements, for a
stub area's internal routers.

In order to take advantage of the OSPF stub area support, default
routing must be used in the stub area.  This is accomplished as follows.
One or more of the stub area's area border routers must advertise a
default route into the stub area via summary advertisements.  These



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summary defaults are flooded throughout the stub area, but no further.
(For this reason these defaults pertain only to the particular stub
area).  These summary default routes will match any destination that is
not explicitly reachable by an intra-area or inter-area path (i.e., AS
external destinations).

An area can be configured as stub when there is a single exit point from
the area, or when the choice of exit point need not be made on a per-
external-destination basis.  For example, area 3 in Figure 6 could be
configured as a stub area, because all external traffic must travel
though its single area border router RT11.  If area 3 were configured as
a stub, router RT11 would advertise a default route for distribution
inside area 3 (in a summary advertisement), instead of flooding the
external advertisements for networks N12-N15 into/throughout the area.

The OSPF protocol ensures that all routers belonging to an area agree on
whether the area has been configured as a stub.  This guarantees that no
confusion will arise in the flooding of external advertisements.

There are a couple of restrictions on the use of stub areas.  Virtual
links cannot be configured through stub areas.  In addition, AS boundary
routers cannot be placed internal to stub areas.


3.7 Partitions of areas

OSPF does not actively attempt to repair area partitions.  When an area
becomes partitioned, each component simply becomes a separate area.  The
backbone then performs routing between the new areas.  Some destinations
reachable via intra-area routing before the partition will now require
inter-area routing.

In the previous section, an area was described as a list of address
ranges.  Any particular address range must still be completely contained
in a single component of the area partition.  This has to do with the
way the area contents are summarized to the backbone.  Also, the
backbone itself must not partition.  If it does, parts of the Autonomous
System will become unreachable.  Backbone partitions can be repaired by
configuring virtual links (see Section 15).

Another way to think about area partitions is to look at the Autonomous
System graph that was introduced in Section 2.  Area IDs can be viewed
as colors for the graph's edges.[1] Each edge of the graph connects to a
network, or is itself a point-to-point network.  In either case, the
edge is colored with the network's Area ID.

A group of edges, all having the same color, and interconnected by
vertices, represents an area.  If the topology of the Autonomous System



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is intact, the graph will have several regions of color, each color
being a distinct Area ID.

When the AS topology changes, one of the areas may become partitioned.
The graph of the AS will then have multiple regions of the same color
(Area ID).  The routing in the Autonomous System will continue to
function as long as these regions of same color are connected by the
single backbone region.











































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4. Functional Summary

A separate copy of OSPF's basic routing algorithm runs in each area.
Routers having interfaces to multiple areas run multiple copies of the
algorithm.  A brief summary of the routing algorithm follows.

When a router starts, it first initializes the routing protocol data
structures.  The router then waits for indications from the lower-level
protocols that its interfaces are functional.

A router then uses the OSPF's Hello Protocol to acquire neighbors.  The
router sends Hello packets to its neighbors, and in turn receives their
Hello packets.  On broadcast and point-to-point networks, the router
dynamically detects its neighboring routers by sending its Hello packets
to the multicast address AllSPFRouters.  On non-broadcast networks, some
configuration information is necessary in order to discover neighbors.
On all multi-access networks (broadcast or non-broadcast), the Hello
Protocol also elects a Designated router for the network.

The router will attempt to form adjacencies with some of its newly
acquired neighbors.  Topological databases are synchronized between
pairs of adjacent routers.  On multi-access networks, the Designated
Router determines which routers should become adjacent.

Adjacencies control the distribution of routing protocol packets.
Routing protocol packets are sent and received only on adjacencies.  In
particular, distribution of topological database updates proceeds along
adjacencies.

A router periodically advertises its state, which is also called link
state.  Link state is also advertised when a router's state changes.  A
router's adjacencies are reflected in the contents of its link state
advertisements.  This relationship between adjacencies and link state
allows the protocol to detect dead routers in a timely fashion.

Link state advertisements are flooded throughout the area.  The flooding
algorithm is reliable, ensuring that all routers in an area have exactly
the same topological database.  This database consists of the collection
of link state advertisements received from each router belonging to the
area.  From this database each router calculates a shortest-path tree,
with itself as root.  This shortest-path tree in turn yields a routing
table for the protocol.


4.1 Inter-area routing

The previous section described the operation of the protocol within a
single area.  For intra-area routing, no other routing information is



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pertinent.  In order to be able to route to destinations outside of the
area, the area border routers inject additional routing information into
the area.  This additional information is a distillation of the rest of
the Autonomous System's topology.

This distillation is accomplished as follows: Each area border router is
by definition connected to the backbone.  Each area border router
summarizes the topology of its attached areas for transmission on the
backbone, and hence to all other area border routers.  A area border
router then has complete topological information concerning the
backbone, and the area summaries from each of the other area border
routers.  From this information, the router calculates paths to all
destinations not contained in its attached areas.  The router then
advertises these paths to its attached areas.  This enables the area's
internal routers to pick the best exit router when forwarding traffic to
destinations in other areas.


4.2 AS external routes

Routers that have information regarding other Autonomous Systems can
flood this information throughout the AS.  This external routing
information is distributed verbatim to every participating router.
There is one exception: external routing information is not flooded into
"stub" areas (see Section 3.6).

To utilize external routing information, the path to all routers
advertising external information must be known throughout the AS
(excepting the stub areas).  For that reason, the locations of these AS
boundary routers are summarized by the (non-stub) area border routers.


4.3 Routing protocol packets

The OSPF protocol runs directly over IP, using IP protocol 89.  OSPF
does not provide any explicit fragmentation/reassembly support.  When
fragmentation is necessary, IP fragmentation/reassembly is used.  OSPF
protocol packets have been designed so that large protocol packets can
generally be split into several smaller protocol packets.  This practice
is recommended; IP fragmentation should be avoided whenever possible.

Routing protocol packets should always be sent with the IP TOS field set
to 0.  If at all possible, routing protocol packets should be given
preference over regular IP data traffic, both when being sent and
received.  As an aid to accomplishing this, OSPF protocol packets should
have their IP precedence field set to the value Internetwork Control
(see [RFC 791]).




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All OSPF protocol packets share a common protocol header that is
described in Appendix A.  The OSPF packet types are listed below in
Table 8.  Their formats are also described in Appendix A.



         Type   Packet  name           Protocol  function
         __________________________________________________________
         1      Hello                  Discover/maintain  neighbors
         2      Database Description   Summarize database contents
         3      Link State Request     Database download
         4      Link State Update      Database update
         5      Link State Ack         Flooding acknowledgment


                        Table 8: OSPF packet types.


OSPF's Hello protocol uses Hello packets to discover and maintain
neighbor relationships.  The Database Description and Link State Request
packets are used in the forming of adjacencies.  OSPF's reliable update
mechanism is implemented by the Link State Update and Link State
Acknowledgment packets.

Each Link State Update packet carries a set of new link state
advertisements one hop further away from their point of origination.  A
single Link State Update packet may contain the link state
advertisements of several routers.  Each advertisement is tagged with
the ID of the originating router and a checksum of its link state
contents.  The five different types of OSPF link state advertisements
are listed below in Table 9.


LS     Advertisement        Advertisement description
type   name
____________________________________________________________________________
1      Router links advs.   Originated by all routers. This
       advs.                advertisement describes the collected
                            states of the router's interfaces to an
                            area. Flooded throughout a single area
                            only.
____________________________________________________________________________
2      Network links        Originated for multi-access networks by
       advs.                the Designated Router. This
                            advertisement contains the list of
                            routers connected to the network.
                            Flooded throughout a single area only.




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LS     Advertisement        Advertisement description
type   name
____________________________________________________________________________
____________________________________________________________________________
3,4    Summary link         Originated by area border routers, and
       advs.                flooded throughout their associated
                            area. Each summary link advertisement
                            describes a route to a destination
                            outside the area, yet still inside the
                            AS (i.e., an inter-area route). Type 3
                            advertisements describe routes to
                            networks. Type 4 advertisements
                            describe routes to AS boundary routers.
____________________________________________________________________________
5      AS external          Originated by AS boundary routers, and
       link advs.           flooded throughout the AS. Each external
                            advertisement describes a route to a
                            destination in another Autonomous
                            System. Default routes for the AS can
                            also be described by AS external advertisements.


                Table 9: OSPF link state advertisements.

As mentioned above, OSPF routing packets (with the exception of Hellos)
are sent only over adjacencies.  Note that this means that all protocol
packets travel a single IP hop, except those that are sent over virtual
adjacencies.  The IP source address of an OSPF protocol packet is one
end of a router adjacency, and the IP destination address is either the
other end of the adjacency or an IP multicast address.


4.4 Basic implementation requirements

An implementation of OSPF requires the following pieces of system
support:


Timers
    Two different kind of timers are required.  The first kind, called
    single shot timers, fire once and cause a protocol event to be
    processed.  The second kind, called interval timers, fire at
    continuous intervals.  These are used for the sending of packets at
    regular intervals.  A good example of this is the regular broadcast
    of Hello packets (on broadcast networks).  The granularity of both
    kinds of timers is one second.

    Interval timers should be implemented to avoid drift.  In some



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    router implementations, packet processing can affect timer
    execution.  When multiple routers are attached to a single network,
    all doing broadcasts, this can lead to the synchronization of
    routing packets (which should be avoided).  If timers cannot be
    implemented to avoid drift, small random amounts should be added
    to/subtracted from the timer interval at each firing.

IP multicast
    Certain OSPF packets use IP multicast.  Support for receiving and
    sending IP multicasts, along with the appropriate lower-level
    protocol support, is required.  These IP multicast packets never
    travel more than one hop.  For information on IP multicast, see [RFC
    1112].

Lower-level protocol support
    The lower level protocols referred to here are the network access
    protocols, such as the Ethernet data link layer.  Indications must
    be passed from from these protocols to OSPF as the network interface
    goes up and down.  For example, on an ethernet it would be valuable
    to know when the ethernet transceiver cable becomes unplugged.

Non-broadcast lower-level protocol support
    Remember that non-broadcast networks are multi-access networks such
    as a X.25 PDN.  On these networks, the Hello Protocol can be aided
    by providing an indication to OSPF when an attempt is made to send a
    packet to a dead or non-existent router.  For example, on a PDN a
    dead router may be indicated by the reception of a X.25 clear with
    an appropriate cause and diagnostic, and this information would be
    passed to OSPF.

List manipulation primitives
    Much of the OSPF functionality is described in terms of its
    operation on lists of link state advertisements.  For example, the
    advertisements that will be retransmitted to an adjacent router
    until acknowledged are described as a list.  Any particular
    advertisement may be on many such lists.  An OSPF implementation
    needs to be able to manipulate these lists, adding and deleting
    constituent advertisements as necessary.

Tasking support
    Certain procedures described in this specification invoke other
    procedures.  At times, these other procedures should be executed
    in-line, that is, before the current procedure is finished.  This is
    indicated in the text by instructions to execute a procedure.  At
    other times, the other procedures are to be executed only when the
    current procedure has finished.  This is indicated by instructions
    to schedule a task.




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4.5 Optional OSPF capabilities

The OSPF protocol defines several optional capabilities.  A router
indicates the optional capabilities that it supports in its OSPF Hello
packets, Database Description packets and in its link state
advertisements.  This enables routers supporting a mix of optional
capabilities to coexist in a single Autonomous System.

Some capabilities must be supported by all routers attached to a
specific area.  In this case, a router will not accept a neighbor's
Hello unless there is a match in reported capabilities (i.e., a
capability mismatch prevents a neighbor relationship from forming).  An
example of this is the external routing capability (see below).

Other capabilities can be negotiated during the database synchronization
process.  This is accomplished by specifying the optional capabilities
in Database Description packets.  A capability mismatch with a neighbor
is this case will result in only a subset of link state advertisements
being exchanged between the two neighbors.

The routing table build process can also be affected by the
presence/absence of optional capabilities.  For example, since the
optional capabilities are reported in link state advertisements, routers
incapable of certain functions can be avoided when building the shortest
path tree.  An example of this is the TOS routing capability (see
below).

The current OSPF optional capabilities are listed below.  See Section
A.2 for more information.


External routing capability
    Entire OSPF areas can be configured as "stubs" (see Section 3.6).
    AS external advertisements will not be flooded into stub areas.
    This capability is represented by the E-bit in the OSPF options
    field (see Section A.2).  In order to ensure consistent
    configuration of stub areas, all routers interfacing to such an area
    must have the E-bit clear in their Hello packets (see Sections 9.5
    and 10.5).

TOS capability
    All OSPF implementations must be able to calculate separate routes
    based on IP Type of Service.  However, to save routing table space
    and processing resources, an OSPF router can be configured to ignore
    TOS when forwarding packets.  In this case, the router calculates
    routes for TOS 0 only.  This capability is represented by the T-bit
    in the OSPF options field (see Section A.2).  TOS-capable routers
    will attempt to avoid non-TOS-capable routers when calculating non-



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    zero TOS paths.


5. Protocol Data Structures

The OSPF protocol is described in this specification in terms of its
operation on various protocol data structures.  The following list
comprises the top-level OSPF data structures.  Any initialization that
needs to be done is noted.  Areas, OSPF interfaces and neighbors also
have associated data structures that are described later in this
specification.


Router ID
    a 32-bit number that uniquely identifies this router in the AS.  One
    possible implementation strategy would be to use the smallest IP
    interface address belonging to the router.

Pointers to area structures
    Each one of the areas to which the router is connected has its own
    data structure.  This data structure describes the working of the
    basic algorithm.  Remember that each area runs a separate copy of
    the basic algorithm.

Pointer to the backbone structure
    The basic algorithm operates on the backbone as if it were an area.
    For this reason the backbone is represented as an area structure.

Virtual links configured
    The virtual links configured with this router as one endpoint.  In
    order to have configured virtual links, the router itself must be an
    area border router.  Virtual links are identified by the Router ID
    of the other endpoint -- which is another area border router.  These
    two endpoint routers must be attached to a common area, called the
    virtual link's transit area.  Virtual links are part of the
    backbone, and behave as if they were unnumbered point-to-point
    networks between the two routers.  A virtual link uses the intra-
    area routing of its transit area to forward packets.  Virtual links
    are brought up and down through the building of the shortest-path
    trees for the transit area.

List of external routes
    These are routes to destinations external to the Autonomous System,
    that have been gained either through direct experience with another
    routing protocol (such as EGP), or through configuration
    information, or through a combination of the two (e.g., dynamic
    external info.  to be advertised by OSPF with configured metric).
    Any router having these external routes is called an AS boundary



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    router.  These routes are advertised by the router to the entire AS
    through AS external link advertisements.

List of AS external link advertisements
    Part of the topological database.  These have have originated from
    the AS boundary routers.  They comprise routes to destinations
    external to the Autonomous System.  Note that, if the router is
    itself an AS boundary router, some of these AS external link
    advertisements have been self originated.

The routing table
    Derived from the topological database.  Each destination that the
    router can forward to is represented by a cost and a set of paths.
    A path is described by its type and next hop.  For more information,
    see Section 11.

TOS capability
    This item indicates whether the router will calculate separate
    routes based on TOS.  This is a configurable parameter.  For more
    information, see Sections 4.5 and 16.9.


Figure 9 shows the collection of data structures present in a typical
router.  The router pictured is RT10, from the map in Figure 6.  Note
that router RT10 has a virtual link configured to router RT11, with Area
2 as the link's transit area.  This is indicated by the dashed line in
Figure 9.  When the virtual link becomes active, through the building of
the shortest path tree for Area 2, it becomes an interface to the
backbone (see the two backbone interfaces depicted in Figure 9).



6. The Area Data Structure

The area data structure contains all the information used to run the
basic routing algorithm.  Remember that each area maintains its own
topological database.  Router interfaces and adjacencies belong to a


                _______________________________________

                (Figure not included in text version.)

                Figure 9: Router RT10's Data Structures
                _______________________________________






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single area.

The backbone has all the properties of an area.  For that reason it is
also represented by an area data structure.  Note that some items in the
structure apply differently to the backbone than to areas.

The area topological (or link state) database consists of the collection
of router links, network links and summary links advertisements that
have originated from the area's routers.  This information is flooded
throughout a single area only.  The list of AS external advertisements
is also considered to be part of each area's topological database.


Area ID
    A 32-bit number identifying the area.  0 is reserved for the area ID
    of the backbone.  If assigning subnetted networks as separate areas,
    the IP network number could be used as the Area ID.

List of component address ranges
    The address ranges that define the area.  Each address range is
    specified by an [address,mask] pair.  Each network is then assigned
    to an area depending on the address range that it falls into
    (specified address ranges are not allowed to overlap).  As an
    example, if an IP subnetted network is to be its own separate OSPF
    area, the area is defined to consist of a single address range - an
    IP network number with its natural (class A, B or C) mask.

Associated router interfaces
    This router's interfaces connecting to the area.  A router interface
    belongs to one and only one area (or the backbone).  For the
    backbone structure this list includes all the virtual adjacencies.
    A virtual adjacency is identified by the router ID of its other
    endpoint; its cost is the cost of the shortest intra-area path that
    exists between the two routers.

List of router links advertisements
    A router links advertisement is generated by each router in the
    area.  It describes the state of the router's interfaces to the
    area.

List of network links advertisements
    One network links advertisement is generated for each transit
    multi-access network in the area.  It describes the set of routers
    currently connected to the network.

List of summary links advertisements
    Summary link advertisements originate from the area's area border
    routers.  They describe routes to destinations internal to the



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    Autonomous System, yet external to the area.

Shortest-path tree
    The shortest-path tree for the area, with this router itself as
    root.  Derived from the collected router links and network links
    advertisements by the Dijkstra algorithm.

Authentication type
    The type of authentication used for this area.  Authentication types
    are defined in Appendix E.  All OSPF packet exchanges are
    authenticated.  Different authentication schemes may be used in
    different areas.

External routing capability
    Whether AS external advertisements will be flooded into/throughout
    the area.  This is a configurable parameter.  If AS external
    advertisements are excluded from the area, the area is called a
    "stub".  Internal to stub areas, routing to external destinations
    will be based solely on a default summary route.  The backbone
    cannot be configured as a stub area.  Also, virtual links cannot be
    configured through stub areas.  For more information, see Section
    3.6.

StubDefaultCost
    If the area has been configured as a stub area, and the router
    itself is an area border router, then the StubDefaultCost indicates
    the cost of the default summary link that the router should
    advertise into the area.  There can be a separate cost configured
    for each IP TOS.  See Section 12.4.3 for more information.


Unless otherwise specified, the remaining sections of this document
refer to the operation of the protocol in a single area.


7. Bringing Up Adjacencies

OSPF creates adjacencies between neighboring routers for the purpose of
exchanging routing information.  Not every two neighboring routers will
become adjacent.  This section covers the generalities involved in
creating adjacencies.  For further details consult Section 10.


7.1 The Hello Protocol

The Hello Protocol is responsible for establishing and maintaining
neighbor relationships.  It also ensures that communication between
neighbors is bidirectional.  Hello packets are sent periodically out all



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router interfaces.  Bidirectional communication is indicated when the
router sees itself listed in the neighbor's Hello Packet.

On multi-access networks, the Hello Protocol elects a Designated Router
for the network.  Among other things, the Designated Router controls
what adjacencies will be formed over the network (see below).

The Hello Protocol works differently on broadcast networks, as compared
to non-broadcast networks.  On broadcast networks, each router
advertises itself by periodically multicasting Hello Packets.  This
allows neighbors to be discovered dynamically.  These Hello Packets
contain the router's view of the Designated Router's identity, and the
list of routers whose Hellos have been seen recently.

On non-broadcast networks some configuration information is necessary
for the operation of the Hello Protocol.  Each router that may
potentially become Designated Router has a list of all other routers
attached to the network.  A router, having Designated Router potential,
sends hellos to all other potential Designated Routers when its
interface to the non-broadcast network first becomes operational.  This
is an attempt to find the Designated Router for the network.  If the
router itself is elected Designated Router, it begins sending hellos to
all other routers attached to the network.

After a neighbor has been discovered, bidirectional communication
ensured, and (if on a multi-access network) a Designated Router elected,
a decision is made regarding whether or not an adjacency should be
formed with the neighbor (see Section 10.4).  An attempt is always made
to establish adjacencies over point-to-point networks and virtual links.
The first step in bringing up an adjacency is to synchronize the
neighbors' topological databases.  This is covered in the next section.


7.2 The Synchronization of Databases

In an SPF-based routing algorithm, it is very important for all routers'
topological databases to stay synchronized.  OSPF simplifies this by
requiring only adjacent routers to remain synchronized.  The
synchronization process begins as soon as the routers attempt to bring
up the adjacency.  Each router describes its database by sending a
sequence of Database Description packets to its neighbor.  Each Database
Description Packet describes a set of link state advertisements
belonging to the database.  When the neighbor sees a link state
advertisement that is more recent than its own database copy, it makes a
note that this newer advertisement should be requested.

This sending and receiving of Database Description packets is called the
"Database Exchange Process".  During this process, the two routers form



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a master/slave relationship.  Each Database Description Packet has a
sequence number.  Database Description Packets sent by the master
(polls) are acknowledged by the slave through echoing of the sequence
number.  Both polls and their responses contain summaries of link state
data.  The master is the only one allowed to retransmit Database
Description Packets.  It does so only at fixed intervals, the length of
which is the configured constant RxmtInterval.

Each Database Description contains an indication that there are more
packets to follow --- the M-bit.  The Database Exchange Process is over
when a router has received and sent Database Description Packets with
the M-bit off.

During and after the Database Exchange Process, each router has a list
of those link state advertisements for which the neighbor has more up-
to-date instances.  These advertisements are requested in Link State
Request Packets.  Link State Request packets that are not satisfied are
retransmitted at fixed intervals of time RxmtInterval.  When the
Database Description Process has completed and all Link State Requests
have been satisfied, the databases are deemed synchronized and the
routers are marked fully adjacent.  At this time the adjacency is fully
functional and is advertised in the two routers' link state
advertisements.

The adjacency is used by the flooding procedure as soon as the Database
Exchange Process begins.  This simplifies database synchronization, and
guarantees that it finishes in a predictable period of time.


7.3 The Designated Router

Every multi-access network has a Designated Router.  The Designated
Router performs two main functions for the routing protocol:

o   The Designated Router originates a network links advertisement on
    behalf of the network.  This advertisement lists the set of routers
    (including the Designated Router itself) currently attached to the
    network.  The Link State ID for this advertisement (see Section
    12.1.4) is the IP interface address of the Designated Router.  The
    IP network number can then be obtained by using the subnet/network
    mask.

o   The Designated router becomes adjacent to all other routers on the
    network.  Since the link state databases are synchronized across
    adjacencies (through adjacency bring-up and then the flooding
    procedure), the Designated Router plays a central part in the
    synchronization process.




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The Designated Router is elected by the Hello Protocol.  A router's
Hello Packet contains its Router Priority, which is configurable on a
per-interface basis.  In general, when a router's interface to a network
first becomes functional, it checks to see whether there is currently a
Designated Router for the network.  If there is, it accepts that
Designated Router, regardless of its Router Priority.  (This makes it
harder to predict the identity of the Designated Router, but ensures
that the Designated Router changes less often.  See below.)  Otherwise,
the router itself becomes Designated Router if it has the highest Router
Priority on the network.  A more detailed (and more accurate)
description of Designated Router election is presented in Section 9.4.

The Designated Router is the endpoint of many adjacencies.  In order to
optimize the flooding procedure on broadcast networks, the Designated
Router multicasts its Link State Update Packets to the address
AllSPFRouters, rather than sending separate packets over each adjacency.

Section 2 of this document discusses the directed graph representation
of an area.  Router nodes are labelled with their Router ID.  Broadcast
network nodes are actually labelled with the IP address of their
Designated Router.  It follows that when the Designated Router changes,
it appears as if the network node on the graph is replaced by an
entirely new node.  This will cause the network and all its attached
routers to originate new link state advertisements.  Until the
topological databases again converge, some temporary loss of
connectivity may result.  This may result in ICMP unreachable messages
being sent in response to data traffic.  For that reason, the Designated
Router should change only infrequently.  Router Priorities should be
configured so that the most dependable router on a network eventually
becomes Designated Router.


7.4 The Backup Designated Router

In order to make the transition to a new Designated Router smoother,
there is a Backup Designated Router for each multi-access network.  The
Backup Designated Router is also adjacent to all routers on the network,
and becomes Designated Router when the previous Designated Router fails.
If there were no Backup Designated Router, when a new Designated Router
became necessary, new adjacencies would have to be formed between the
router and all other routers attached to the network.  Part of the
adjacency forming process is the synchronizing of topological databases,
which can potentially take quite a long time.  During this time, the
network would not be available for transit data traffic.  The Backup
Designated obviates the need to form these adjacencies, since they
already exist.  This means the period of disruption in transit traffic
lasts only as long as it take to flood the new link state advertisements
(which announce the new Designated Router).



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The Backup Designated Router does not generate a network links
advertisement for the network.  (If it did, the transition to a new
Designated Router would be even faster.  However, this is a tradeoff
between database size and speed of convergence when the Designated
Router disappears.)

The Backup Designated Router is also elected by the Hello Protocol.
Each Hello Packet has a field that specifies the Backup Designated
Router for the network.

In some steps of the flooding procedure, the Backup Designated Router
plays a passive role, letting the Designated Router do more of the work.
This cuts down on the amount of local routing traffic.  See Section 13.3
for more information.


7.5 The graph of adjacencies

An adjacency is bound to the network that the two routers have in
common.  If two routers have multiple networks in common, they may have
multiple adjacencies between them.

One can picture the collection of adjacencies on a network as forming an
undirected graph.  The vertices consist of routers, with an edge joining
two routers if they are adjacent.  The graph of adjacencies describes
the flow of routing protocol packets, and in particular Link State
Updates, through the Autonomous System.

Two graphs are possible, depending on whether the common network is
multi-access.  On physical point-to-point networks (and virtual links),
the two routers joined by the network will be adjacent after their
databases have been synchronized.  On multi-access networks, both the
Designated Router and the Backup Designated Router are adjacent to all
other routers attached to the network, and these account for all
adjacencies.

These graphs are shown in Figure 10.  It is assumed that router RT7 has
become the Designated Router, and router RT3 the Backup Designated
Router, for the network N2.  The Backup Designated Router performs a
lesser function during the flooding procedure than the Designated Router
(see Section 13.3).  This is the reason for the dashed lines connecting
the Backup Designated Router RT3.


8. Protocol Packet Processing

This section discusses the general processing of routing protocol
packets.  It is very important that the router topological databases



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remain synchronized.  For this reason, routing protocol packets should
get preferential treatment over ordinary data packets, both in sending
and receiving.

Routing protocol packets are sent along adjacencies only (with the
exception of Hello packets, which are used to discover the adjacencies).
This means that all protocol packets travel a single IP hop, except
those sent over virtual links.

All routing protocol packets begin with a standard header.  The sections
below give the details on how to fill in and verify this standard
header.  Then, for each packet type, the section is listed that gives
more details on that particular packet type's processing.



8.1 Sending protocol packets

When a router sends a routing protocol packet, it fills in the fields of
that standard header as follows.  For more details on the header format
consult Section A.3.1:


Version #
    Set to 2, the version number of the protocol as documented in this
    specification.

Packet type
    The type of OSPF packet, such as Link state Update or Hello Packet.

Packet length
    The length of the entire OSPF packet in bytes, including the
    standard header.

Router ID
    The identity of the router itself (who is originating the packet).


                 ______________________________________

                 (Figure not included in text version.)

                  Figure 10: The graph of adjacencies
                   Figure 11: Interface state changes
                 ______________________________________






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Area ID
    The area that the packet is being sent into.

Checksum
    The standard IP 16-bit one's complement checksum of the entire OSPF
    packet, excluding the 64-bit authentication field.  This checksum
    should be calculated before handing the packet to the appropriate
    authentication procedure.

Autype and Authentication
    Each OSPF packet exchange is authenticated.  Authentication types
    are assigned by the protocol and documented in Appendix E.  A
    different authentication scheme can be used for each OSPF area.  The
    64-bit authentication field is set by the appropriate authentication
    procedure (determined by Autype).  This procedure should be the last
    called when forming the packet to be sent.  The setting of the
    authentication field is determined by the packet contents and the
    authentication key (which is configurable on a per-interface basis).


The IP destination address for the packet is selected as follows.  On
physical point-to-point networks, the IP destination is always set to
the the address AllSPFRouters.  On all other network types (including
virtual links), the majority of OSPF packets are sent as unicasts, i.e.,
sent directly to the other end of the adjacency.  In this case, the IP
destination is just the neighbor IP address associated with the other
end of the adjacency (see Section 10).  The only packets not sent as
unicasts are on broadcast networks; on these networks Hello packets are
sent to the multicast destination AllSPFRouters, the Designated Router
and its Backup send both Link State Update Packets and Link State
Acknowledgment Packets to the multicast address AllSPFRouters, while all
other routers send both their Link State Update and Link State
Acknowledgment Packets to the multicast address AllDRouters.

Retransmissions of Link State Update packets are ALWAYS sent as
unicasts.

The IP source address should be set to the IP address of the sending
interface.  Interfaces to unnumbered point-to-point networks have no
associated IP address.  On these interfaces, the IP source should be set
to any of the other IP addresses belonging to the router.  For this
reason, there must be at least one IP address assigned to the router.[2]
Note that, for most purposes, virtual links act precisely the same as
unnumbered point-to-point networks.  However, each virtual link does
have an interface IP address (discovered during the routing table build
process) which is used as the IP source when sending packets over the
virtual link.




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For more information on the format of specific packet types, consult the
sections listed in Table 10.



         Type   Packet name            detailed section (transmit)
         _________________________________________________________
         1      Hello                  Section  9.5
         2      Database description   Section 10.8
         3      Link state request     Section 10.9
         4      Link state update      Section 13.3
         5      Link state ack         Section 13.5


             Table 10: Sections describing packet transmission.



8.2 Receiving protocol packets

Whenever a protocol packet is received by the router it is marked with
the interface it was received on.  For routers that have virtual links
configured, it may not be immediately obvious which interface to
associate the packet with.  For example, consider the router RT11
depicted in Figure 6.  If RT11 receives an OSPF protocol packet on its
interface to network N8, it may want to associate the packet with the
interface to area 2, or with the virtual link to router RT10 (which is
part of the backbone).  In the following, we assume that the packet is
initially associated with the non-virtual  link.[3]

In order for the packet to be accepted at the IP level, it must pass a
number of tests, even before the packet is passed to OSPF for
processing:


o   The IP checksum must be correct.

o   The packet's IP destination address must be the IP address of the
    receiving interface, or one of the IP multicast addresses
    AllSPFRouters or AllDRouters.

o   The IP protocol specified must be OSPF (89).

o   Locally originated packets should not be passed on to OSPF.  That
    is, the source IP address should be examined to make sure this is
    not a multicast packet that the router itself generated.





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Next, the OSPF packet header is verified.  The fields specified in the
header must match those configured for the receiving interface.  If they
do not, the packet should be discarded:


o   The version number field must specify protocol version 2.

o   The 16-bit checksum of the OSPF packet's contents must be verified.
    Remember that the 64-bit authentication field must be excluded from
    the checksum calculation.

o   The Area ID found in the OSPF header must be verified.  If both of
    the following cases fail, the packet should be discarded.  The Area
    ID specified in the header must either:

    (1) Match the Area ID of the receiving interface.  In this case, the
        packet has been sent over a single hop.  Therefore, the packet's
        IP source address must be on the same network as the receiving
        interface.  This can be determined by comparing the packet's IP
        source address to the interface's IP address, after masking both
        addresses with the interface mask.

    (2) Indicate the backbone.  In this case, the packet has been sent
        over a virtual link.  The receiving router must be an area
        border router, and the router ID specified in the packet (the
        source router) must be the other end of a configured virtual
        link.  The receiving interface must also attach to the virtual
        link's configured transit area.  If all of these checks succeed,
        the packet is accepted and is from now on associated with the
        virtual link (and the backbone area).

o   Packets whose IP destination is AllDRouters should only be accepted
    if the state of the receiving interface is DR or Backup (see Section
    9.1).

o   The Authentication type specified must match the authentication type
    specified for the associated area.


Next, the packet must be authenticated.  This depends on the
authentication type specified (see Appendix E).  The authentication
procedure may use an Authentication key, which can be configured on a
per-interface basis.  If the authentication fails, the packet should be
discarded.

If the packet type is Hello, it should then be further processed by the
Hello Protocol (see Section 10.5).  All other packet types are
sent/received only on adjacencies.  This means that the packet must have



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been sent by one of the router's active neighbors.  If the receiving
interface is a multi-access network (either broadcast or non-broadcast)
the sender is identified by the IP source address found in the packet's
IP header.  If the receiving interface is a point-to-point link or a
virtual link, the sender is identified by the Router ID (source router)
found in the packet's OSPF header.  The data structure associated with
the receiving interface contains the list of active neighbors.  Packets
not matching any active neighbor are discarded.

At this point all received protocol packets are associated with an
active neighbor.  For the further input processing of specific packet
types, consult the sections listed in Table 11.



          Type   Packet name            detailed section (receive)
          ________________________________________________________
          1      Hello                  Section 10.5
          2      Database description   Section 10.6
          3      Link state request     Section 10.7
          4      Link state update      Section 13
          5      Link state ack         Section 13.7


              Table 11: Sections describing packet reception.



9. The Interface Data Structure

An OSPF interface is the connection between a router and a network.
There is a single OSPF interface structure for each attached network;
each interface structure has at most one IP interface address (see
below).  The support for multiple addresses on a single network is a
matter for future consideration.

An OSPF interface can be considered to belong to the area that contains
the attached network.  All routing protocol packets originated by the
router over this interface are labelled with the interface's Area ID.
One or more router adjacencies may develop over an interface.  A
router's link state advertisements reflect the state of its interfaces
and their associated adjacencies.

The following data items are associated with an interface.  Note that a
number of these items are actually configuration for the attached
network; those items must be the same for all routers connected to the
network.




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Type
    The kind of network to which the interface attaches.  Its value is
    either broadcast, non-broadcast yet still multi-access, point-to-
    point or virtual link.

State
    The functional level of an interface.  State determines whether or
    not full adjacencies are allowed to form over the interface.  State
    is also reflected in the router's link state advertisements.

IP interface address
    The IP address associated with the interface.  This appears as the
    IP source address in all routing protocol packets originated over
    this interface.  Interfaces to unnumbered point-to-point networks do
    not have an associated IP address.

IP interface mask
    This indicates the portion of the IP interface address that
    identifies the attached network.  This is often referred to as the
    subnet mask.  Masking the IP interface address with this value
    yields the IP network number of the attached network.

Area ID
    The Area ID to which the attached network belongs.  All routing
    protocol packets originating from the interface are labelled with
    this Area ID.

HelloInterval
    The length of time, in seconds, between the Hello packets that the
    router sends on the interface.  Advertised in Hello packets sent out
    this interface.

RouterDeadInterval
    The number of seconds before the router's neighbors will declare it
    down, when they stop hearing the router's hellos.  Advertised in
    Hello packets sent out this interface.

InfTransDelay
    The estimated number of seconds it takes to transmit a Link State
    Update Packet over this interface.  Link state advertisements
    contained in the update packet will have their age incremented by
    this amount before transmission.  This value should take into
    account transmission and propagation delays; it must be greater than
    zero.

Router Priority
    An 8-bit unsigned integer.  When two routers attached to a network
    both attempt to become Designated Router, the one with the highest



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    Router Priority takes precedence.  A router whose Router Priority is
    set to 0 is ineligible to become Designated Router on the attached
    network.  Advertised in Hello packets sent out this interface.

Hello Timer
    An interval timer that causes the interface to send a Hello packet.
    This timer fires every HelloInterval seconds.  Note that on non-
    broadcast networks a separate Hello packet is sent to each qualified
    neighbor.

Wait Timer
    A single shot timer that causes the interface to exit the Waiting
    state, and as a consequence select a Designated Router on the
    network.  The length of the timer is RouterDeadInterval seconds.

List of neighboring routers
    The other routers attached to this network.  On multi-access
    networks, this list is formed by the Hello Protocol.  Adjacencies
    will be formed to some of these neighbors.  The set of adjacent
    neighbors can be determined by an examination of all of the
    neighbors' states.

Designated Router
    The Designated Router selected for the attached network.  The
    Designated Router is selected on all multi-access networks by the
    Hello Protocol.  Two pieces of identification are kept for the
    Designated Router: its Router ID and its interface IP address on the
    network.  The Designated Router advertises link state for the
    network.  The network link state advertisement is labelled with the
    Designated Router's IP address.  This item is initialized to 0,
    which indicates the lack of a Designated Router.

Backup Designated Router
    The Backup Designated Router is also selected on all multi-access
    networks by the Hello Protocol.  All routers on the attached network
    become adjacent to both the Designated Router and the Backup
    Designated Router.  The Backup Designated Router becomes Designated
    Router when the current Designated Router fails.  Initialized to 0
    indicating the lack of a Backup Designated Router.

Interface output cost(s)
    The cost of sending a packet on the interface, expressed in the link
    state metric.  This is advertised as the link cost for this
    interface in the router links advertisement.  There may be a
    separate cost for each IP Type of Service.  The cost of an interface
    must be greater than zero.





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RxmtInterval
    The number of seconds between link state advertisement
    retransmissions, for adjacencies belonging to this interface.  Also
    used when retransmitting Database Description and Link State Request
    Packets.

Authentication key
    This configured data allows the authentication procedure to generate
    and/or verify the authentication field in the OSPF header.  The
    authentication key can be configured on a per-interface basis.  For
    example, if the authentication type indicates simple password, the
    authentication key would be a 64-bit password.  This key would be
    inserted directly into the OSPF header when originating routing
    protocol packets, and there could be a separate password for each
    network.


9.1 Interface states

The various states that router interface may attain is documented in
this section.  The states are listed in order of progressing
functionality.  For example, the inoperative state is listed first,
followed by a list of intermediate states before the final, fully
functional state is achieved.  The specification makes use of this
ordering by sometimes making references such as "those interfaces in
state greater than X".

Figure 11 shows the graph of interface state changes.  The arcs of the
graph are labelled with the event causing the state change.  These
events are documented in Section 9.2.  The interface state machine is
described in more detail in Section 9.3.


Down
    This is the initial interface state.  In this state, the lower-level
    protocols have indicated that the interface is unusable.  No
    protocol traffic at all will be sent or received on such a
    interface.  In this state, interface parameters should be set to
    their initial values.  All interface timers should be disabled, and
    there should be no adjacencies associated with the interface.

Loopback
    In this state, the router's interface to the network is looped back.
    The interface may be looped back in hardware or software.  The
    interface will be unavailable for regular data traffic.  However, it
    may still be desirable to gain information on the quality of this
    interface, either through sending ICMP pings to the interface or
    through something like a bit error test.  For this reason, IP



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    packets may still be addressed to an interface in Loopback state.
    To facilitate this, such interfaces are advertised in router links
    advertisements as single host routes, whose destination is the IP
    interface address.[4]

Waiting
    In this state, the router is trying to determine the identity of the
    Backup Designated Router for the network.  To do this, the router
    monitors the Hellos it receives.  The router is not allowed to elect
    a Backup Designated Router nor Designated Router until it
    transitions out of Waiting state.  This prevents unnecessary changes
    of (Backup) Designated Router.

Point-to-point
    In this state, the interface is operational, and connects either to
    a physical point-to-point network or to a virtual link.  Upon
    entering this state, the router attempts to form an adjacency with
    the neighboring router.  Hellos are sent to the neighbor every
    HelloInterval seconds.

DR Other
    The interface is to a multi-access network on which another router
    has been selected to be the Designated Router.  In this state, the
    router itself has not been selected Backup Designated Router either.
    The router forms adjacencies to both the Designated Router and the
    Backup Designated Router (if they exist).

Backup
    In this state, the router itself is the Backup Designated Router on
    the attached network.  It will be promoted to Designated Router when
    the present Designated Router fails.  The router establishes
    adjacencies to all other routers attached to the network.  The
    Backup Designated Router performs slightly different functions
    during the Flooding Procedure, as compared to the Designated Router
    (see Section 13.3).  See Section 7.4 for more details on the
    functions performed by the Backup Designated Router.

DR  In this state, this router itself is the Designated Router on the
    attached network.  Adjacencies are established to all other routers
    attached to the network.  The router must also originate a network
    links advertisement for the network node.  The advertisement will
    contain links to all routers (including the Designated Router
    itself) attached to the network.  See Section 7.3 for more details
    on the functions performed by the Designated Router.







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9.2 Events causing interface state changes

State changes can be effected by a number of events.  These events are
pictured as the labelled arcs in Figure 11.  The label definitions are
listed below.  For a detailed explanation of the effect of these events
on OSPF protocol operation, consult Section 9.3.


Interface Up
    Lower-level protocols have indicated that the network interface is
    operational.  This enables the interface to transition out of Down
    state.  On virtual links, the interface operational indication is
    actually a result of the shortest path calculation (see Section
    16.7).

Wait Timer
    The Wait timer has fired, indicating the end of the waiting period
    that is required before electing a (Backup) Designated Router.

Backup seen
    The router has detected the existence or non-existence of a Backup
    Designated Router for the network.  This is done in one of two ways.
    First, a Hello Packet may be received from a neighbor claiming to be
    itself the Backup Designated Router.  Alternatively, a Hello Packet
    may be received from a neighbor claiming to be itself the Designated
    Router, and indicating that there is no Backup.  In either case
    there must be bidirectional communication with the neighbor, i.e.,
    the router must also appear in the neighbor's Hello Packet.  This
    event signals an end to the Waiting state.

Neighbor Change
    There has been a change in the set of bidirectional neighbors
    associated with the interface.  The (Backup) Designated Router needs
    to be recalculated.  The following neighbor changes lead to the
    Neighbor Change event.  For an explanation of neighbor states, see
    Section 10.1.

    o   Bidirectional communication has been established to a neighbor.
        In other words, the state of the neighbor has transitioned to
        2-Way or higher.

    o   There is no longer bidirectional communication with a neighbor.
        In other words, the state of the neighbor has transitioned to
        Init or lower.

    o   One of the bidirectional neighbors is newly declaring itself as
        either Designated Router or Backup Designated Router.  This is
        detected through examination of that neighbor's Hello Packets.



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    o   One of the bidirectional neighbors is no longer declaring itself
        as Designated Router, or is no longer declaring itself as Backup
        Designated Router.  This is again detected through examination
        of that neighbor's Hello Packets.

    o   The advertised Router Priority for a bidirectional neighbor has
        changed.  This is again detected through examination of that
        neighbor's Hello Packets.

Loop Ind
    An indication has been received that the interface is now looped
    back to itself.  This indication can be received either from network
    management or from the lower level protocols.

Unloop Ind
    An indication has been received that the interface is no longer
    looped back.  As with the Loop Ind event, this indication can be
    received either from network management or from the lower level
    protocols.

Interface Down
    Lower-level protocols indicate that this interface is no longer
    functional.  No matter what the current interface state is, the new
    interface state will be Down.


9.3 The Interface state machine

A detailed description of the interface state changes follows.  Each
state change is invoked by an event (Section 9.2).  This event may
produce different effects, depending on the current state of the
interface.  For this reason, the state machine below is organized by
current interface state and received event.  Each entry in the state
machine describes the resulting new interface state and the required set
of additional actions.

When an interface's state changes, it may be necessary to originate a
new router links advertisement.  See Section 12.4 for more details.

Some of the required actions below involve generating events for the
neighbor state machine.  For example, when an interface becomes
inoperative, all neighbor connections associated with the interface must
be destroyed.  For more information on the neighbor state machine, see
Section 10.3.


 State(s):  Down




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    Event:  Interface Up

New state:  Depends on action routine

   Action:  Start the interval Hello Timer, enabling the periodic
            sending of Hello packets out the interface.  If the attached
            network is a physical point-to-point network or virtual
            link, the interface state transitions to Point-to-Point.
            Else, if the router is not eligible to become Designated
            Router the interface state transitions to DR other.

            Otherwise, the attached network is multi-access and the
            router is eligible to become Designated Router.  In this
            case, in an attempt to discover the attached network's
            Designated Router the interface state is set to Waiting and
            the single shot Wait Timer is started.  If in addition the
            attached network is non-broadcast, examine the configured
            list of neighbors for this interface and generate the
            neighbor event Start for each neighbor that is also eligible
            to become Designated Router.


 State(s):  Waiting

    Event:  Backup Seen

New state:  Depends upon action routine.

   Action:  Calculate the attached network's Backup Designated Router
            and Designated Router, as shown in Section 9.4.  As a result
            of this calculation, the new state of the interface will be
            either DR other, Backup or DR.


 State(s):  Waiting

    Event:  Wait Timer

New state:  Depends upon action routine.

   Action:  Calculate the attached network's Backup Designated Router
            and Designated Router, as shown in Section 9.4.  As a result
            of this calculation, the new state of the interface will be
            either DR other, Backup or DR.


 State(s):  DR Other, Backup or DR




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    Event:  Neighbor Change

New state:  Depends upon action routine.

   Action:  Recalculate the attached network's Backup Designated Router
            and Designated Router, as shown in Section 9.4.  As a result
            of this calculation, the new state of the interface will be
            either DR other, Backup or DR.


 State(s):  Any State

    Event:  Interface Down

New state:  Down

   Action:  All interface variables are reset, and interface timers
            disabled.  Also, all neighbor connections associated with
            the interface are destroyed.  This is done by generating the
            event KillNbr on all associated neighbors (see Section
            10.2).


 State(s):  Any State

    Event:  Loop Ind

New state:  Loopback

   Action:  Since this interface is no longer connected to the attached
            network the actions associated with the above Interface Down
            event are executed.


 State(s):  Loopback

    Event:  Unloop Ind

New state:  Down

   Action:  No actions are necessary.  For example, the interface
            variables have already been reset upon entering the Loopback
            state.  Note that reception of an Interface Up event is
            necessary before the interface again becomes fully
            functional.






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9.4 Electing the Designated Router

This section describes the algorithm used for calculating a network's
Designated Router and Backup Designated Router.  This algorithm is
invoked by the Interface state machine.  The initial time a router runs
the election algorithm for a network, the network's Designated Router
and Backup Designated Router are initialized to 0.0.0.0.  This indicates
the lack of both a Designated Router and a Backup Designated Router.

The Designated Router election algorithm proceeds as follows: Call the
router doing the calculation Router X.  The list of neighbors attached
to the network and having established bidirectional communication with
Router X is examined.  This list is precisely the collection of Router
X's neighbors (on this network) whose state is greater than or equal to
2-Way (see Section 10.1).  Router X itself is also considered to be on
the list.  Discard all routers from the list that are ineligible to
become Designated Router.  (Routers having Router Priority of 0 are
ineligible to become Designated Router.)  The following steps are then
executed, considering only those routers that remain on the list:


(1) Note the current values for the network's Designated Router and
    Backup Designated Router.  This is used later for comparison
    purposes.

(2) Calculate the new Backup Designated Router for the network as
    follows.  Only those routers on the list that have not declared
    themselves to be Designated Router are eligible to become Backup
    Designated Router.  If one or more of these routers have declared
    themselves Backup Designated Router (i.e., they are currently
    listing themselves as Backup Designated Router, but not as
    Designated Router, in their Hello Packets) the one having highest
    Router Priority is declared to be Backup Designated Router.  In case
    of a tie, the one having the highest Router ID is chosen.  If no
    routers have declared themselves Backup Designated Router, choose
    the router having highest Router Priority, (again excluding those
    routers who have declared themselves Designated Router), and again
    use the Router ID to break ties.

(3) Calculate the new Designated Router for the network as follows.  If
    one or more of the routers have declared themselves Designated
    Router (i.e., they are currently listing themselves as Designated
    Router in their Hello Packets) the one having highest Router
    Priority is declared to be Designated Router.  In case of a tie, the
    one having the highest Router ID is chosen.  If no routers have
    declared themselves Designated Router, promote the new Backup
    Designated Router to Designated Router.




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(4) If Router X is now newly the Designated Router or newly the Backup
    Designated Router, or is now no longer the Designated Router or no
    longer the Backup Designated Router, repeat steps 2 and 3, and then
    proceed to step 5.  For example, if Router X is now the Designated
    Router, when step 2 is repeated X will no longer be eligible for
    Backup Designated Router election.  Among other things, this will
    ensure that no router will declare itself both Backup Designated
    Router and Designated Router.[5]

(5) As a result of these calculations, the router itself may now be
    Designated Router or Backup Designated Router.  See Sections 7.3 and
    7.4 for the additional duties this would entail.  The router's
    interface state should be set accordingly.  If the router itself is
    now Designated Router, the new interface state is DR.  If the router
    itself is now Backup Designated Router, the new interface state is
    Backup.  Otherwise, the new interface state is DR Other.

(6) If the attached network is non-broadcast, and the router itself has
    just become either Designated Router or Backup Designated Router, it
    must start sending hellos to those neighbors that are not eligible
    to become Designated Router (see Section 9.5.1).  This is done by
    invoking the neighbor event Start for each neighbor having a Router
    Priority of 0.

(7) If the above calculations have caused the identity of either the
    Designated Router or Backup Designated Router to change, the set of
    adjacencies associated with this interface will need to be modified.
    Some adjacencies may need to be formed, and others may need to be
    broken.  To accomplish this, invoke the event AdjOK?  on all
    neighbors whose state is at least 2-Way.  This will cause their
    eligibility for adjacency to be reexamined (see Sections 10.3 and
    10.4).


The reason behind the election algorithm's complexity is the desire for
an orderly transition from Backup Designated Router to Designated
Router, when the current Designated Router fails.  This orderly
transition is ensured through the introduction of hysteresis: no new
Backup router can be chosen until the old Backup accepts its new
Designated Router responsibilities.

If Router X is not itself eligible to become Designated Router, it is
possible that neither a Backup Designated Router nor a Designated Router
will be selected in the above procedure.  Note also that if Router X is
the only attached router that is eligible to become Designated Router,
it will select itself as Designated Router and there will be no Backup
Designated Router for the network.




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9.5 Sending Hello packets

Hello packets are sent out each functioning router interface.  They are
used to discover and maintain neighbor relationships.[6] On multi-access
networks, hellos are also used to elect the Designated Router and Backup
Designated Router, and in that way determine what adjacencies should be
formed.

The format of a Hello packet is detailed in Section A.3.2.  The Hello
Packet contains the router's Router Priority (used in choosing the
Designated Router), and the interval between Hello broadcasts
(HelloInterval).  The Hello Packet also indicates how often a neighbor
must be heard from to remain active (RouterDeadInterval).  Both
HelloInterval and RouterDeadInterval must be the same for all routers
attached to a common network.  The Hello packet also contains the IP
address mask of the attached network (Network Mask).  On unnumbered
point-to-point networks and on virtual links this field should be set to
0.

The Hello packet's Options field describes the router's optional OSPF
capabilities.  There are currently two optional capabilities defined
(see Sections 4.5 and A.2).  The T-bit of the Options field should be
set if the router is capable of calculating separate routes for each IP
TOS.  The E-bit should be set if and only if the attached area is
capable of processing AS external advertisements (i.e., it is not a stub
area).  If the E-bit is set incorrectly the neighboring routers will
refuse to accept the Hello Packet (see Section 10.5).  The rest of the
Hello Packet's Options field should be set to zero.

In order to ensure two-way communication between adjacent routers, the
Hello packet contains the list of all routers from which hellos have
been seen recently.  The Hello packet also contains the router's current
choice for Designated Router and Backup Designated Router.  A value of 0
in these fields means that one has not yet been selected.

On broadcast networks and physical point-to-point networks, Hello
packets are sent every HelloInterval seconds to the IP multicast address
AllSPFRouters.  On virtual links, Hello packets are sent as unicasts
(addressed directly to the other end of the virtual link) every
HelloInterval seconds.  On non-broadcast networks, the sending of Hello
packets is more complicated.  This will be covered in the next section.


9.5.1 Sending Hello packets on non-broadcast networks

Static configuration information is necessary in order for the Hello
Protocol to function on non-broadcast networks (see Section C.5).  Every
attached router which is eligible to become Designated Router has a



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configured list of all of its neighbors on the network.  Each listed
neighbor is labelled with its Designated Router eligibility.

The interface state must be at least Waiting for any hellos to be sent.
Hellos are then sent directly (as unicasts) to some subset of a router's
neighbors.  Sometimes an hello is sent periodically on a timer; at other
times it is sent as a response to a received hello.  A router's hello-
sending behavior varies depending on whether the router itself is
eligible to become Designated Router.

If the router is eligible to become Designated Router, it must
periodically send hellos to all neighbors that are also eligible.  In
addition, if the router is itself the Designated Router or Backup
Designated Router, it must also send periodic hellos to all other
neighbors.  This means that any two eligible routers are always
exchanging hellos, which is necessary for the correct operation of the
Designated Router election algorithm.  To minimize the number of hellos
sent, the number of eligible routers on a non-broadcast network should
be kept small.

If the router is not eligible to become Designated Router, it must
periodically send hellos to both the Designated Router and the Backup
Designated Router (if they exist).  It must also send an hello in reply
to an hello received from any eligible neighbor (other than the current
Designated Router and Backup Designated Router).  This is needed to
establish an initial bidirectional relationship with any potential
Designated Router.

When sending Hello packets periodically to any neighbor, the interval
between hellos is determined by the neighbor's state.  If the neighbor
is in state Down, hellos are sent every PollInterval seconds.
Otherwise, hellos are sent every HelloInterval seconds.


10. The Neighbor Data Structure

An OSPF router converses with its neighboring routers.  Each separate
conversation is described by a "neighbor data structure".  Each
conversation is bound to a particular OSPF router interface, and is
identified either by the neighboring router's OSPF router ID or by its
Neighbor IP address (see below).  Thus if the OSPF router and another
router have multiple attached networks in common, multiple conversations
ensue, each described by a unique neighbor data structure.  Each
separate conversation is loosely referred to in the text as being a
separate "neighbor".

The neighbor data structure contains all information pertinent to the
forming or formed adjacency between the two neighbors.  (However,



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remember that not all neighbors become adjacent.)  An adjacency can be
viewed as a highly developed conversation between two routers.


State
    The functional level of the neighbor conversation.  This is
    described in more detail in Section 10.1.

Inactivity Timer
    A single shot timer whose firing indicates that no Hello Packet has
    been seen from this neighbor recently.  The length of the timer is
    RouterDeadInterval seconds.

Master/Slave
    When the two neighbors are exchanging databases, they form a Master
    Slave relationship.  The Master sends the first Database Description
    Packet, and is the only part that is allowed to retransmit.  The
    slave can only respond to the master's Database Description Packets.
    The master/slave relationship is negotiated in state ExStart.

Sequence Number
    A 32-bit number identifying individual Database Description packets.
    When the neighbor state ExStart is entered, the sequence number
    should be set to a value not previously seen by the neighboring
    router.  One possible scheme is to use the machine's time of day
    counter.  The sequence number is then incremented by the master with
    each new Database Description packet sent.  The slave's sequence
    number indicates the last packet received from the master.  Only one
    packet is allowed outstanding at a time.

Neighbor ID
    The OSPF Router ID of the neighboring router.  The neighbor ID is
    learned when Hello packets are received from the neighbor, or is
    configured if this is a virtual adjacency (see Section C.4).

Neighbor priority
    The Router Priority of the neighboring router.  Contained in the
    neighbor's Hello packets, this item is used when selecting the
    Designated Router for the attached network.

Neighbor IP address
    The IP address of the neighboring router's interface to the attached
    network.  Used as the Destination IP address when protocol packets
    are sent as unicasts along this adjacency.  Also used in router
    links advertisements as the Link ID for the attached network if the
    neighboring router is selected to be Designated Router (see Section
    12.4.1).  The neighbor IP address is learned when Hello packets are
    received from the neighbor.  For virtual links, the neighbor IP



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    address is learned during the routing table build process (see
    Section 15).

Neighbor Options
    The optional OSPF capabilities supported by the neighbor.  Learned
    during the Database Exchange process (see Section 10.6).  The
    neighbor's optional OSPF capabilities are also listed in its Hello
    packets.  This enables received Hellos to be rejected (i.e.,
    neighbor relationships will not even start to form) if there is a
    mismatch in certain crucial OSPF capabilities (see Section 10.5).
    The optional OSPF capabilities are documented in Section 4.5.

Neighbor's Designated Router
    The neighbor's idea of the Designated Router.  If this is the
    neighbor itself, this is important in the local calculation of the
    Designated Router.  Defined only on multi-access networks.

Neighbor's Backup Designated Router
    The neighbor's idea of the Backup Designated Router.  If this is the
    neighbor itself, this is important in the local calculation of the
    Backup Designated Router.  Defined only on multi-access networks.


The next set of variables are lists of link state advertisements.  These
lists describe subsets of the area topological database.  There can be
five distinct types of link state advertisements in an area topological
database: router links, network links, and type 3 and 4 summary links
(all stored in the area data structure), and AS external links (stored
in the global data structure).


Link state retransmission list
    The list of link state advertisements that have been flooded but not
    acknowledged on this adjacency.  These will be retransmitted at
    intervals until they are acknowledged, or until the adjacency is
    destroyed.

Database summary list
    The complete list of link state advertisements that make up the area
    topological database, at the moment the neighbor goes into Database
    Exchange state.  This list is sent to the neighbor in Database
    Description packets.

Link state request list
    The list of link state advertisements that need to be received from
    this neighbor in order to synchronize the two neighbors' topological
    databases.  This list is created as Database Description packets are
    received, and is then sent to the neighbor in Link State Request



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    packets.  The list is depleted as appropriate Link State Update
    packets are received.


10.1 Neighbor states

The state of a neighbor (really, the state of a conversation being held
with a neighboring router) is documented in the following sections.  The
states are listed in order of progressing functionality.  For example,
the inoperative state is listed first, followed by a list of
intermediate states before the final, fully functional state is
achieved.  The specification makes use of this ordering by sometimes
making references such as "those neighbors/adjacencies in state greater
than X".  Figures 12 and 13 show the graph of neighbor state changes.
The arcs of the graphs are labelled with the event causing the state
change.  The neighbor events are documented in Section 10.2.


The graph in Figure 12 show the state changes effected by the Hello
Protocol.  The Hello Protocol is responsible for neighbor acquisition
and maintenance, and for ensuring two way communication between
neighbors.

The graph in Figure 13 shows the forming of an adjacency.  Not every two
neighboring routers become adjacent (see Section 10.4).  The adjacency
starts to form when the neighbor is in state ExStart.  After the two
routers discover their master/slave status, the state transitions to
Exchange.  At this point the neighbor starts to be used in the flooding
procedure, and the two neighboring routers begin synchronizing their
databases.  When this synchronization is finished, the neighbor is in
state Full and we say that the two routers are fully adjacent.  At this
point the adjacency is listed in link state advertisements.

For a more detailed description of neighbor state changes, together with
the additional actions involved in each change, see Section 10.3.



         _____________________________________________________

                (Figures not included in text version.)

          Figure 12: Neighbor state changes (Hello Protocol)
         Figure 13: Neighbor state changes (Database Exchange)
         _____________________________________________________






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Down
    This is the initial state of a neighbor conversation.  It indicates
    that there has been no recent information received from the
    neighbor.  On non-broadcast networks, Hello packets may still be
    sent to "Down" neighbors, although at a reduced frequency (see
    Section 9.5.1).

Attempt
    This state is only valid for neighbors attached to non-broadcast
    networks.  It indicates that no recent information has been received
    from the neighbor, but that a more concerted effort should be made
    to contact the neighbor.  This is done by sending the neighbor Hello
    packets at intervals of HelloInterval (see Section 9.5.1).

Init
    In this state, an Hello packet has recently been seen from the
    neighbor.  However, bidirectional communication has not yet been
    established with the neighbor (i.e., the router itself did not
    appear in the neighbor's Hello packet).  All neighbors in this state
    (or higher) are listed in the Hello packets sent from the associated
    interface.

2-Way
    In this state, communication between the two routers is
    bidirectional.  This has been assured by the operation of the Hello
    Protocol.  This is the most advanced state short of beginning
    adjacency establishment.  The (Backup) Designated Router is selected
    from the set of neighbors in state 2-Way or greater.

ExStart
    This is the first step in creating an adjacency between the two
    neighboring routers.  The goal of this step is to decide which
    router is the master, and to decide upon the initial sequence
    number.  Neighbor conversations in this state or greater are called
    adjacencies.

Exchange
    In this state the router is describing its entire link state
    database by sending Database Description packets to the neighbor.
    Each Database Description Packet has a sequence number, and is
    explicitly acknowledged.  Only one Database Description Packet is
    allowed outstanding at any one time.  In this state, Link State
    Request Packets may also be sent asking for the neighbor's more
    recent advertisements.  All adjacencies in Exchange state or greater
    are used by the flooding procedure.  In fact, these adjacencies are
    fully capable of transmitting and receiving all types of OSPF
    routing protocol packets.




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Loading
    In this state, Link State Request packets are sent to the neighbor
    asking for the more recent advertisements that have been discovered
    (but not yet received) in the Exchange state.

Full
    In this state, the neighboring routers are fully adjacent.  These
    adjacencies will now appear in router links and network links
    advertisements.


10.2 Events causing neighbor state changes

State changes can be effected by a number of events.  These events are
shown in the labels of the arcs in Figures 12 and 13.  The label
definitions are as follows:


Hello Received
    A Hello packet has been received from a neighbor.

Start
    This is an indication that Hello Packets should now be sent to the
    neighbor at intervals of HelloInterval seconds.  This event is
    generated only for neighbors associated with non-broadcast networks.

2-Way Received
    Bidirectional communication has been realized between the two
    neighboring routers.  This is indicated by this router seeing itself
    in the other's Hello packet.

NegotiationDone
    The Master/Slave relationship has been negotiated, and sequence
    numbers have been exchanged.  This signals the start of the
    sending/receiving of Database Description packets.  For more
    information on the generation of this event, consult Section 10.8.

Exchange Done
    Both routers have successfully transmitted a full sequence of
    Database Description packets.  Each router now knows what parts of
    its link state database are out of date.  For more information on
    the generation of this event, consult Section 10.8.

BadLSReq
    A Link State Request has been received for a link state
    advertisement not contained in the database.  This indicates an
    error in the synchronization process.




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Loading Done
    Link State Updates have been received for all out-of-date portions
    of the database.  This is indicated by the Link state request list
    becoming empty after the Database Description Process has completed.

AdjOK?
    A decision must be made (again) as to whether an adjacency should be
    established/maintained with the neighbor.  This event will start
    some adjacencies forming, and destroy others.


The following events cause well developed neighbors to revert to lesser
states.  Unlike the above events, these events may occur when the
neighbor conversation is in any of a number of states.


Seq Number Mismatch
    A Database Description packet has been received that either a) has
    an unexpected sequence number, b) unexpectedly has the Init bit set
    or c) has an Options field differing from the last Options field
    received in a Database Description packet.  Any of these conditions
    indicate that some error has occurred during adjacency
    establishment.

1-Way
    An Hello packet has been received from the neighbor, in which this
    router is not mentioned.  This indicates that communication with the
    neighbor is not bidirectional.

KillNbr
    This  is  an  indication that  all  communication  with  the
    neighbor  is now  impossible,  forcing  the  neighbor  to  revert
    to  Down  state.

Inactivity Timer
    The inactivity Timer has fired.  This means that no Hello packets
    have been seen recently from the neighbor.  The neighbor reverts to
    Down state.

LLDown
    This is an indication from the lower level protocols that the
    neighbor is now unreachable.  For example, on an X.25 network this
    could be indicated by an X.25 clear indication with appropriate
    cause and diagnostic fields.  This event forces the neighbor into
    Down state.






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10.3 The Neighbor state machine

A detailed description of the neighbor state changes follows.  Each
state change is invoked by an event (Section 10.2).  This event may
produce different effects, depending on the current state of the
neighbor.  For this reason, the state machine below is organized by
current neighbor state and received event.  Each entry in the state
machine describes the resulting new neighbor state and the required set
of additional actions.

When an neighbor's state changes, it may be necessary to rerun the
Designated Router election algorithm.  This is determined by whether the
interface Neighbor Change event is generated (see Section 9.2).  Also,
if the Interface is in DR state (the router is itself Designated
Router), changes in neighbor state may cause a new network links
advertisement to be originated (see Section 12.4).

When the neighbor state machine needs to invoke the interface state
machine, it should be done as a scheduled task (see Section 4.4).  This
simplifies things, by ensuring that neither state machine will be
executed recursively.


 State(s):  Down

    Event:  Start

New state:  Attempt

   Action:  Send an hello to the neighbor (this neighbor is always
            associated with a non-broadcast network) and start the
            inactivity timer for the neighbor.  The timer's later firing
            would indicate that communication with the neighbor was not
            attained.


 State(s):  Attempt

    Event:  Hello Received

New state:  Init

   Action:  Restart the inactivity timer for the neighbor, since the
            neighbor has now been heard from.


 State(s):  Down




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    Event:  Hello Received

New state:  Init

   Action:  Start the inactivity timer for the neighbor.  The timer's
            later firing would indicate that the neighbor is dead.


 State(s):  Init or greater

    Event:  Hello Received

New state:  No state change.

   Action:  Restart the inactivity timer for the neighbor, since the
            neighbor has again been heard from.


 State(s):  Init

    Event:  2-Way Received

New state:  Depends upon action routine.

   Action:  Determine whether an adjacency should be established with
            the neighbor (see Section 10.4).  If not, the new neighbor
            state is 2-Way.

            Otherwise (an adjacency should be established) the neighbor
            state transitions to ExStart.  Upon entering this state, the
            router increments the sequence number for this neighbor.  If
            this is the first time that an adjacency has been attempted,
            the sequence number should be assigned some unique value
            (like the time of day clock).  It then declares itself
            master (sets the master/slave bit to master), and starts
            sending Database Description Packets, with the initialize
            (I), more (M) and master (MS) bits set.  This Database
            Description Packet should be otherwise empty.  This Database
            Description Packet should be retransmitted at intervals of
            RxmtInterval until the next state is entered (see Section
            10.8).


 State(s):  ExStart

    Event:  NegDone





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New state:  Exchange

   Action:  The router must list the contents of its entire area link
            state database in the neighbor Database summary list.  The
            area link state database consists of the router links,
            network links and summary links contained in the area
            structure, along with the AS external links contained in the
            global structure.  AS external link advertisements are
            omitted from a virtual neighbor's Database summary list.  AS
            external advertisements are omitted from the Database
            summary list if the area has been configured as a stub (see
            Section 3.6).  Advertisements whose age is equal to MaxAge
            are instead added to the neighbor's Link state
            retransmission list.  A summary of the Database summary list
            will be sent to the neighbor in Database Description
            packets.  Each Database Description Packet has a sequence
            number, and is explicitly acknowledged.  Only one Database
            Description Packet is allowed outstanding at any one time.
            For more detail on the sending and receiving of Database
            Description packets, see Sections 10.8 and 10.6.


 State(s):  Exchange

    Event:  Exchange Done

New state:  Depends upon action routine.

   Action:  If the neighbor Link state request list is empty, the new
            neighbor state is Full.  No other action is required.  This
            is an adjacency's final state.

            Otherwise, the new neighbor state is Loading.  Start (or
            continue) sending Link State Request packets to the neighbor
            (see Section 10.9).  These are requests for the neighbor's
            more recent advertisements (which were discovered but not
            yet received in the Exchange state).  These advertisements
            are listed in the Link state request list associated with
            the neighbor.


 State(s):  Loading

    Event:  Loading Done

New state:  Full





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   Action:  No action required.  This is an adjacency's final state.


 State(s):  2-Way

    Event:  AdjOK?

New state:  Depends upon action routine.

   Action:  Determine whether an adjacency should be formed with the
            neighboring router (see Section 10.4).  If not, the neighbor
            state remains at 2-Way.  Otherwise, transition the neighbor
            state to ExStart and perform the actions associated with the
            above state machine entry for state Init and event 2-Way
            Received.


 State(s):  ExStart or greater

    Event:  AdjOK?

New state:  Depends upon action routine.

   Action:  Determine whether the neighboring router should still be
            adjacent.  If yes, there is no state change and no further
            action is necessary.

            Otherwise, the (possibly partially formed) adjacency must be
            destroyed.  The neighbor state transitions to 2-Way.  The
            Link state retransmission list, Database summary list and
            Link state request list are cleared of link state
            advertisements.


 State(s):  Exchange or greater

    Event:  Seq Number Mismatch

New state:  ExStart

   Action:  The (possibly partially formed) adjacency is torn down, and
            then an attempt is made at reestablishment.  The neighbor
            state first transitions to ExStart.  The Link state
            retransmission list, Database summary list and Link state
            request list are cleared of link state advertisements.  Then
            the router increments the sequence number for this neighbor,
            declares itself master (sets the master/slave bit to
            master), and starts sending Database Description Packets,



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            with the initialize (I), more (M) and master (MS) bits set.
            This Database Description Packet should be otherwise empty
            (see Section 10.8).


 State(s):  Exchange or greater

    Event:  BadLSReq

New state:  ExStart

   Action:  The action for event BadLSReq is exactly the same as for the
            neighbor event SeqNumberMismatch.  The (possibly partially
            formed) adjacency is torn down, and then an attempt is made
            at reestablishment.  For more information, see the neighbor
            state machine entry that is invoked when event
            SeqNumberMismatch is generated in state Exchange or greater.


 State(s):  Any state

    Event:  KillNbr

New state:  Down

   Action:  The Link state retransmission list, Database summary list
            and Link state request list are cleared of link state
            advertisements.  Also, the inactivity timer is disabled.


 State(s):  Any state

    Event:  LLDown

New state:  Down

   Action:  The Link state retransmission list, Database summary list
            and Link state request list are cleared of link state
            advertisements.  Also, the inactivity timer is disabled.


 State(s):  Any state

    Event:  Inactivity Timer

New state:  Down





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   Action:  The Link state retransmission list, Database summary list
            and Link state request list are cleared of link state
            advertisements.


 State(s):  2-Way or greater

    Event:  1-Way Received

New state:  Init

   Action:  The Link state retransmission list, Database summary list
            and Link state request list are cleared of link state
            advertisements.


 State(s):  2-Way or greater

    Event:  2-Way received

New state:  No state change.

   Action:  No action required.


 State(s):  Init

    Event:  1-Way received

New state:  No state change.

   Action:  No action required.


10.4 Whether to become adjacent

Adjacencies are established with some subset of the router's neighbors.
Routers connected by point-to-point networks and virtual links always
become adjacent.  On multi-access networks, all routers become adjacent
to both the Designated Router and the Backup Designated Router.

The adjacency-forming decision occurs in two places in the neighbor
state machine.  First, when bidirectional communication is initially
established with the neighbor, and secondly, when the identity of the
attached network's (Backup) Designated Router changes.  If the decision
is made to not attempt an adjacency, the state of the neighbor
communication stops at 2-Way.




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An adjacency should be established with a (bidirectional) neighbor when
at least one of the following conditions holds:


o   The underlying network type is point-to-point

o   The underlying network type is virtual link

o   The router itself is the Designated Router

o   The router itself is the Backup Designated Router

o   The neighboring router is the Designated Router

o   The neighboring router is the Backup Designated Router


10.5 Receiving Hello packets

This section explains the detailed processing of a received Hello
packet.  (See Section A.3.2 for the format of Hello packets.)  The
generic input processing of OSPF packets will have checked the validity
of the IP header and the OSPF packet header.  Next, the values of the
Network Mask, HelloInt, and DeadInt fields in the received Hello packet
must be checked against the values configured for the receiving
interface.  Any mismatch causes processing to stop and the packet to be
dropped.  In other words, the above fields are really describing the
attached network's configuration.  Note that the value of the Network
Mask field should not be checked in Hellos received on unnumbered serial
lines or on virtual links.

The receiving interface attaches to a single OSPF area (this could be
the backbone).  The setting of the E-bit found in the Hello Packet's
option field must match this area's external routing capability.  If AS
external advertisements are not flooded into/throughout the area (i.e,
the area is a "stub") the E-bit must be clear in received hellos,
otherwise the E-bit must be set.  A mismatch causes processing to stop
and the packet to be dropped.  The setting of the rest of the bits in
the Hello Packet's option field should be ignored.

At this point, an attempt is made to match the source of the Hello
Packet to one of the receiving interface's neighbors.  If the receiving
interface is a multi-access network (either broadcast or non-broadcast)
the source is identified by the IP source address found in the Hello's
IP header.  If the receiving interface is a point-to-point link or a
virtual link, the source is identified by the Router ID found in the
Hello's OSPF packet header.  The interface's current list of neighbors
is contained in the interface's data structure.  If a matching neighbor



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structure cannot be found, (i.e., this is the first time the neighbor
has been detected), one is created.  The initial state of a newly
created neighbor is set to Down.

When receiving an Hello Packet from a neighbor on a multi-access network
(broadcast or non-broadcast), set the neighbor structure's Neighbor ID
equal to the Router ID found in the packet's OSPF header.  When
receiving an Hello on a point-to-point network (but not on a virtual
link) set the neighbor structure's Neighbor IP address to the packet's
IP source address.

Now the rest of the Hello Packet is examined, generating events to be
given to the neighbor and interface state machines.  These state
machines are specified either to be executed or scheduled (see Section
4.4).  For example, by specifying below that the neighbor state machine
be executed in line, several neighbor state transitions may be effected
by a single received Hello:


o   Each Hello Packet causes the neighbor state machine to be executed
    with the event Hello Received.

o   Then the list of neighbors contained in the Hello Packet is
    examined.  If the router itself appears in this list, the neighbor
    state machine should be executed with the event 2-Way Received.
    Otherwise, the neighbor state machine should be executed with the
    event 1-Way Received, and the processing of the packet stops.

o   Next, the Hello packet's Router Priority field is examined.  If this
    field is different than the one previously received from the
    neighbor, the receiving interface's state machine is scheduled with
    the event NeighborChange.  In any case, the Router Priority field in
    the neighbor data structure should be set accordingly.

o   Next the Designated Router field in the Hello Packet is examined.
    If the neighbor is both declaring itself to be Designated Router
    (Designated Router field = neighbor IP address) and the Backup
    Designated Router field in the packet is equal to 0.0.0.0 and the
    receiving interface is in state Waiting, the receiving interface's
    state machine is scheduled with the event BackupSeen.  Otherwise, if
    the neighbor is declaring itself to be Designated Router and it had
    not previously, or the neighbor is not declaring itself Designated
    Router where it had previously, the receiving interface's state
    machine is scheduled with the event NeighborChange.  In any case,
    the Designated Router item in the neighbor structure is set
    accordingly.





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o   Finally, the Backup Designated Router field in the Hello Packet is
    examined.  If the neighbor is declaring itself to be Backup
    Designated Router (Backup Designated Router field = neighbor IP
    address) and the receiving interface is in state Waiting, the
    receiving interface's state machine is scheduled with the event
    BackupSeen.  Otherwise, if the neighbor is declaring itself to be
    Backup Designated Router and it had not previously, or the neighbor
    is not declaring itself Backup Designated Router where it had
    previously, the receiving interface's state machine is scheduled
    with the event NeighborChange.  In any case, the Backup Designated
    Router item in the neighbor structure is set accordingly.


10.6 Receiving Database Description Packets

This section explains the detailed processing of a received Database
Description packet.  The incoming Database Description Packet has
already been associated with a neighbor and receiving interface by the
generic input packet processing (Section 8.2).  The further processing
of the Database Description Packet depends on the neighbor state.  If
the neighbor's state is Down or Attempt the packet should be ignored.
Otherwise, if the state is:


Init
    The neighbor state machine should be executed with the event 2-Way
    Received.  This causes an immediate state change to either state 2-
    Way or state Exstart.  The processing of the current packet should
    then continue in this new state.

2-Way
    The packet should be ignored.  Database description packets are used
    only for the purpose of bringing up adjacencies.[7]

ExStart
    If the received packet matches one of the following cases, then the
    neighbor state machine should be executed with the event
    NegotiationDone (causing the state to transition to Exchange), the
    packet's Options field should be recorded in the neighbor
    structure's Neighbor Options field and the packet should be accepted
    as next in sequence and processed further (see below).  Otherwise,
    the packet should be ignored.

    o   The initialize(I), more (M) and master(MS) bits are set, the
        contents of the packet are empty, and the neighbor's Router ID
        is larger than the router's own.  In this case the router is now
        Slave.  Set the master/slave bit to slave, and set the sequence
        number to that specified by the master.



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    o   The initialize(I) and master(MS) bits are off, the packet's
        sequence number equals the router's own sequence number
        (indicating acknowledgment) and the neighbor's Router ID is
        smaller than the router's own.  In this case the router is
        Master.

Exchange
    If the state of the MS-bit is inconsistent with the master/slave
    state of the connection, generate the neighbor event Seq Number
    Mismatch and stop processing the packet.  Otherwise:

    o   If the initialize(I) bit is set, generate the neighbor event Seq
        Number Mismatch and stop processing the packet.

    o   If the packet's Options field indicates a different set of
        optional OSPF capabilities than were previously received from
        the neighbor (recorded in the Neighbor Options field of the
        neighbor structure), generate the neighbor event Seq Number
        Mismatch and stop processing the packet.

    o   If the router is master, and the packet's sequence number equals
        the router's own sequence number (this packet is the next in
        sequence) the packet should be accepted and its contents
        processed (below).

    o   If the router is master, and the packet's sequence number is one
        less than the router's sequence number, the packet is a
        duplicate.  Duplicates should be discarded by the master.

    o   If the router is slave, and the packet's sequence number is one
        more than the router's own sequence number (this packet is the
        next in sequence) the packet should be accepted and its contents
        processed (below).

    o   If the router is slave, and the packet's sequence number is
        equal to the router's sequence number, the packet is a
        duplicate.  The slave must respond to duplicates by repeating
        the last Database Description packet that it sent.

    o   Else, generate the neighbor event Seq Number Mismatch and stop
        processing the packet.

Loading or Full
    In this state, the router has sent and received an entire sequence
    of Database Descriptions.  The only packets received should be
    duplicates (see above).  In particular, the packet's Options field
    should match the set of optional OSPF capabilities previously
    indicated by the neighbor (stored in the neighbor structure's



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    neighbor Options field).  Any other packets received, including the
    reception of a packet with the Initialize(I) bit set, should
    generate the neighbor event Seq Number Mismatch.[8] Duplicates
    should be discarded by the master.  The slave must respond to
    duplicates by repeating the last Database Description packet that it
    sent.


When the router accepts a received Database Description Packet as the
next in sequence the packet contents are processed as follows.  For each
link state advertisement listed, the advertisement's LS type is checked
for validity.  If the LS type is unknown (e.g., not one of the LS types
1-5 defined by this specification), or if this is a AS external
advertisement (LS type = 5) and the neighbor is associated with a stub
area, generate the neighbor event Seq Number Mismatch and stop
processing the packet.  Otherwise, the router looks up the advertisement
in its database to see whether it also has an instance of the link state
advertisement.  If it does not, or if the database copy is less recent
(see Section 13.1), the link state advertisement is put on the Link
state request list so that it can be requested (immediately or at some
later time) in Link State Request Packets.

When the router accepts a received Database Description Packet as the
next in sequence, it also performs the following actions, depending on
whether it is master or slave:


Master
    Increments the sequence number.  If the router has already sent its
    entire sequence of Database Descriptions, and the just accepted
    packet has the more bit (M) set to 0, the neighbor event Exchange
    Done is generated.  Otherwise, it should send a new Database
    Description to the slave.

Slave
    Sets the sequence number to the sequence number appearing in the
    received packet.  The slave must send a Database Description in
    reply.  If the received packet has the more bit (M) set to 0, and
    the packet to be sent by the slave will have the M-bit set to 0
    also, the neighbor event Exchange Done is generated.  Note that the
    slave always generates this event before the master.


10.7 Receiving Link State Request Packets

This section explains the detailed processing of received Link State
Request packets.  Received Link State Request Packets specify a list of
link state advertisements that the neighbor wishes to receive.  Link



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state Request Packets should be accepted when the neighbor is in states
Exchange, Loading, or Full.  In all other states Link State Request
Packets should be ignored.

Each link state advertisement specified in the Link State Request packet
should be located in the router's database, and copied into Link State
Update packets for transmission to the neighbor.  These link state
advertisements should NOT be placed on the Link state retransmission
list for the neighbor.  If a link state advertisement cannot be found in
the database, something has gone wrong with the synchronization
procedure, and neighbor event BadLSReq should be generated.


10.8 Sending Database Description Packets

This section describes how Database Description Packets are sent to a
neighbor.  The router's optional OSPF capabilities (see Section 4.5) are
transmitted to the neighbor in the Options field of the Database
Description packet.  The router should maintain the same set of optional
capabilities throughout the Database Exchange and flooding procedures.
If for some reason the router's optional capabilities change, the
Database Exchange procedure should be restarted by reverting to neighbor
state ExStart.  There are currently two optional capabilities defined.
The T-bit should be set if and only if the router is capable of
calculating separate routes for each IP TOS.  The E-bit should be set if
and only if the attached network belongs to a non-stub area.  The rest
of the Options field should be set to zero.

The sending of Database Description packets depends on the neighbor's
state.  In state ExStart the router sends empty Database Description
packets, with the initialize (I), more (M) and master (MS) bits set.
These packets are retransmitted every RxmtInterval seconds.

In state Exchange the Database Description Packets actually contain
summaries of the link state information contained in the router's
database.  Each link state advertisement in the area's topological
database (at the time the neighbor transitions into Exchange state) is
listed in the neighbor Database summary list.  When a new Database
Description Packet is to be sent, the packet's sequence number is
incremented, and the (new) top of the Database summary list is described
by the packet.  Items are removed from the Database summary list when
the previous packet is acknowledged.

In state Exchange, the determination of when to send a packet depends on
whether the router is master or slave:






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Master
    Packets are sent when either a) the slave acknowledges the previous
    packet by echoing the sequence number or b) RxmtInterval seconds
    elapse without an acknowledgment, in which case the previous packet
    is retransmitted.

Slave
    Packets are sent only in response to packets received from the
    master.  If the packet received from the master is new, a new packet
    is sent, otherwise the previous packet is resent.


In states Loading and Full the slave must resend its last packet in
response to duplicate packets received from the master.  For this reason
the slave must wait RouterDeadInterval seconds before freeing the last
packet.  Reception of a packet from the master after this interval will
generate a Seq Number Mismatch neighbor event.


10.9 Sending Link State Request Packets

In neighbor states Exchange or Loading, the Link state request list
contains a list of those link state advertisements that need to be
obtained from the neighbor.  To request these advertisements, a router
sends the neighbor the beginning of the Link state request list,
packaged in a Link State Request packet.

When the neighbor responds to these requests with the proper Link State
Update packet(s), the Link state request list is truncated and a new
Link State Request packet is sent.  This process continues until the
link state request list becomes empty.  Unsatisfied Link State Requests
are retransmitted at intervals of RxmtInterval.  There should be at most
one Link State Request packet outstanding at any one time.

When the Link state request list becomes empty, and the neighbor state
is Loading (i.e., a complete sequence of Database Description packets
has been received from the neighbor), the Loading Done neighbor event is
generated.


10.10 An Example

Figure 14 shows an example of an adjacency forming.  Routers RT1 and RT2
are both connected to a broadcast network.  It is assumed that RT2 is
the Designated Router for the network, and that RT2 has a higher Router
ID that router RT1.

The neighbor state changes realized by each router are listed on the



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sides of the figure.


At the beginning of Figure 14, router RT1's interface to the network
becomes operational.  It begins sending hellos, although it doesn't know
the identity of the Designated Router or of any other neighboring
routers.  Router RT2 hears this hello (moving the neighbor to Init
state), and in its next hello indicates that it is itself the Designated
Router and that it has heard hellos from RT1.  This in turn causes RT1
to go to state ExStart, as it starts to bring up the adjacency.

RT1 begins by asserting itself as the master.  When it sees that RT2 is
indeed the master (because of RT2's higher Router ID), RT1 transitions
to slave state and adopts its neighbor's sequence number.  Database
Description packets are then exchanged, with polls coming from the
master (RT2) and responses from the slave (RT1).  This sequence of
Database Description Packets ends when both the poll and associated
response has the M-bit off.

In this example, it is assumed that RT2 has a completely up to date
database.  In that case, RT2 goes immediately into Full state.  RT1 will
go into Full state after updating the necessary parts of its database.
This is done by sending Link State Request Packets, and receiving Link
State Update Packets in response.  Note that, while RT1 has waited until
a complete set of Database Description Packets has been received (from
RT2) before sending any Link State Request Packets, this need not be the
case.  RT1 could have interleaved the sending of Link State Request
Packets with the reception of Database Description Packets.


11. The Routing Table Structure

The routing table data structure contains all the information necessary
to forward an IP data packet toward its destination.  Each routing table
entry describes the collection of best paths to a particular
destination.  When forwarding an IP data packet, the routing table entry
providing the best match for the packet's IP destination is located.


                ________________________________________

                 (Figure not included in text version.)

                Figure 14: An adjacency bring-up example
                ________________________________________






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The matching routing table entry then provides the next hop towards the
packet's destination.  OSPF also provides for the existence of a default
route (Destination ID = DefaultDestination).  When the default route
exists, it matches all IP destinations (although any other matching
entry is a better match).  Finding the routing table entry that best
matches an IP destination is further described in Section 11.1.

There is a single routing table in each router.  Two sample routing
tables are described in Sections 11.2 and 11.3.  The building of the
routing table is discussed in Section 16.

The rest of this section defines the fields found in a routing table
entry.  The first set of fields describes the routing table entry's
destination.


Destination Type
    The destination can be one of three types.  Only the first type,
    Network, is actually used when forwarding IP data traffic.  The
    other destinations are used solely as intermediate steps in the
    routing table build process.

    Network
        A range of IP addresses, to which IP data traffic may be
        forwarded.  This includes IP networks (class A, B, or C), IP
        subnets, and single IP hosts.  The default route also falls in
        this category.

    Area border router
        Routers that are connected to multiple OSPF areas.  Such routers
        originate summary link advertisements.  These routing table
        entries are used when calculating the inter-area routes (see
        Section 16.2).  These routing table entries may also be
        associated with configured virtual links.

    AS boundary router
        Routers that originate AS external link advertisements.  These
        routing table entries are used when calculating the AS external
        routes (see Section 16.4).

Destination ID
    The destination's identifier or name.  This depends on the
    destination's type.  For networks, the identifier is their
    associated IP address.  For all other types, the identifier is the
    OSPF Router ID.[9]

Address Mask
    Only defined for networks.  The network's IP address together with



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    its address mask defines a range of IP addresses.  For IP subnets,
    the address mask is referred to as the subnet mask.  For host
    routes, the mask is "all ones" (0xffffffff).

Optional Capabilities
    When the destination is a router (either an area border router or an
    AS boundary router) this field indicates the optional OSPF
    capabilities supported by the destination router.  The two optional
    capabilities currently defined by this specification are the ability
    to route based on IP TOS and the ability to process AS external
    advertisements.  For a further discussion of OSPF's optional
    capabilities, see Section 4.5.


The set of paths to use for a destination may vary based on IP Type of
Service and the OSPF area to which the paths belong.  This means that
there may be multiple routing table entries for the same destination,
depending on the values of the next two fields.


Type of Service
    There can be a separate set of routes for each IP Type of Service.
    The encoding of TOS in OSPF link state advertisements is described
    in Section 12.3.

Area
    This field indicates the area whose link state information has led
    to the routing table entry's collection of paths.  This is called
    the entry's associated area.  For sets of AS external paths, this
    field is not defined.  For destinations of type "area border
    router", there may be separate sets of paths (and therefore separate
    routing table entries) associated with each of several areas.  This
    will happen when two area border routers share multiple areas in
    common.  For all other destination types, only the set of paths
    associated with the best area (the one providing the shortest route)
    is kept.


The rest of the routing table entry describes the set of paths to the
destination.  The following fields pertain to the set of paths as a
whole.  In other words, each one of the paths contained in a routing
table entry is of the same path-type and cost (see below).


Path-type
    There are four possible types of paths used to route traffic to the
    destination, listed here in order of preference: intra-area, inter-
    area, type 1 external or type 2 external.  Intra-area paths indicate



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    destinations belonging to one of the router's attached areas.
    Inter-area paths are paths to destinations in other OSPF areas.
    These are discovered through the examination of received summary
    link advertisements.  AS external paths are paths to destinations
    external to the AS.  These are detected through the examination of
    received AS external link advertisements.

Cost
    The link state cost of the path to the destination.  For all paths
    except type 2 external paths this describes the entire path's cost.
    For Type 2 external paths, this field describes the cost of the
    portion of the path internal to the AS.  This cost is calculated as
    the sum of the costs of the path's constituent links.

Type 2 cost
    Only valid for type 2 external paths.  For these paths, this field
    indicates the cost of the path's external portion.  This cost has
    been advertised by an AS boundary router, and is the most
    significant part of the total path cost.  For example, an external
    type 2 path with type 2 cost of 5 is always preferred over a path
    with type 2 cost of 10, regardless of the cost of the two paths'
    internal components.

Link State Origin
    Valid only for intra-area paths, this field indicates the link state
    advertisement (router links or network links) that directly
    references the destination.  For example, if the destination is a
    transit network, this is the transit network's network links
    advertisement.  If the destination is a stub network, this is the
    router links advertisement for the attached router.  The
    advertisement is discovered during the shortest-path tree
    calculation (see Section 16.1).  Multiple advertisements may
    reference the destination, however a tie-breaking scheme always
    reduces the choice to a single advertisement.

    This field is for informational purposes only.  The advertisement
    could be used as a root for an SPF calculation when building a
    reverse path forwarding tree.  This is beyond the scope of this
    specification.


When multiple paths of equal path-type and cost exist to a destination
(called elsewhere "equal-cost" paths), they are stored in a single
routing table entry.  Each one of the "equal-cost" paths is
distinguished by the following fields:






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Next hop
    The outgoing router interface to use when forwarding traffic to the
    destination.  On multi-access networks, the next hop also includes
    the IP address of the next router (if any) in the path towards the
    destination.  This next router will always be one of the adjacent
    neighbors.

Advertising router
    Valid only for inter-area and AS external paths.  This field
    indicates the Router ID of the router advertising the summary link
    or AS external link that led to this path.


11.1 Routing table lookup

When an IP data packet is received, an OSPF router finds the routing
table entry that best matches the packet's destination. This routing
table entry then provides the outgoing interface and next hop router to
use in forwarding the packet. This section describes the process of
finding the best matching routing table entry. The process consists of a
number of steps, wherein the collection of routing table entries is
progressively pruned. In the end, the single routing table entry
remaining is the called best match.

Note that the steps described below may fail to produce a best match
routing table entry (i.e., all existing routing table entries are pruned
for some reason or another). In this case, the packet's IP destination
is considered unreachable. Instead of being forwarded, the packet should
be dropped and an ICMP destination unreachable message should be
returned to the packet's source.


(1) Select the complete set of "matching" routing table entries from the
    routing table.  Each routing table entry describes a (set of)
    path(s) to a range of IP addresses. If the data packet's IP
    destination falls into an entry's range of IP addresses, the routing
    table entry is called a match. (It is quite likely that multiple
    entries will match the data packet.  For example, a default route
    will match all packets.)

(2) Suppose that the packet's IP destination falls into one of the
    router's configured area address ranges (see Section 3.5), and that
    the particular area address range is active. This means that there
    are one or more reachable (by intra-area paths) networks contained
    in the area address range. The packet's IP destination is then
    required to belong to one of these constituent networks. For this
    reason, only matching routing table entries with path-type of
    intra-area are considered (all others are pruned). If no such



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    matching entries exist, the destination is unreachable (see above).
    Otherwise, skip to step 4.

(3) Reduce the set of matching entries to those having the most
    preferential path-type (see Section 11). OSPF has a four level
    hierarchy of paths. Intra-area paths are the most preferred,
    followed in order by inter-area, Type 1 external and Type 2 external
    paths.

(4) Select the remaining routing table entry that provides the longest
    (most specific) match. Another way of saying this is to choose the
    remaining entry that specifies the narrowest range of IP
    addresses.[10] For example, the entry for the address/mask pair of
    (128.185.1.0, 0xffffff00) is more specific than an entry for the
    pair (128.185.0.0, 0xffff0000). The default route is the least
    specific match, since it matches all destinations.

(5) At this point, there may still be multiple routing table entries
    remaining. Each routing entry will specify the same range of IP
    addresses, but a different IP Type of Service. Select the routing
    table entry whose TOS value matches the TOS found in the packet
    header. If there is no routing table entry for this TOS, select the
    routing table entry for TOS 0. In other words, packets requesting
    TOS X are routed along the TOS 0 path if a TOS X path does not
    exist.


11.2 Sample routing table, without areas

Consider the Autonomous System pictured in Figure 2.  No OSPF areas have
been configured.  A single metric is shown per outbound interface,
indicating that routes will not vary based on TOS.  The calculation
router RT6's routing table proceeds as described in Section 2.1.  The
resulting routing table is shown in Table 12.  Destination types are
abbreviated: Network as "N", area border router as "BR" and AS boundary
router as "ASBR".

There are no instances of multiple equal-cost shortest paths in this
example.  Also, since there are no areas, there are no inter-area paths.

Routers RT5 and RT7 are AS boundary routers.  Intra-area routes have
been calculated to routers RT5 and RT7.  This allows external routes to
be calculated to the destinations advertised by RT5 and RT7 (i.e.,
networks N12, N13, N14 and N15).  It is assumed all AS external
advertisements originated by RT5 and RT7 are advertising type 1 external
metrics.  This results in type 1 external paths being calculated to
destinations N12-N15.




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11.3 Sample routing table, with areas

Consider the previous example, this time split into OSPF areas.  An OSPF
area configuration is pictured in Figure 6.  Router RT4's routing table
will be described for this area configuration.  Router RT4 has a
connection to Area 1 and a backbone connection.  This causes Router RT4
to view the AS as the concatenation of the two graphs shown in Figures 7
and 8.  The resulting routing table is displayed in Table 13.

Again, routers RT5 and RT7 are AS boundary routers.  Routers RT3, RT4,
RT7, RT10 and RT11 are area border routers.  Note that there are two
routing entries (in this case having identical paths) for router RT7, in
its dual capacities as an area border router and an AS boundary router.
Note also that there are two routing entries for the area border router
RT3, since it has two areas in common with RT4 (Area 1 and the
backbone).

Backbone paths have been calculated to all area border routers (BR).
These are used when determining the inter-area routes.  Note that all of


Type   Dest   Area   Path  Type        Cost   Next Hop(s)   Adv. Router(s)
__________________________________________________________________________
N      N1     0      intra-area        10     RT3           *
N      N2     0      intra-area        10     RT3           *
N      N3     0      intra-area        7      RT3           *
N      N4     0      intra-area        8      RT3           *
N      Ib     0      intra-area        7      *             *
N      Ia     0      intra-area        12     RT10          *
N      N6     0      intra-area        8      RT10          *
N      N7     0      intra-area        12     RT10          *
N      N8     0      intra-area        10     RT10          *
N      N9     0      intra-area        11     RT10          *
N      N10    0      intra-area        13     RT10          *
N      N11    0      intra-area        14     RT10          *
N      H1     0      intra-area        21     RT10          *
ASBR   RT5    0      intra-area        6      RT5           *
ASBR   RT7    0      intra-area        8      RT10          *
__________________________________________________________________________
N      N12    *      type 1 external   10     RT10          RT7
N      N13    *      type 1 external   14     RT5           RT5
N      N14    *      type 1 external   14     RT5           RT5
N      N15    *      type 1 external   17     RT10          RT7


   Table 12: The routing table for Router RT6 (no configured areas).





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the inter-area routes are associated with the backbone; this is always
the case when the router is itself an area border router.  Routing
information is condensed at area boundaries.  In this example, we assume
that Area 3 has been defined so that networks N9-N11 and the host route
to H1 are all condensed to a single route when advertised to the
backbone (by router RT11).  Note that the cost of this route is the
minimum of the set of costs to its individual components.

There is a virtual link configured between routers RT10 and RT11.
Without this configured virtual link, RT11 would be unable to advertise
a route for networks N9-N11 and host H1 into the backbone, and there
would not be an entry for these networks in router RT4's routing table.

In this example there are two equal-cost paths to network N12.  However,
they both use the same next hop (Router RT5).



Router RT4's routing table would improve (i.e., some of the paths in the
routing table would become shorter) if an additional virtual link were
configured between router RT4 and router RT3.  The new virtual link
would itself be associated with the first entry for area border router
RT3 in Table 13 (an intra-area path through Area 1).  This would yield a
cost of 1 for the virtual link.  The routing table entries changes that
would be caused by the addition of this virtual link are shown in Table
14.



12. Link State Advertisements

Each router in the Autonomous System originates one or more link state
advertisements.  There are five distinct types of link state
advertisements, which are described in Section 4.3.  The collection of
link state advertisements forms the link state or topological database.
Each separate type of advertisement has a separate function.  Router
links and network links advertisements describe how an area's routers
and networks are interconnected.  Summary link advertisements provide a
way of condensing an area's routing information.  AS external
advertisements provide a way of transparently advertising externally-
derived routing information throughout the Autonomous System.

Each link state advertisement begins with a standard 20-byte header.
This link state header is discussed below.







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Type   Dest        Area   Path  Type        Cost   Next Hop(s)   Adv. Router(s)
_______________________________________________________________________________
N      N1          1      intra-area        4      RT1           *
N      N2          1      intra-area        4      RT2           *
N      N3          1      intra-area        1      *             *
N      N4          1      intra-area        3      RT3           *
BR     RT3         1      intra-area        1      *             *
_______________________________________________________________________________
N      Ib          0      intra-area        22     RT5           *
N      Ia          0      intra-area        27     RT5           *
BR     RT3         0      intra-area        21     RT5           *
BR     RT7         0      intra-area        14     RT5           *
BR     RT10        0      intra-area        22     RT5           *
BR     RT11        0      intra-area        25     RT5           *
ASBR   RT5         0      intra-area        8      *             *
ASBR   RT7         0      intra-area        14     RT5           *
_______________________________________________________________________________
N      N6          0      inter-area        15     RT5           RT7
N      N7          0      inter-area        19     RT5           RT7
N      N8          0      inter-area        18     RT5           RT7
N      N9-N11,H1   0      inter-area        26     RT5           RT11
_______________________________________________________________________________
N      N12         *      type 1 external   16     RT5           RT5,RT7
N      N13         *      type 1 external   16     RT5           RT5
N      N14         *      type 1 external   16     RT5           RT5
N      N15         *      type 1 external   23     RT5           RT7


     Table 13: Router RT4's routing table in the presence of areas.


Type   Dest        Area   Path  Type   Cost   Next Hop(s)   Adv. Router(s)
__________________________________________________________________________
N      Ib          0      intra-area   16     RT3           *
N      Ia          0      intra-area   21     RT3           *
BR     RT3         0      intra-area   1      *             *
BR     RT10        0      intra-area   16     RT3           *
BR     RT11        0      intra-area   19     RT3           *
__________________________________________________________________________
N      N9-N11,H1   0      inter-area   20     RT3           RT11


      Table 14: Changes resulting from an additional virtual link.

12.1 The Link State Header




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The link state header contains the LS type, Link State ID and
Advertising Router fields.  The combination of these three fields
uniquely identifies the link state advertisement.

There may be several instances of an advertisement present in the
Autonomous System, all at the same time.  It must then be determined
which instance is more recent.  This determination is made be examining
the LS sequence, LS checksum and LS age fields.  These fields are also
contained in the 20-byte link state header.

Several of the OSPF packet types list link state advertisements.  When
the instance is not important, an advertisement is referred to by its LS
type, Link State ID and Advertising Router (see Link State Request
Packets).  Otherwise, the LS sequence number, LS age and LS checksum
fields must also be referenced.

A detailed explanation of the fields contained in the link state header
follows.


12.1.1 LS age

This field is the age of the link state advertisement in seconds.  It
should be processed as an unsigned 16-bit integer.  It is set to 0 when
the link state advertisement is originated.  It must be incremented by
InfTransDelay on every hop of the flooding procedure.  Link state
advertisements are also aged as they are held in each router's database.

The age of a link state advertisement is never incremented past MaxAge.
Advertisements having age MaxAge are not used in the routing table
calculation.  When an advertisement's age first reaches MaxAge, it is
reflooded.  A link state advertisement of age MaxAge is finally flushed
from the database when it is no longer contained on any neighbor Link
state retransmission lists.  This indicates that it has been
acknowledged by all adjacent neighbors.  For more information on the
aging of link state advertisements, consult Section 14.

Ages are examined when a router receives two instances of a link state
advertisement, both having identical sequence numbers and checksums.  An
instance of age MaxAge is then always accepted as most recent; this
allows old advertisements to be flushed quickly from the routing domain.
Otherwise, if the ages differ by more than MaxAgeDiff, the instance
having the smaller age is accepted as most recent.[11] See Section 13.1
for more details.







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12.1.2 Options

The options field in the link state header indicates which optional
capabilities are associated with the advertisement.  OSPF's optional
capabilities are described in Section 4.5.  There are currently two
optional capabilities defined; they are represented by the T-bit and E-
bit found in the options field.  The rest of the options field should be
set to zero.

The E-bit represents OSPF's external routing capability.  This bit
should be set in all advertisements associated with the backbone, and
all advertisements associated with non-stub areas (see Section 3.6).  It
should also be set in all AS external advertisements.  It should be
reset in all router links, network links and summary link advertisements
associated with a stub area.  For all link state advertisements, the
setting of the E-bit is for informational purposes only; it does not
affect the routing table calculation.

The T-bit represents OSPF's TOS routing capability.  This bit should be
set in a router links advertisement if and only if the router is capable
of calculating separate routes for each IP TOS (see Section 2.4).  The
T-bit should always be set in network links advertisements.  It should
be set in summary link and AS external link advertisements if and only
if the advertisement describes paths for all TOS values, instead of just
the TOS 0 path.  Note that, with the T-bit set, there may still be only
a single metric in the advertisement (the TOS 0 metric).  This would
mean that paths for non-zero TOS exist, but are equivalent to the TOS 0
path.  A link state advertisement's T-bit is examined when calculating
the routing table's non-zero TOS paths (see Section 16.9).


12.1.3 LS type

The LS type field dictates the format and function of the link state
advertisement.  Advertisements of different types have different names
(e.g., router links or network links).  All advertisement types, except
the AS external link advertisements (LS type = 5), are flooded
throughout a single area only.  AS external link advertisements are
flooded throughout the entire Autonomous System, excluding stub areas
(see Section 3.6).  Each separate advertisement type is briefly
described below in Table 15.


           LS Type   Advertisement description
           __________________________________________________
           1         These are the router links
                     advertisements. They describe the




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           LS Type   Advertisement description
           __________________________________________________
                     collected states of the router's
                     interfaces. For more information,
                     consult Section 12.4.1.
           __________________________________________________
           2         These are the network links
                     advertisements. They describe the set
                     of routers attached to the network. For
                     more information, consult
                     Section 12.4.2.
           __________________________________________________
           3 or 4    These are the summary link
                     advertisements. They describe
                     inter-area routes, and enable the
                     condensation of routing information at
                     area borders. Originated by area border
                     routers, the Type 3 advertisements
                     describe routes to networks while the
                     Type 4 advertisements describe routes to
                     AS boundary routers.
           __________________________________________________
           5         These are the AS external link
                     advertisements. Originated by AS
                     boundary routers, they describe routes
                     to destinations external to the
                     Autonomous System. A default route for
                     the Autonomous System can also be
                     described by an AS external link
                     advertisement.


               Table 15: OSPF link state advertisements.


12.1.4 Link State ID

This field identifies the piece of the routing domain that is being
described by the advertisement.  Depending on the advertisement's LS
type, the Link State ID takes on the values listed in Table 16.











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   LS Type   Link State ID
   ______________________________________________________________________
   1         The originating router's Router ID.
   2         The IP interface address of the network's Designated Router.
   3         The destination network's IP address.
   4         The Router ID of the described AS boundary router.
   5         The destination network's IP address.


                Table 16: The advertisement's Link State ID.


When the link state advertisement is describing a network, the Link
State ID is either the network's IP address (as in type 3 summary link
advertisements and in AS external link advertisements) or the network's
IP address is easily derivable from the Link State ID (note that masking
a network links advertisement's Link State ID with the network's subnet
mask yields the network's IP address).  When the link state
advertisement is describing a router, the Link State ID is always the
described router's OSPF Router ID.

When an AS external advertisement (LS Type = 5) is describing a default
route, its Link State ID is set to DefaultDestination (0.0.0.0).


12.1.5 Advertising Router

This field specifies the OSPF Router ID of the advertisement's
originator.  For router links advertisements, this field is identical to
the Link State ID field.  Network link advertisements are originated by
the network's Designated Router.  Summary link advertisements are
originated by area border routers.  Finally, AS external link
advertisements are originated by AS boundary routers.


12.1.6 LS sequence number

The sequence number field is a signed 32-bit integer.  It is used to
detect old and duplicate link state advertisements.  The space of
sequence numbers is linearly ordered.  The larger the sequence number
(when compared as signed 32-bit integers) the more recent the
advertisement.  To describe to sequence number space more precisely, let
N refer in the discussion below to the constant 2**31.

The sequence number -N (0x80000000) is reserved (and unused).  This
leaves -N + 1 (0x80000001) as the smallest (and therefore oldest)
sequence number.  A router uses this sequence number the first time it



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originates any link state advertisement.  Afterwards, the
advertisement's sequence number is incremented each time the router
originates a new instance of the advertisement.  When an attempt is made
to increment the sequence number past the maximum value of of N - 1
(0x7fffffff), the current instance of the advertisement must first be
flushed from the routing domain.  This is done by prematurely aging the
advertisement (see Section 14.1) and reflooding it.  As soon as this
flood has been acknowledged by all adjacent neighbors, a new instance
can be originated with sequence number of -N + 1 (0x80000001).

The router may be forced to promote the sequence number of one of its
advertisements when a more recent instance of the advertisement is
unexpectedly received during the flooding process.  This should be a
rare event.  This may indicate that an out-of-date advertisement,
originated by the router itself before its last restart/reload, still
exists in the Autonomous System.  For more information see Section 13.4.

,uh "12.1.7 LS checksum"

This field is the checksum of the complete contents of the
advertisement, excepting the age field.  The age field is excepted so
that an advertisement's age can be incremented without updating the
checksum.  The checksum used is the same that is used for ISO
connectionless datagrams; it is commonly referred to as the Fletcher
checksum.  It is documented in Annex C of [RFC 994].  The link state
header also contains the length of the advertisement in bytes;
subtracting the size of the age field (two bytes) yields the amount of
data to checksum.

The checksum is used to detect data corruption of an advertisement.
This corruption can occur while an advertisement is being flooded, or
while it is being held in a router's memory.  The LS checksum field
cannot take on the value of zero; the occurrence of such a value should
be considered a checksum failure.  In other words, calculation of the
checksum is not optional.

The checksum of a link state advertisement is verified in two cases: a)
when it is received in a Link State Update Packet and b) at times during
the aging of the link state database.  The detection of a checksum
failure leads to separate actions in each case.  See Sections 13 and 14
for more details.

Whenever the LS sequence number field indicates that two instances of an
advertisement are the same, the LS checksum field is examined.  If there
is a difference, the instance with the larger checksum is considered to
be most recent.[12] See Section 13.1 for more details.





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12.2 The link state database

A router has a separate link state database for every area to which it
belongs.  The link state database has been referred to elsewhere in the
text as the topological database.  All routers belonging to the same
area have identical topological databases for the area.

The databases for each individual area are always dealt with separately.
The shortest path calculation is performed separately for each area (see
Section 16).  Components of the area topological database are flooded
throughout the area only.  Finally, when an adjacency (belonging to Area
A) is being brought up, only the database for Area A is synchronized
between the two routers.

The area database is composed of router links advertisements, network
links advertisements, and summary link advertisements (all listed in the
area data structure).  In addition, external routes (AS external
advertisements) are included in all non-stub area databases (see Section
3.6).

An implementation of OSPF must be able to access individual pieces of an
area database.  This lookup function is based on an advertisement's LS
type, Link State ID and Advertising Router.[13] There will be a single
instance (the most up-to-date) of each link state advertisement in the
database.  The database lookup function is invoked during the link state
flooding procedure (Section 13) and the routing table calculation
(Section 16).  In addition, using this lookup function the router can
determine whether it has itself ever originated a particular link state
advertisement, and if so, with what LS sequence number.

A link state advertisement is added to a router's database when either
a) it is received during the flooding process (Section 13) or b) it is
originated by the router itself (Section 12.4).  A link state
advertisement is deleted from a router's database when either a) it has
been overwritten by a newer instance during the flooding process
(Section 13) or b) the router originates a newer instance of one of its
self-originated advertisements (Section 12.4) or c) the advertisement
ages out and is flushed from the routing domain (Section 14).  Whenever
a link state advertisement is deleted from the database it must also be
removed from all neighbors' Link state retransmission lists (see Section
10).


12.3 Representation of TOS

All OSPF link state advertisements (with the exception of network links
advertisements) specify metrics.  In router links advertisements, the
metrics indicate the costs of the described interfaces.  In summary link



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and AS external link advertisements, the metric indicates the cost of
the described path.  In all of these advertisements, a separate metric
can be specified for each IP TOS.  TOS is encoded in an OSPF link state
advertisement as the following mapping of the Delay (D), Throughput (T)
and Reliability (R) flags found in the IP packet header's TOS field (see
[RFC 791]).



                         OSPF encoding   D   T   R
                         _________________________
                         0               0   0   0
                         4               0   0   1
                         8               0   1   0
                         12              0   1   1
                         16              1   0   0
                         20              1   0   1
                         24              1   1   0
                         28              1   1   1


                    Table 17: Representing TOS in OSPF.


Each OSPF link state advertisement must specify the TOS 0 metric.  Other
TOS metrics, if they appear, must appear in order of increasing TOS
encoding.  For example, the TOS 8 (high throughput) metric must always
appear before the TOS 16 (low delay) metric when both are specified.  If
a metric for some non-zero TOS is not specified, its cost defaults to
the cost for TOS 0, unless the T-bit is reset in the advertisement's
options field (see Section 12.1.2 for more details).

Note that if more TOS types are defined in a future IP architecture,
OSPF's TOS encoding can be extended in a straightforward manner.


12.4 Originating link state advertisements

A router may originate many types of link state advertisements.  A
router originates a router links advertisement for each area to which it
belongs.  If the router is also the Designated Router for any of its
attached networks, it will originate a network links advertisement for
that network.

Area border routers originate a single summary links advertisement for
each known inter-area destination.  AS boundary routers originate a
single AS external links advertisement for each known AS external
destination.  Destinations are advertised one at a time so that the



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change in any single route can be flooded without reflooding the entire
collection of routes.  During the flooding procedure, many link state
advertisements can be carried by a single Link State Update packet.

As an example, consider router RT4 in Figure 6.  It is an area border
router, having a connection to Area 1 and the backbone.  Router RT4
originates 5 distinct link state advertisements into the backbone (one
router links, and one summary link for each of the networks N1-N4).
Router RT4 will also originate 8 distinct link state advertisements into
Area 1 (one router links and seven summary link advertisements as
pictured in Figure 7).  If RT4 has been selected as Designated Router
for network N3, it will also originate a network links advertisement for
N3 into Area 1.

In this same figure, router RT5 will be originating 3 distinct AS
external link advertisements (one for each of the networks N12-N14).
These will be flooded throughout the entire AS, assuming that none of
the areas have been configured as stubs.  However, if area 3 has been
configured as a stub area, the external advertisements for networks
N12-N14 will not be flooded into area 3 (see Section 3.6).  Instead,
router RT11 would originate a default summary link advertisement that
would be flooded throughout area 3 (see Section 12.4.3).  This instructs
all of area 3's internal routers to send their AS external traffic to
RT11.

Whenever a new instance of a link state advertisement is originated, its
LS sequence number is incremented, its LS age is set to 0, its LS
checksum is calculated, and the advertisement is added to the link state
database and flooded out the appropriate interfaces.  See Section 13.2
for details concerning the installation of the advertisement into the
link state database.  See Section 13.3 for details concerning the
flooding of newly originated advertisements.


The eight events that cause a new instance of a link state advertisement
to be originated are:


(1) The LS refresh timer firing.  There is a LS refresh timer for each
    link state advertisement that the router has originated.  The LS
    refresh timer is an interval timer, with length LSRefreshTimer.  The
    LS refresh timer guarantees periodic originations regardless of any
    other events that cause new instances.  This periodic updating of
    link state advertisements adds robustness to the link state
    algorithm.  Link state advertisements that solely describe
    unreachable destinations should not be refreshed, but should instead
    be flushed from the routing domain (see Section 14.1).




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When whatever is being described by a link state advertisement changes,
a new advertisement is originated.  Two instances of the same link state
advertisement may not be originated within the time period
MinLSInterval.  This may require that the generation of the next
instance to be delayed by up to MinLSInterval.  The following changes
may cause a router to originate a new instance of an advertisement.
These changes should cause new originations only if the contents of the
new advertisement would be different.


(2) An interface's state changes (see Section 9.1).  This may mean that
    it is necessary to produce a new instance of the router links
    advertisement.

(3) An attached network's Designated Router changes.  A new router links
    advertisement should be originated.  Also, if the router itself is
    now the Designated Router, a new network links advertisement should
    be produced.

(4) One of the neighboring routers changes to/from the FULL state.  This
    may mean that it is necessary to produce a new instance of the
    router links advertisement.  Also, if the router is itself the
    Designated Router for the attached network, a new network links
    advertisement should be produced.


The next three events concern area border routers only.


(5) An intra-area route has been added/deleted/modified in the routing
    table.  This may cause a new instance of a summary links
    advertisement (for this route) to be originated in each attached
    area (this includes the backbone).

(6) An inter-area route has been added/deleted/modified in the routing
    table.  This may cause a new instance of a summary links
    advertisement (for this route) to be originated in each attached
    area (but NEVER for the backbone).

(7) The router becomes newly attached to an area.  The router must then
    originate summary link advertisements into the newly attached area
    for all pertinent intra-area and inter-area routes in its routing
    table.  See Section 12.4.3 for more details.


The last event concerns AS boundary routers only.





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(8) An external route gained through direct experience with an external
    routing protocol (like EGP) changes.  This will cause the AS
    boundary router to originate a new instance of an external links
    advertisement.


The construction of each type of the link state advertisement is
explained below.  In general, these sections describe the contents of
the advertisement body (i.e., the part coming after the 20-byte
advertisement header).  For information concerning the building of the
link state advertisement header, see Section 12.1.



12.4.1 Router links

A router originates a router links advertisement for each area that it
belongs to.  Such an advertisement describes the collected states of the
router's links to the area.  The advertisement is flooded throughout the
particular area, and no further.

The format of a router links advertisement is shown in Appendix A
(Section A.4.2).  The first 20 bytes of the advertisement consist of the
generic link state header that was discussed in Section 12.1.  Router
links advertisements have LS type = 1.  The router indicates whether it
is willing to calculate separate routes for each IP TOS by setting (or
resetting) the T-bit of the link state advertisement's Options field.

A router also indicates whether it is an area border router, or an AS
boundary router, by setting the appropriate bits in its router links
advertisements.  This enables paths to those types of routers to be
saved in the routing table, for later processing of summary link
advertisements and AS external link advertisements.

The router links advertisement then describes the router's working
connections (links) to the area.  Each link is typed according to the


               _________________________________________

                (Figure not included in text version.)

               Figure 15: Area 1 with IP addresses shown
                 Figure 16: Forwarding address example
               _________________________________________






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kind of attached network.  Each link is also labelled with its Link ID.
This ID gives a name to the entity that is on the other end of the link.
Table 18 summarizes the values used for the type and Link ID fields.



Link type   Description               Link ID
____________________________________________________________________________
1           Point-to-point link       Neighbor Router ID
2           Link to transit network   Interface address of Designated Router
3           Link to stub network      IP network number
4           Virtual link              Neighbor Router ID


       Table 18: Link descriptions in the router links advertisement.


In addition, the Link Data field is specified for each link.  This field
gives 32 bits of extra information for the link.  For links to routers
and transit networks, this field specifies the IP interface address of
the associated router interface (this is needed by the routing table
calculation, see Section 16.3).  For links to stub networks, this field
specifies the network's IP address mask.

Finally, the cost of using the link for output (possibly specifying a
different cost for each type of service) is specified.  The output cost
of a link is configurable.  It must always be non-zero.

To further describe the process of building the list of link records,
suppose a router wishes to build router links advertisement for an Area
A.  The router examines its collection of interface data structures.
For each interface, the following steps are taken:


o   If the attached network does not belong to Area A, no links are
    added to the advertisement, and the next interface should be
    examined.

o   Else, if the state of the interface is Down, no links are added.

o   Else, if the state of the interface is Point-to-Point, then add
    links according to the following:

    -   If the neighboring router is fully adjacent, add a Type 1 link
        (point-to-point) if this is an interface to a point-to-point
        network, or add a type 4 link (virtual link) if this is a
        virtual link.  The Link ID should be set to the Router ID of the
        neighboring router, and the Link Data should specify the



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        interface IP address.

    -   If this is a numbered point-to-point network (i.e, not a virtual
        link and not an unnumbered point-to-point network) and the
        neighboring router's IP address is known, add a Type 3 link
        (stub network) whose Link ID is the neighbor's IP address, whose
        Link Data is the mask 0xffffffff indicating a host route, and
        whose cost is the interface's configured output cost.

o   Else if the state of the interface is Loopback, add a Type 3 link
    (stub network) as long as this is not an interface to an unnumbered
    serial line.  The Link ID should be set to the IP interface address,
    the Link Data set to the mask 0xffffffff (indicating a host route),
    and the cost set to 0.

o   Else if the state of the interface is Waiting, add a Type 3 link
    (stub network) whose Link ID is the IP network number of the
    attached network and whose Link Data is the attached network's
    address mask.

o   Else, there has been a Designated Router selected for the attached
    network.  If the router is fully adjacent to the Designated Router,
    or if the router itself is Designated Router and is fully adjacent
    to at least one other router, add a single Type 2 link (transit
    network) whose whose link ID is the IP interface address of the
    attached network's Designated Router (which may be the router
    itself) and whose Link Data is the interface IP address.  Otherwise,
    add a link as if the interface state were Waiting (see above).


Unless otherwise specified, the cost of each link generated by the above
procedure is equal to the output cost of the associated interface.  Note
that in the case of serial lines, multiple links may be generated by a
single interface.

After consideration of all the router interfaces, host links are added
to the advertisement by examining the list of attached hosts.  A host
route is represented as a Type 3 link (stub network) whose link ID is
the host's IP address and whose Link Data is the mask of all ones
(0xffffffff).

As an example, consider the router links advertisements generated by
router RT3, as pictured in Figure 6.  The area containing router RT3
(Area 1) has been redrawn, with actual network addresses, in Figure 15.
Assume that the last byte of all of RT3's interface addresses is 3,
giving it the interface addresses 192.1.1.3 and 192.1.4.3, and that the
other routers have similar addressing schemes.  In addition, assume that
all links are functional, and that Router IDs are assigned as the



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smallest IP interface address.

RT3 originates two router links advertisements, one for Area 1 and one
for the backbone.  Assume that router RT4 has been selected as the
Designated router for network 192.1.1.0.  RT3's router links
advertisement for Area 1 is then shown below.  It indicates that RT3 has
two connections to Area 1, the first a link to the transit network
192.1.1.0 and the second a link to the stub network 192.1.4.0.  Note
that the transit network is identified by the IP interface of its
Designated Router (i.e., the Link ID = 192.1.1.4 which is the Designated
Router RT4's IP interface to 192.1.1.0).  Note also that RT3 has
indicated that it is capable of calculating separate routes based on IP
TOS, through setting the T-bit in the Options field.  It has also
indicated that it is an area border router.

       ; RT3's router links advertisement for Area 1

       LS age = 0                     ;always true on origination
       Options = (T-bit|E-bit)        ;TOS-capable
       LS type = 1                    ;indicates router links
       Link State ID = 192.1.1.3      ;RT3's Router ID
       Advertising Router = 192.1.1.3 ;RT3's Router ID
       bit E = 0                      ;not an AS boundary router
       bit B = 1                      ;RT3 is an area border router
       #links = 2
               Link ID = 192.1.1.4    ;IP address of Designated Router
               Link Data = 192.1.1.3  ;RT3's IP interface to net
               Type = 2               ;connects to transit network
               # other metrics = 0
               TOS 0 metric = 1

               Link ID = 192.1.4.0    ;IP Network number
               Link Data = 0xffffff00 ;Network mask
               Type = 3               ;connects to stub network
               # other metrics = 0
               TOS 0 metric = 2

Next RT3's router links advertisement for the backbone is shown.  It
indicates that RT3 has a single attachment to the backbone.  This
attachment is via an unnumbered point-to-point link to router RT6.  RT3
has again indicated that it is TOS-capable, and that it is an area
border router.

       ; RT3's router links advertisement for the backbone

       LS age = 0                     ;always true on origination
       Options = (T-bit|E-bit)        ;TOS-capable
       LS type = 1                    ;indicates router links



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       Link State ID = 192.1.1.3      ;RT3's router ID
       Advertising Router = 192.1.1.3 ;RT3's router ID
       bit E = 0                      ;not an AS boundary router
       bit B = 1                      ;RT3 is an area border router
       #links = 1
               Link ID = 18.10.0.6    ;Neighbor's Router ID
               Link Data = 0.0.0.0    ;Interface to unnumbered SL
               Type = 1               ;connects to router
               # other metrics = 0
               TOS 0 metric = 8

Even though router RT3 has indicated that it is TOS-capable in the above
examples, only a single metric (the TOS 0 metric) has been specified for
each interface.  Different metrics can be specified for each TOS.  The
encoding of TOS in OSPF link state advertisements is described in
Section 12.3.

As an example, suppose the point-to-point link between routers RT3 and
RT6 in Figure 15 is a satellite link.  The AS administrator may want to
encourage the use of the line for high bandwidth traffic.  This would be
done by setting the metric artificially low for that TOS.  Router RT3
would then originate the following router links advertisement for the
backbone (IP TOS 8 = high bandwidth):

       ; RT3's router links advertisement for the backbone

       LS age = 0                  ;always true on origination
       Options = (T-bit|E-bit)     ;TOS-capable
       LS type = 1                 ;indicates router links
       Link State ID = 192.1.1.3   ;RT3's Router ID
       Advertising Router = 192.1.1.3
       bit E = 0                   ;not an AS boundary router
       bit B = 1                   ;RT3 is an area border router
       #links = 1
               Link ID = 18.10.0.6 ; Neighbor's Router ID
               Link Data = 0.0.0.0 ;Interface to unnumbered SL
               Type = 1            ;connects to router
               # other metrics = 1
               TOS 0 metric = 8
                       TOS = 8     ;High bandwidth
                       metric = 1  ;traffic preferred


12.4.2 Network links

A network links advertisement is generated for every transit multi-
access network.  (A transit network is a network having two or more
attached routers).  The network links advertisement describes all the



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routers that are attached to the network.

The Designated Router for the network originates the advertisement.  The
Designated Router originates the advertisement only if it is fully
adjacent to at least one other router on the network.  The network links
advertisement is flooded throughout the area that contains the transit
network, and no further.  The networks links advertisement lists those
routers that are fully adjacent to the Designated Router; each fully
adjacent router is identified by its OSPF Router ID.  The Designated
Router includes itself in this list.

The Link State ID for a network links advertisement is the IP interface
address of the Designated Router.  This value, masked by the network's
address mask (which is also contained in the network links
advertisement) yields the network's IP address.

A router that has formerly been the Designated Router for a network, but
is no longer, should flush the network links advertisement that it had
previously originated.  This advertisement is no longer used in the
routing table calculation.  It is flushed by prematurely incrementing
the advertisement's age to MaxAge and reflooding (see Section 14.1).

As an example of a network links advertisement, again consider the area
configuration in Figure 6.  Network links advertisements are originated
for network N3 in Area 1, networks N6 and N8 in Area 2, and network N9
in Area 3.  Assuming that router RT4 has been selected as the Designated
Router for network N3, the following network links advertisement is
generated by RT4 on behalf of network N3 (see Figure 15 for the address
assignments):

       ; network links advertisement for network N3

       LS age = 0                     ;always true on origination
       Options = (T-bit|E-bit)        ;TOS-capable
       LS type = 2                    ;indicates network links
       Link State ID = 192.1.1.4      ;IP address of Designated Router
       Advertising Router = 192.1.1.4 ;RT4's Router ID
       Network Mask = 0xffffff00
               Attached Router = 192.1.1.4    ;Router ID
               Attached Router = 192.1.1.1    ;Router ID
               Attached Router = 192.1.1.2    ;Router ID
               Attached Router = 192.1.1.3    ;Router ID


12.4.3 Summary links

Each summary link advertisement describes a route to a single
destination.  Summary link advertisements are flooded throughout a



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single area only.  The destination described is one that is external to
the area, yet still belonging to the Autonomous System.

The DefaultDestination can also be specified in summary link
advertisements.  This is used when implementing OSPF's stub area
functionality (see Section 3.6).  In a stub area, instead of importing
external routes each area border router originates a "default summary
link" (Link State ID = DefaultDestination) into the area.

Summary link advertisements are originated by area border routers.  The
precise summary routes to advertise into an area are determined by
examining the routing table structure (see Section 11).  Only intra-area
routes are advertised into the backbone.  Both intra-area and inter-area
routes are advertised into the other areas.

To determine which routes to advertise into an attached Area A, each
routing table entry is processed as follows:


o   Only Destination types of network and AS boundary router are
    advertised in summary link advertisements.  If the routing table
    entry's Destination type is area border router, examine the next
    routing table entry.

o   AS external routes are never advertised in summary link
    advertisements.  If the routing table entry has Path-type type 1
    external or type 2 external, examine the next routing table entry.

o   Else, if the area associated with this set of paths is the Area A
    itself, do not generate a summary link advertisement for the
    route.[14]

o   Else, if the destination of this route is an AS boundary router,
    generate a Type 4 link state advertisement for the destination, with
    Link State ID equal to the AS boundary router's ID and metric equal
    to the routing table entry's cost.  These advertisements should not
    be generated if area A has been configured as a stub area.

o   Else, the Destination type is network.  If this is an inter-area
    route, generate a Type 3 advertisement for the destination, with
    Link State ID equal to the network's address and metric equal to the
    routing table cost.

o   The one remaining case is an intra-area route to a network.  This
    means that the network is contained in one of the router's directly
    attached areas.  In general, this information must be condensed
    before appearing in summary link advertisements.  Remember that an
    area has been defined as a list of address ranges, each range



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    consisting of an [address,mask] pair.  A single Type 3 advertisement
    must be made for each range, with Link State ID equal to the range's
    address and cost equal to the smallest cost of any of the component
    networks.

    If virtual links are being used to provide/increase connectivity of
    the backbone, routing information concerning the backbone networks
    should not be condensed before being summarized into the virtual
    links' transit areas.  In other words, the backbone ranges should be
    ignored when originating summary links into these areas.  The
    existence of virtual links can be determined during the shortest
    path calculation for the backbone (see Section 16.1).


In addition, if area A has been configured as a stub area and the router
is an area border router, it should advertise a default summary link
into Area A.  The Link State ID for the advertisement should be set to
DefaultDestination, and the metric set to the (per-area) configurable
parameter StubDefaultCost.

If a router advertises a summary advertisement for a destination which
then becomes unreachable, the router must then flush the advertisement
from the routing domain by setting its age to MaxAge and reflooding (see
Section 14.1).  Also, if the destination is still reachable, yet can no
longer be advertised according to the above procedure (e.g., it is now
an inter-area route, when it used to be an intra-area route associated
with some non-backbone area; it would thus no longer be advertisable to
the backbone), the advertisement should also be flushed from the routing
domain.

For an example of summary link advertisements, consider again the area
configuration in Figure 6.  Routers RT3, RT4, RT7, RT10 and RT11 are all
area border routers, and therefore are originating summary links
advertisements.  Consider in particular router RT4.  Its routing table
was calculated as the example in Section 11.3.  RT4 originates summary
link advertisements into both the backbone and Area 1.  Into the
backbone, router RT4 originates separate advertisements for each of the
networks N1-N4.  Into Area 1, router RT4 originates separate
advertisements for networks N6-N8 and the AS boundary routers RT5,RT7.
It also condenses host routes Ia and Ib into a single summary
advertisement.  Finally, the routes to networks N9,N10,N11 and host H9
are advertised by a single summary link.  This condensation was
originally performed by the router RT11.

These advertisements are illustrated graphically in Figures 7 and 8.
Two of the summary link advertisements originated by router RT4 follow.
The actual IP addresses for the networks and routers in question have
been assigned in Figure 15.



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       ; summary link advertisement for network N1,
       ; originated by router RT4 into the backbone

       LS age = 0                  ;always true on origination
       Options = (T-bit|E-bit)     ;TOS-capable
       LS type = 3                 ;indicates summary link to IP net
       Link State ID = 192.1.2.0   ;N1's IP network number
       Advertising Router = 192.1.1.4       ;RT4's ID
               TOS = 0
               metric = 4

       ; summary link advertisement for AS boundary router RT7
       ; originated by router RT4 into Area 1

       LS age = 0                  ;always true on origination
       Options = (T-bit|E-bit)     ;TOS-capable
       LS type = 4                 ;indicates summary link to ASBR
       Link State ID = router RT7's ID
       Advertising Router = 192.1.1.4       ;RT4's ID
               TOS = 0
               metric = 14

Summary link advertisements pertain to a single destination (IP network
or AS boundary router).  However, for a single destination there may be
separate sets of paths, and therefore separate routing table entries,
for each Type of Service.  All these entries must be considered when
building the summary link advertisement for the destination; a single
advertisement must specify the separate costs (if they exist) for each
TOS.  The encoding of TOS in OSPF link state advertisements is described
in Section 12.3.

Clearing the T-bit in the Options field of a summary link advertisement
indicates that there is a TOS 0 path to the destination, but no paths
for non-zero TOS.  This can happen when non-TOS capable routers exist in
the routing domain (see Section 2.4).


12.4.4 AS external links

AS external link advertisements describe routes to destinations external
to the Autonomous System.  Most AS external link advertisements describe
routes to specific external destinations.  However, a default route for
the Autonomous System can be described in an AS external advertisement
by setting the advertisement's Link State ID to DefaultDestination
(0.0.0.0).  AS external link advertisements are originated by AS
boundary routers.  An AS boundary router originates a single AS external
link advertisement for each external route that it has learned, either
through another routing protocol (such as EGP), or through configuration



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information.

In general, AS external link advertisements are the only type of link
state advertisements that are flooded throughout the entire Autonomous
System; all other types of link state advertisements are specific to a
single area.  However, AS external advertisements are not flooded
into/throughout stub areas (see Section 3.6).  This enables a reduction
in link state database size for routers internal to stub areas.

The metric that is advertised for an external route can be one of two
types.  Type 1 metrics are comparable to the link state metric.  Type 2
metrics are assumed to be larger than the cost of any intra-AS path.  As
with summary link advertisements, if separate paths exist based on TOS,
separate TOS costs can be included in the AS external link
advertisement.  The encoding of TOS in OSPF link state advertisements is
described in Section 12.3.  If the T-bit of the advertisement's Options
field is clear, no non-zero TOS paths to the destination exist.

If a router advertises an AS external link advertisement for a
destination which then becomes unreachable, the router must then flush
the advertisement from the routing domain by setting its age to MaxAge
and reflooding (see Section 14.1).

For an example of AS external link advertisements, consider once again
the AS pictured in Figure 6.  There are two AS boundary routers: RT5 and
RT7.  Router RT5 originates three external link advertisements, for
networks N12-N14.  Router RT7 originates two external link
advertisements, for networks N12 and N15.  Assume that RT7 has learned
its route to N12 via EGP, and that it wishes to advertise a Type 2
metric to the AS.  RT7 would then originate the following advertisement
for N12:

       ; AS external link advertisement for network N12,
       ; originated by router RT7

       LS age = 0                  ;always true on origination
       Options = (T-bit|E-bit)     ;TOS-capable
       LS type = 5                 ;indicates AS external link
       Link State ID = N12's IP network number
       Advertising Router = Router RT7's ID
               bit E = 1           ;Type 2 metric
               TOS = 0
               metric = 2
               Forwarding address = 0.0.0.0

In the above example, the forwarding address field has been set to
0.0.0.0, indicating that packets for the external destination should be
forwarded to the advertising OSPF router (RT7).  This is not always



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desirable.  Consider the example pictured in Figure 16.  There are three
OSPF routers (RTA, RTB and RTC) connected to a common network.  Only one
of these routers, RTA, is exchanging EGP information with the non-OSPF
router RTX.  RTA must then originate AS external link state
advertisements for those destinations it has learned from RTX.  By using
the AS external advertisement's forwarding address field, RTA can
specify that packets for these destinations be forwarded directly to
RTX.  Without this feature, routers RTB and RTC would take an extra hop
to get to these destinations.

Note that when the forwarding address field is non-zero, it should point
to a router belonging to another Autonomous System.

A forwarding address can also be specified for the default route.  For
example, in figure 16 RTA may want to specify that all externally-
destined packets should by default be forwarded to its EGP peer RTX.
The resulting AS external link advertisement is pictured below.  Note
that the Link State ID is set to DefaultDestination.

       ; Default route, originated by router RTA
       ; Packets forwarded through RTX

       LS age = 0                  ;always true on origination
       Options = (T-bit|E-bit)     ;TOS-capable
       LS type = 5                 ;indicates AS external link
       Link State ID = DefaultDestination  ; default route
       Advertising Router = Router RTA's ID
               bit E = 1           ;Type 2 metric
               TOS = 0
               metric = 1
               Forwarding address = RTX's IP address

In figure 16, suppose instead that both RTA and RTB exchange EGP
information with RTX.  In this case, RTA and RTB would originate the
same set of external advertisements.  These advertisements, if they
specify the same metric, would be functionally equivalent since they
would specify the same destination and forwarding address (RTX).  This
leads to a clear duplication of effort.  If only one of RTA or RTB
originated the set of external advertisements, the routing would remain
the same, and the size of the link state database would decrease.
However, it must be unambiguously defined as to which router originates
the advertisements (otherwise neither may, or the identity of the
originator may oscillate).  The following rule is thereby established:
if two routers, both reachable from one another, originate functionally
equivalent AS external advertisements (i.e., same destination, cost and
non-zero forwarding address), then the advertisement originated by the
router having the highest OSPF Router ID is used.  The router having the
lower OSPF Router ID can then flush its advertisement.  Flushing a link



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state advertisement is discussed in Section 14.1.


13. The Flooding Procedure

Link State Update packets provide the mechanism for flooding link state
advertisements.  A Link State Update packet may contain several distinct
advertisements, and floods each advertisement one hop further from its
point of origination.  To make the flooding procedure reliable, each
advertisement must be acknowledged separately.  Acknowledgments are
transmitted in Link State Acknowledgment packets.  Many separate
acknowledgments can be grouped together into a single packet.

The flooding procedure starts when a Link State Update packet has been
received.  Many consistency checks have been made on the received packet
before being handed to the flooding procedure (see Section 8.2).  In
particular, the Link State Update packet has been associated with a
particular neighbor, and a particular area.  If the neighbor is in a
lesser state than Exchange, the packet should be dropped without further
processing.

All types of link state advertisements, other than AS external links,
are associated with a specific area.  However, link state advertisements
do not contain an area field.  A link state advertisement's area must be
deduced from the Link State Update packet header.

For each link state advertisement contained in the packet, the following
steps are taken:


(1) Validate the advertisement's link state checksum.  If the checksum
    turns out to be invalid, discard the advertisement and get the next
    one from the Link State Update packet.

(2) Examine the link state advertisement's LS type.  If the LS type is
    unknown, discard the advertisement and get the next one from the
    Link State Update Packet.  This specification defines LS Types 1-5
    (see Section 4.3).

(3) Else if this is a AS external advertisement (LS type = 5), and the
    area has been configured as a stub area, discard the advertisement
    and get the next one from the Link State Update Packet.  AS external
    advertisements are not flooded into/throughout stub areas (see
    Section 3.6).

(4) Else if the advertisement's age is equal to MaxAge, and there is
    currently no instance of the advertisement in the router's link
    state database, then take the following actions:



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    (a) Acknowledge the receipt of the advertisement by sending a Link
        State Acknowledgment packet back to the sending neighbor (see
        Section 13.5).

    (b) Purge all outstanding requests for equal or previous instances
        of the advertisement from the sending neighbor's Link State
        Request list (see Section 10).

    (c) If the sending neighbor is in state Exchange or in state
        Loading, then install the MaxAge advertisement in the link state
        database.  Otherwise, simply discard the advertisement.  In
        either case, examine the next advertisement (if any) listed in
        the Link State Update packet.

(5) Otherwise, find the instance of this advertisement that is currently
    contained in the router's link state database.  If there is no
    database copy, or the received advertisement is more recent than the
    database copy (see Section 13.1 below for the determination of which
    advertisement is more recent) the following steps must be performed:

    (a) If there is already a database copy, and if the database copy
        was installed less than MinLSInterval seconds ago, discard the
        new advertisement (without acknowledging it) and examine the
        next advertisement (if any) listed in the Link State Update
        packet.

    (b) Otherwise immediately flood the new advertisement out some
        subset of the router's interfaces (see Section 13.3).  In some
        cases (e.g., the state of the receiving interface is DR and the
        advertisement was received from a router other than the Backup
        DR) the advertisement will be flooded back out the receiving
        interface.  This occurrence should be noted for later use by the
        acknowledgment process (Section 13.5).

    (c) Remove the current database copy from all neighbors' Link state
        retransmission lists.

    (d) Install the new advertisement in the link state database
        (replacing the current database copy).  This may cause the
        routing table calculation to be scheduled.  In addition,
        timestamp the new advertisement with the current time (i.e., the
        time it was received).  The flooding procedure cannot overwrite
        the newly installed advertisement until MinLSInterval seconds
        have elapsed.  The advertisement installation process is
        discussed further in Section 13.2.

    (e) Possibly acknowledge the receipt of the advertisement by sending
        a Link State Acknowledgment packet back out the receiving



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        interface.  This is explained below in Section 13.5.

    (f) If this new link state advertisement indicates that it was
        originated by this router itself, the router must advance the
        advertisement's link state sequence number, and issue a new
        instance of the advertisement (see Section 13.4).

(6) Else, if there is an instance of the advertisement on the sending
    neighbor's Link state request list, an error has occurred in the
    Database Description process.  In this case, restart the Database
    Description process by generating the neighbor event BadLSReq for
    the sending neighbor and stop processing the Link State Update
    packet.

(7) Else, if the received advertisement is the same instance as the
    database copy (i.e., neither one is more recent) the following two
    steps should be performed:

    (a) If the advertisement is listed in the Link state retransmission
        list for the receiving adjacency, the router itself is expecting
        an acknowledgment for this advertisement.  The router should
        treat the received advertisement as an acknowledgment, by
        removing the advertisement from the Link state retransmission
        list.  This is termed an "implied acknowledgment".  Its
        occurrence should be noted for later use by the acknowledgment
        process (Section 13.5).

    (b) Possibly acknowledge the receipt of the advertisement by sending
        a Link State Acknowledgment packet back out the receiving
        interface.  This is explained below in Section 13.5.

(8) Else, the database copy is more recent.  Note an unusual event to
    network management, discard the advertisement and process the next
    link state advertisement contained in the packet.


13.1 Determining which link state is newer

When a router encounters two instances of a link state advertisement, it
must determine which is more recent.  This occurred above when comparing
a received advertisement to the database copy.  This comparison must
also be done during the database exchange procedure which occurs during
adjacency bring-up.

A link state advertisement is identified by its LS type, Link State ID
and Advertising Router.  For two instances of the same advertisement,
the LS sequence number, LS age, and LS checksum fields are used to
determine which instance is more recent:



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o   The advertisement having the newer LS sequence number is more
    recent.  See Section 12.1.6 for an explanation of the LS sequence
    number space.  If both instances have the same LS sequence number,
    then:

o   If the two instances have different LS checksums, then the instance
    having the larger LS checksum (when considered as a 16-bit unsigned
    integer) is considered more recent.

o   Else, if only one of the instances is of age MaxAge, the instance of
    age MaxAge is considered to be more recent.

o   Else, if the ages of the two instances differ by more than
    MaxAgeDiff, the instance having the smaller (younger) age is
    considered to be more recent.

o   Else, the two instances are considered to be identical.


13.2 Installing link state advertisements in the database

Installing a new link state advertisement in the database, either as the
result of flooding or a newly self originated advertisement, may cause
the routing table structure to be recalculated.  The contents of the new
advertisement should be compared to the old instance, if present.  If
there is no difference, there is no need to recalculate the routing
table.  (Note that even if the contents are the same, the LS checksum
will probably be different, since the checksum covers the LS sequence
number.)

If the contents are different, the following pieces of the routing table
must be recalculated, depending on the LS type field:


Router links, network links
    The entire routing table must be recalculated, starting with the
    shortest path calculations for each area (not just the area whose
    topological database has changed).  The reason that the shortest
    path calculation cannot be restricted to the single changed area has
    to do with the fact that AS boundary routers may belong to multiple
    areas.  A change in the area currently providing the best route may
    force the router to use an intra-area route provided by a different
    area.[15]

Summary link
    The best route to the destination described by the summary link
    advertisement must be re-examined (see Section 16.5).  If this
    destination is an AS boundary router, it may also be necessary to



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    re-examine all the AS external link advertisements.

AS external link
    The best route to the destination described by the AS external link
    advertisement must be re-examined (see Section 16.6).


Also, any old instance of the advertisement must be removed from the
database when the new advertisement is installed.  This old instance
must also be removed from all neighbors' Link state retransmission lists
(see Section 10).


13.3 Next step in the flooding procedure

When a new (and more recent) advertisement has been received, it must be
flooded out some set of the router's interfaces.  This section describes
the second part of flooding procedure (the first part being the
processing that occurred in Section 13), namely, selecting the outgoing
interfaces and adding the advertisement to the appropriate neighbors'
Link state retransmission lists.  Also included in this part of the
flooding procedure is the maintenance of the neighbors' Link state
request lists.

This section is equally applicable to the flooding of an advertisement
that the router itself has just originated (see Section 12.4).  For
these advertisements, this section provides the entirety of the flooding
procedure (i.e., the processing of Section 13 is not performed, since,
for example, the advertisement has not been received from a neighbor and
therefore does not need to be acknowledged).

Depending upon the advertisement's LS type, the advertisement can be
flooded out only certain interfaces.  These interfaces, defined by the
following, are called the eligible interfaces:


AS external links (LS Type = 5)
    AS external links are flooded throughout the entire AS, with the
    exception of stub areas (see Section 3.6).  The eligible interfaces
    are all the router's interfaces, excluding virtual links and those
    interfaces attaching to stub areas.

All other types
    All other types are specific to a single area (Area A).  The
    eligible interfaces are all those interfaces attaching to the Area
    A.  If Area A is the backbone, this includes all the virtual links.





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Link state databases must remain synchronized over all adjacencies
associated with the above eligible interfaces.  This is accomplished by
executing the following steps on each eligible interface.  It should be
noted that this procedure may decide not to flood a link state
advertisement out a particular interface, if there is a high probability
that the attached neighbors have already received the advertisement.
However, in these cases the flooding procedure must be absolutely sure
that the neighbors eventually do receive the advertisement, so the
advertisement is still added to each adjacency's Link state
retransmission list.  For each eligible interface:


(1) Each of the neighbors attached to this interface are examined, to
    determine whether they must receive the new advertisement.  The
    following steps are executed for each neighbor:

    (a) If the neighbor is in a lesser state than Exchange, it does not
        participate in flooding, and the next neighbor should be
        examined.

    (b) Else, if the adjacency is not yet full (neighbor state is
        Exchange or Loading), examine the Link state request list
        associated with this adjacency.  If there is an instance of the
        new advertisement on the list, it indicates that the neighboring
        router has an instance of the advertisement already.  Compare
        the new advertisement to the neighbor's copy:

        o   If the new advertisement is less recent, then try the next
            neighbor.

        o   If the two copies are the same instance, then delete the
            advertisement from the Link state request list, and try the
            next neighbor.[16]

        o   Else, the new advertisement is more recent.  Delete the
            advertisement from the Link state request list.

    (c) If the new advertisement was received from this neighbor, try
        the next neighbor.

    (d) At this point we are not positive that the new neighbor has an
        up-to-date instance of this new advertisement.  Add the new
        advertisement to the Link state retransmission list for the
        adjacency.  This ensures that the flooding procedure is
        reliable; the advertisement will be retransmitted at intervals
        until an acknowledgment is seen from the neighbor.





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(2) The router must now decide whether to flood the new link state
    advertisement out this interface.  If in the previous step, the link
    state advertisement was NOT added to any of the Link state
    retransmission lists, there is no need to flood the advertisement
    and the next interface should be examined.

(3) If the new advertisement was received on this interface, and it was
    received from either the Designated Router or the Backup Designated
    Router, chances are all the neighbors have received the
    advertisement already.  Therefore, examine the next interface.

(4) If the new advertisement was received on this interface, and the
    interface state is Backup (i.e., the router itself is the Backup
    Designated Router), examine the next interface.  The Designated
    Router will do the flooding on this interface.  If the Designated
    Router fails, this router will end up retransmitting the updates.

(5) If this step is reached, the advertisement must be flooded out the
    interface.  Send a Link State Update packet (with the new
    advertisement as contents) out the interface.  The advertisement's
    LS age must be incremented by InfTransDelay (which must be > 0) when
    copied into the outgoing packet (until the LS age field reaches its
    maximum value of MaxAge).

    On broadcast networks, the Link State Update packets are multicast.
    The destination IP address specified for the Link State Update
    Packet depends on the state of the interface.  If the interface
    state is DR or Backup, the address AllSPFRouters should be used.
    Otherwise, the address AllDRouters should be used.

    On non-broadcast, multi-access networks, separate Link State Update
    packets must be sent, as unicasts, to each adjacent neighbor (i.e.,
    those in state Exchange or greater).  The destination IP addresses
    for these packets are the neighbors' IP addresses.


13.4 Receiving self-originated link state

It is a common occurrence to receive a self-originated link state
advertisement via the flooding procedure.  If the advertisement received
is a newer instance than the last instance that the router actually
originated, the router must take special action.

The reception of such an advertisement indicates that there are link
state advertisements in the routing domain that were originated before
the last time the router was restarted.  In this case, the router must
advance the sequence number for the advertisement one past the received
sequence number, and originate a new instance of the advertisement.



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Note also that if the type of the advertisement is Summary link or AS
external link, the router may no longer have an (advertisable) route to
the destination.  In this case, the advertisement should be flushed from
the routing domain by incrementing the advertisement's LS age to MaxAge
and reflooding (see Section 14.1).


13.5 Sending Link State Acknowledgment packets

Each newly received link state advertisement must be acknowledged.  This
is usually done by sending Link State Acknowledgment packets.  However,
acknowledgments can also be accomplished implicitly by sending Link
State Update packets (see step 7a of Section 13).

Many acknowledgments may be grouped together into a single Link State
Acknowledgment packet.  Such a packet is sent back out the interface
that has received the advertisements.  The packet can be sent in one of
two ways: delayed and sent on an interval timer, or sent directly (as a
unicast) to a particular neighbor.  The particular acknowledgment
strategy used depends on the circumstances surrounding the receipt of
the advertisement.

Sending delayed acknowledgments accomplishes several things: it
facilitates the packaging of multiple acknowledgments in a single
packet; it enables a single packet to indicate acknowledgments to
several neighbors at once (through multicasting); and it randomizes the
acknowledgment packets sent by the various routers attached to a multi-
access network.  The fixed interval between a router's delayed
transmissions must be short (less than RxmtInterval) or needless
retransmissions will ensue.

Direct acknowledgments are sent to a particular neighbor in response to
the receipt of duplicate link state advertisements.  These
acknowledgments are sent as unicasts, and are sent immediately when the
duplicate is received.

The precise procedure for sending Link State Acknowledgment packets is
described in Table 19.  The circumstances surrounding the receipt of the
advertisement are listed in the left column.  The acknowledgment action
then taken is listed in one of the two right columns.  This action
depends on the state of the concerned interface; interfaces in state
Backup behave differently from interfaces in all other states.


                                     Action taken in state
     Circumstances          Backup               All other states
     ______________________________________________________________




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                                     Action taken in state
     Circumstances          Backup               All other states
     ______________________________________________________________
Advertisement  has     No  acknowledgment   No  acknowledgment
been  flooded back     sent.                sent.
out receiving  in-
terface  (see Sec-
tion 13, step 5b).
______________________________________________________________
Advertisement   is     Delayed       ack-   Delayed       ack-
more  recent  than     nowledgment   sent   nowledgment sent.
database copy, but     if   advertisement
was   not  flooded     received  from DR,
back out receiving     otherwise do noth-
interface              ing
______________________________________________________________
Advertisement is a     Delayed       ack-   No  acknowledgment
duplicate, and was     nowledgment   sent   sent.
treated as an  im-     if   advertisement
plied  acknowledg-     received  from DR,
ment (see  Section     otherwise do noth-
13, step 7a).          ing
______________________________________________________________
Advertisement is a     Direct acknowledg-   Direct acknowledg-
duplicate, and was     ment sent.           ment sent.
not treated as  an
implied       ack-
nowledgment.
______________________________________________________________
Advertisement's age    Direct acknowledg-   Direct acknowledg-
is equal to MaxAge,    ment sent.           ment sent.
and there is no
current instance of
the advertisement in
the link state
database (see
Section 13, step 4).


             Table 19: Sending link state acknowledgements.

Delayed acknowledgments must be delivered to all adjacent routers
associated with the interface.  On broadcast networks, this is
accomplished by sending the delayed Link State Acknowledgment packets as
multicasts.  The Destination IP address used depends on the state of the
interface.  If the state is DR or Backup, the destination AllSPFRouters
is used.  In other states, the destination AllDRouters is used.  On
non-broadcast networks, delayed acks must be unicast separately over



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each adjacency (neighbor whose state is >= Exchange).

The reasoning behind sending the above packets as multicasts is best
explained by an example.  Consider the network configuration depicted in
Figure 15.  Suppose RT4 has been elected as DR, and RT3 as Backup for
the network N3.  When router RT4 floods a new advertisement to network
N3, it is received by routers RT1, RT2, and RT3.  These routers will not
flood the advertisement back onto net N3, but they still must ensure
that their topological databases remain synchronized with their adjacent
neighbors.  So RT1, RT2, and RT4 are waiting to see an acknowledgment
from RT3.  Likewise, RT4 and RT3 are both waiting to see acknowledgments
from RT1 and RT2.  This is best achieved by sending the acknowledgments
as multicasts.

The reason that the acknowledgment logic for Backup DRs is slightly
different is because they perform differently during the flooding of
link state advertisements (see Section 13.3, step 4).



13.6 Retransmitting link state advertisements

Advertisements flooded out an adjacency are placed on the adjacency's
Link state retransmission list.  In order to ensure that flooding is
reliable, these advertisements are retransmitted until they are
acknowledged.  The length of time between retransmissions is a
configurable per-interface value, RxmtInterval.  If this is set too low
for an interface, needless retransmissions will ensue.  If the value is
set too high, the speed of the flooding, in the face of lost packets,
may be affected.

Several retransmitted advertisements may fit into a single Link State
Update packet.  When advertisements are to be retransmitted, only the
number fitting in a single Link State Update packet should be
transmitted.  Another packet of retransmissions can be sent when some of
the advertisements are acknowledged, or on the next firing of the
retransmission timer.

Link State Update Packets carrying retransmissions are always sent as
unicasts (directly to the physical address of the neighbor).  They are
never sent as multicasts.  Each advertisement's LS age must be
incremented by InfTransDelay (which must be > 0) when copied into the
outgoing packet (until the LS age field reaches its maximum value of
MaxAge).

If the adjacent router goes down, retransmissions may occur until the
adjacency is destroyed by OSPF's Hello Protocol.  When the adjacency is
destroyed, the Link state retransmission list is cleared.



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13.7 Receiving link state acknowledgments

Many consistency checks have been made on a received Link State
Acknowledgment packet before it is handed to the flooding procedure.  In
particular, it has been associated with a particular neighbor.  If this
neighbor is in a lesser state than Exchange, the packet is discarded.

Otherwise, for each acknowledgment in the packet, the following steps
are performed:


o   Does the advertisement acknowledged have an instance on the Link
    state retransmission list for the neighbor?  If not, examine the
    next acknowledgment.  Otherwise:

o   If the acknowledgment is for the same instance that is contained on
    the list, remove the item from the list and examine the next
    acknowledgment.  Otherwise:

o   Log the questionable acknowledgment, and examine the next one.


14. Aging The Link State Database

Each link state advertisement has an age field.  The age is expressed in
seconds.  An advertisement's age field is incremented while it is
contained in a router's database.  Also, when copied into a Link State
Update Packet for flooding out a particular interface, the
advertisement's age is incremented by InfTransDelay.

An advertisement's age is never incremented past the value MaxAge.
Advertisements having age MaxAge are not used in the routing table
calculation.  As a router ages its link state database, an
advertisement's age may reach MaxAge.[17] At this time, the router must
attempt to flush the advertisement from the routing domain.  This is
done simply by reflooding the MaxAge advertisement just as if it was a
newly originated advertisement (see Section 13.3).

When a Database summary list for a newly adjacent neighbor is formed,
any MaxAge advertisements present in the link state database are added
to the neighbor's Link state retransmission list instead of the
neighbor's Database summary list.  See Section 10.3 for more details.

A MaxAge advertisement is removed entirely from the router's link state
database when a) it is no longer contained on any neighbor Link state
retransmission lists and b) none of the router's neighbors are in states
Exchange or Loading.




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When, in the process of aging the link state database, an
advertisement's age hits a multiple of CheckAge, its checksum should be
verified.  If the checksum is incorrect, a program or memory error has
been detected, and at the very least the router itself should be
restarted.


14.1 Premature aging of advertisements

A link state advertisement can be flushed from the routing domain by
setting its age to MaxAge and reflooding the advertisement.  This
procedure follows the same course as flushing an advertisement whose age
has naturally reached the value MaxAge (see Section 14).  In particular,
the MaxAge advertisement is removed from the router's link state
database as soon as a) it is no longer contained on any neighbor Link
state retransmission lists and b) none of the router's neighbors are in
states Exchange or Loading.  We call the setting of an advertisement's
age to MaxAge premature aging.

Premature aging is used when it is time for a self-originated
advertisement's sequence number field to wrap.  At this point, the
current advertisement instance (having LS sequence number of 0x7fffffff)
must be prematurely aged and flushed from the routing domain before a
new instance with sequence number 0x80000001 can be originated.  See
Section 12.1.6 for more information.

Premature aging can also be used when, for example, one of the router's
previously advertised external routes is no longer reachable.  In this
circumstance, the router can flush its external advertisement from the
routing domain via premature aging.  This procedure is preferable to the
alternative, which is to originate a new advertisement for the
destination specifying a metric of LSInfinity.

A router may only prematurely age its own (self-originated) link state
advertisements.  These are the link state advertisements having the
router's own OSPF Router ID in the Advertising Router field.


15. Virtual Links

The single backbone area (Area ID = 0) cannot be disconnected, or some
areas of the Autonomous System will become unreachable.  To
establish/maintain connectivity of the backbone, virtual links can be
configured through non-backbone areas.  Virtual links serve to connect
separate components of the backbone.  The two endpoints of a virtual
link are area border routers.  The virtual link must be configured in
both routers.  The configuration information in each router consists of
the other virtual endpoint (the other area border router), and the non-



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backbone area the two routers have in common (called the transit area).
Virtual links cannot be configured through stub areas (see Section 3.6).

The virtual link is treated as if it were an unnumbered point-to-point
network (belonging to the backbone) joining the two area border routers.
An attempt is made to establish an adjacency over the virtual link.
When this adjacency is established, the virtual link will be included in
backbone router links advertisements, and OSPF packets pertaining to the
backbone area will flow over the adjacency.  Such an adjacency has been
referred to as a "virtual adjacency".

In each endpoint router, the cost and viability of the virtual link is
discovered by examining the routing table entry for the other endpoint
router.  (The entry's associated area must be the configured transit
area).  Actually, there may be a separate routing table entry for each
Type of Service.  These are called the virtual link's corresponding
routing table entries.  The Interface Up event occurs for a virtual link
when its corresponding TOS 0 routing table entry becomes reachable.
Conversely, the Interface Down event occurs when its TOS 0 routing table
entry becomes unreachable.[18] In other words, the virtual link's
viability is determined by the existence of an intra-area path, through
the transit area, between the two endpoints.  The other details
concerning virtual links are as follows:

o   AS external links are NEVER flooded over virtual adjacencies.  This
    would be duplication of effort, since the same AS external links are
    already flooded throughout the virtual link's transit area.  For
    this same reason, AS external link advertisements are not summarized
    over virtual adjacencies during the database exchange process.

o   The cost of a virtual link is NOT configured.  It is defined to be
    the cost of the intra-area path between the two defining area border
    routers.  This cost appears in the virtual link's corresponding
    routing table entry.  When the cost of a virtual link changes, a new
    router links advertisement should be originated for the backbone
    area.

o   Just as the virtual link's cost and viability are determined by the
    routing table build process (through construction of the routing
    table entry for the other endpoint), so are the IP interface address
    for the virtual interface and the virtual neighbor's IP address.
    These are used when sending protocol packets over the virtual link.

o   In each endpoint's router links advertisement for the backbone, the
    virtual link is represented as a link having link type 4, Link ID
    set to the virtual neighbor's OSPF Router ID and Link Data set to
    the virtual interface's IP address.  See Section 12.4.1 for more
    information.  Also, it may be the case that there is a TOS 0 path,



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    but no non-zero TOS paths to the other endpoint router.  In this
    case, non-zero TOS costs must be set to LSInfinity in the router
    links advertisement.

o   When virtual links are configured for the backbone, information
    concerning backbone networks should not be condensed before being
    summarized for the transit areas.  In other words, each backbone
    network should be advertised in a separate summary link
    advertisement, regardless of the backbone's configured area address
    ranges.  See Section 12.4.3 for more information.

o   The time between link state retransmissions, RxmtInterval, is
    configured for a virtual link.  This should be well over the
    expected round-trip delay between the two routers.  This may be hard
    to estimate for a virtual link.  It is better to err on the side of
    making it too large.


16. Calculation Of The Routing Table

This section details the OSPF routing table calculation.  Using its
attached areas' link state databases as input, a router runs the
following algorithm, building its routing table step by step.  At each
step, the router must access individual pieces of the link state
databases (e.g., a router links advertisement originated by a certain
router).  This access is performed by the lookup function discussed in
Section 12.2.  The lookup process may return a link state advertisement
whose LS age is equal to MaxAge.  Such an advertisement should not be
used in the routing table calculation, and is treated just as if the
lookup process had failed.

The OSPF routing table's organization is explained in Section 11.  Two
examples of the routing table build process are presented in Sections
11.2 and 11.3.  This process can be broken into the following steps:


(1) The present routing table is invalidated.  The routing table is
    built again from scratch.  The old routing table is saved so that
    changes in routing table entries can be identified.

(2) The intra-area routes are calculated by building the shortest path
    tree for each attached area.  In particular, all routing table
    entries whose Destination type is "area border router" are
    calculated in this step.  This step is described in two parts.  At
    first the tree is constructed by only considering those links
    between routers and transit networks.  Then the stub networks are
    incorporated into the tree.




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(3) The inter-area routes are calculated, through examination of summary
    link advertisements.  If the router is attached to multiple areas
    (i.e., it is an area border router), only backbone summary link
    advertisements are examined.

(4) For those routing entries whose next hop is over a virtual link, a
    real (physical) next hop is calculated.  The real next hop will be
    on one of the router's directly attached networks.  This step only
    concerns routers having configured virtual links.

(5) Routes to external destinations are calculated, through examination
    of AS external link advertisements.  The location of the AS boundary
    routers (which originate the AS external link advertisements) has
    been determined in steps 2-4.


Steps 2-5 are explained in further detail below.  The explanations
describe the calculations for TOS 0 only.  It may also be necessary to
perform each step (separately) for each of the non-zero TOS values.[19]
For more information concerning the building of non-zero TOS routes see
Section 16.9.

Changes made to routing table entries as a result of these calculations
can cause the OSPF protocol to take further actions.  For example, a
change to an intra-area route will cause an area border router to
originate new summary link advertisements (see Section 12.4).  See
Section 16.7 for a complete list of the OSPF protocol actions resulting
from routing table table changes.


16.1 Calculating the shortest-path tree for an area

This calculation yields the set of intra-area routes associated with an
area (called hereafter Area A).  A router calculates the shortest-path
tree using itself as the root.[20] The formation of the shortest path
tree is done here in two stages.  In the first stage, only links between
routers and transit networks are considered.  Using the Dijkstra
algorithm, a tree is formed from this subset of the link state database.
In the second stage, leaves are added to the tree by considering the
links to stub networks.

The procedure will be explained using the graph terminology that was
introduced in Section 2.  The area's link state database is represented
as a directed graph.  The graph's vertices are routers, transit networks
and stub networks.  The first stage of the procedure concerns only the
transit vertices (routers and transit networks) and their connecting
links.  Throughout the shortest path calculation, the following data is
also associated with each transit vertex:



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Vertex (node) ID
    A 32-bit number uniquely identifying the vertex.  For router
    vertices this is the OSPF Router ID.  For network vertices, this is
    the IP address of the network's Designated Router.

A link state advertisement
    Each transit vertex has an associated link state advertisement.  For
    router vertices, this is a router links advertisement.  For transit
    networks, this is a network links advertisement (which is actually
    originated by the network's Designated Router).  In any case, the
    advertisement's Link State ID is always equal to the above Vertex
    ID.

List of next hops
    The list of next hops for the current shortest paths from the root
    to this vertex.  There can be multiple shortest paths due to the
    equal-cost multipath capability.  Each next hop indicates the
    outgoing router interface to use when forwarding traffic to the
    destination.  On multi-access networks, the next hop also includes
    the IP address of the next router (if any) in the path towards the
    destination.

Distance from root
    The link state cost of the current shortest path(s) from the root to
    the vertex.  The link state cost of a path is calculated as the sum
    of the costs of the path's constituent links (as advertised in
    router links and network links advertisements).  One path is said to
    be "shorter" than another if it has a smaller link state cost.


The first stage of the procedure can now be summarized as follows.  At
each iteration of the algorithm, there is a list of candidate vertices.
The shortest paths from the root to these vertices have not
(necessarily) been found.  The candidate vertex closest to the root is
added to the shortest-path tree, removed from the candidate list, and
its adjacent vertices are examined for possible addition to/modification
of the candidate list.  The algorithm then iterates again.  It
terminates when the candidate list becomes empty.

The following steps describe the first stage in detail.  Remember that
we are computing the shortest path tree for Area A.  All references to
link state database lookup below are from Area A's database.


(1) Initialize the algorithm's data structures.  Clear the list of
    candidate vertices.  Initialize the shortest-path tree to only the
    root (which is the router doing the calculation).




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(2) Call the vertex just added to the tree vertex V.  Examine the link
    state advertisement associated with vertex V.  This is a lookup in
    the area link state database based on the Vertex ID.  Each link
    described by the advertisement gives the cost to an adjacent vertex.
    For each described link, (say it joins vertex V to vertex W):

    (a) If this is a link to a stub network, examine the next link in
        V's advertisement.  Links to stub networks will be considered in
        the second stage of the shortest path calculation.

    (b) Otherwise, W is a transit vertex (router or transit network).
        Look up the vertex W's link state advertisement (router links or
        network links) in Area A's link state database.  If the
        advertisement does not exist, or its age is equal to MaxAge, or
        it does not have a link back to vertex V, examine the next link
        in V's advertisement.  Both ends of a link must advertise the
        link before it will be used for data traffic.[21]

    (c) If vertex W is already on the shortest-path tree, examine the
        next link in the advertisement.

    (d) If the cost of the link (from V to W) is LSInfinity, the link
        should not be used for data traffic.  In this case, examine the
        next link in the advertisement.

    (e) Calculate the link state cost D of the resulting path from the
        root to vertex W.  D is equal to the sum of the link state cost
        of the (already calculated) shortest path to vertex V and the
        advertised cost of the link between vertices V and W.  If D is:

        o   Greater than the value that already appears for vertex W on
            the candidate list, then examine the next link.

        o   Equal to the value that appears for vertex W on the the
            candidate list, calculate the set of next hops that result
            from using the advertised link.  Input to this calculation
            is the destination (W), and its parent (V).  This
            calculation is shown in Section 16.1.1.  This set of hops
            should be added to the next hop values that appear for W on
            the candidate list.

        o   Less than the value that appears for vertex W on the the
            candidate list, or if W does not yet appear on the candidate
            list, then set the entry for W on the candidate list to
            indicate a distance of D from the root.  Also calculate the
            list of next hops that result from using the advertised
            link, setting the next hop values for W accordingly.  The
            next hop calculation is described in Section 16.1.1; it



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            takes as input the destination (W) and its parent (V).

(3) If at this step the candidate list is empty, the shortest-path tree
    (of transit vertices) has been completely built and this stage of
    the algorithm terminates.  Otherwise, choose the vertex belonging to
    the candidate list that is closest to the root, and add it to the
    shortest-path tree (removing it from the candidate list in the
    process).

(4) Possibly modify the routing table.  For those routing table entries
    modified, the associated area will be set to Area A, the path type
    will be set to intra-area, and the cost will be set to the newly
    discovered shortest path's calculated distance.

    If the newly added vertex is an area border router, a routing table
    entry is added whose destination type is "area border router".  The
    Options field found in the associated router links advertisement is
    copied into the routing table entry's Optional capabilities field.

    If the newly added vertex is an AS boundary router, the routing
    table entry of type "AS boundary router" for the destination is
    located.  Since routers can belong to more than one area, it is
    possible that several sets of intra-area paths exist to the AS
    boundary router, each set using a different area.  However, the AS
    boundary router's routing table entry must indicate a set of paths
    which utilize a single area.  The area leading to the routing table
    entry is selected as follows: A set of intra-area paths having no
    virtual next hops is always preferred over a set of intra-area paths
    in which some virtual next hops appear[22] ; all other things being
    equal the set of paths having lower cost is preferred.  Note that
    whenever an AS boundary router's routing table entry is
    added/modified, the Options found in the associated router links
    advertisement is copied into the routing table entry's Optional
    capabilities field.

    If the newly added vertex is a transit network, the routing table
    entry for the network is located.  The entry's destination ID is the
    IP network number, which can be obtained by masking the Vertex ID
    (Link State ID) with its associated subnet mask (found in the
    associated network links advertisement).  If the routing table entry
    already exists (i.e., there is already an intra-area route to the
    destination installed in the routing table), multiple vertices have
    mapped to the same IP network.  For example, this can occur when a
    new Designated Router is being established.  In this case, the
    current routing table entry should be overwritten if and only if the
    newly found path is just as short and the current routing table
    entry's Link State Origin has a smaller Link State ID than the newly
    added vertex' link state advertisement.



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    If there is no routing table entry for the network (the usual case),
    a routing table entry for the IP network should be added.  The
    routing table entry's Link State origin should be set to the newly
    added vertex' link state advertisement.

(5) Iterate the algorithm by returning to Step 2.


The stub networks are added to the tree in the procedure's second stage.
In this stage, all router vertices are again examined.  Those that have
been determined to be unreachable in the above first phase are
discarded.  For each reachable router vertex (call it V), the associated
router links advertisement is found in the link state database.  Each
stub network link appearing in the advertisement is then examined, and
the following steps are executed:


(1) If the cost of the stub network link is LSInfinity, the link should
    not be used for data traffic.  In this case, go on to examine the
    next stub network link in the advertisement.

(2) Otherwise, Calculate the distance D of stub network from the root.
    D is equal to the distance from the root to the router vertex
    (calculated in stage 1), plus the stub network link's advertised
    cost.  Compare this distance to the current best cost to the stub
    network.  This is done by looking up the network's current routing
    table entry.  If the calculated distance D is larger, go on to
    examine the next stub network link in the advertisement.

(3) If this step is reached, the stub network's routing table entry must
    be updated.  Calculate the set of next hops that would result from
    using the stub network link.  This calculation is shown in Section
    16.1.1; input to this calculation is the destination (the stub
    network) and the parent vertex (the router vertex).  If the distance
    D is the same as the current routing table cost, simply add this set
    of next hops to the routing table entry's list of next hops.  In
    this case, the routing table already has a Link State origin.  If
    this Link State origin is a router links advertisement whose Link
    State ID is smaller than V's Router ID, reset the Link State origin
    to V's router links advertisement.

    Otherwise D is smaller than the routing table cost.  Overwrite the
    current routing table entry by setting the routing table entry's
    cost to D, and by setting the entry's list of next hops to the newly
    calculated set.  Set the routing table entry's Link State origin to
    V's router links advertisement.  Then go on to examine the next stub
    network link.




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For all routing table entries added/modified in the second stage, the
associated area will be set to Area A and the path type will be set to
intra-area.  When the list of reachable router links is exhausted, the
second stage is completed.  At this time, all intra-area routes
associated with Area A have been determined.

The specification does not require that the above two stage method be
used to calculate the shortest path tree.  However, if another algorithm
is used, an identical tree must be produced.  For this reason, it is
important to note that links between transit vertices must be
bidirectional in ordered to be included in the above tree.  It should
also be mentioned that algorithms exist for incrementally updating the
shortest-path tree (see [BBN]).


16.1.1 The next hop calculation

This section explains how to calculate the current set of next hops to
use for a destination.  Each next hop consists of the outgoing interface
to use in forwarding packets to the destination together with the next
hop router (if any).  The next hop calculation is invoked each time a
shorter path to the destination is discovered.  This can happen in
either stage of the shortest-path tree calculation (see Section 16.1).
In stage 1 of the shortest-path tree calculation a shorter path is found
as the destination is added to the candidate list, or when the
destination's entry on the candidate list is modified (Step 2e of Stage
1).  In stage 2 a shorter path is discovered each time the destination's
routing table entry is modified (Step 3 of Stage 2).

The set of next hops to use for the destination may be recalculated
several times during the shortest-path tree calculation, as shorter and
shorter paths are discovered.  In the end, the destination's routing
table entry will always reflect the next hops resulting from the
absolute shortest path(s).

Input to the next hop calculation is a) the destination and b) its
parent in the current shortest path between the root (the calculating
router) and the destination.  The parent is always a transit vertex
(i.e., always a router or a transit network).

If there is at least one intervening router in the current shortest path
between the destination and the root, the destination simply inherits
the set of next hops from the parent.  Otherwise, there are two cases.
In the first case, the parent vertex is the root (the calculating router
itself).  This means that the destination is either a directly c