<pre>Network Working Group                                     J. Parker, Ed.
Request for Comments: 3719                             Axiowave Networks
Category: Informational                                    February 2004


           <span class="h1">Recommendations for Interoperable Networks using</span>
           <span class="h1">Intermediate System to Intermediate System (IS-IS)</span>

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   This document discusses a number of differences between the
   Intermediate System to Intermediate System (IS-IS) protocol as
   described in ISO 10589 and the protocol as it is deployed today.
   These differences are discussed as a service to those implementing,
   testing, and deploying the IS-IS Protocol.  A companion document
   discusses differences between the protocol described in <a href="./rfc1195">RFC 1195</a> and
   the protocol as it is deployed today for routing IP traffic.

Table of Contents

   <a href="#section-1">1</a>.  Introduction. . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-2">2</a>
   <a href="#section-2">2</a>.  Constants That Are Variable . . . . . . . . . . . . . . . . .  <a href="#page-2">2</a>
   <a href="#section-3">3</a>.  Variables That Are Constant . . . . . . . . . . . . . . . . .  <a href="#page-4">4</a>
   <a href="#section-4">4</a>.  Alternative Metrics . . . . . . . . . . . . . . . . . . . . .  <a href="#page-6">6</a>
   <a href="#section-5">5</a>.  ReceiveLSPBufferSize. . . . . . . . . . . . . . . . . . . . .  <a href="#page-6">6</a>
   <a href="#section-6">6</a>.  Padding Hello PDUs. . . . . . . . . . . . . . . . . . . . . .  <a href="#page-8">8</a>
   <a href="#section-7">7</a>.  Zero Checksum . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-9">9</a>
   <a href="#section-8">8</a>.  Purging Corrupted LSPs. . . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
   <a href="#section-9">9</a>.  Checking System ID in Received point-to-point IIH PDUs. . . . <a href="#page-10">10</a>
   <a href="#section-10">10</a>. Doppelganger LSPs . . . . . . . . . . . . . . . . . . . . . . <a href="#page-11">11</a>
   <a href="#section-11">11</a>. Generating a Complete Set of CSNPs. . . . . . . . . . . . . . <a href="#page-11">11</a>
   <a href="#section-12">12</a>. Overload Bit. . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-12">12</a>
   <a href="#section-13">13</a>. Security Considerations . . . . . . . . . . . . . . . . . . . <a href="#page-13">13</a>
   <a href="#section-14">14</a>. References. . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-13">13</a>
   <a href="#section-15">15</a>. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
   <a href="#section-16">16</a>. Author's  Address . . . . . . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
   <a href="#section-17">17</a>. Full Copyright Statement. . . . . . . . . . . . . . . . . . . <a href="#page-15">15</a>




<span class="grey">Parker                       Informational                      [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc3719">RFC 3719</a>           Interoperable Networks using IS-IS      February 2004</span>


<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

         In theory, there is no difference between theory and practice.
         But in practice, there is.
                                    Jan L.A. van de Snepscheut

   Interior Gateway Protocols such as IS-IS are designed to provide
   timely information about the best routes in a routing domain.  The
   original design of IS-IS, as described in ISO 10589 [<a href="#ref-1" title="&quot;Intermediate system to Intermediate system routeing information exchange protocol for use in conjunction with the Protocol for providing the Connectionless-mode Network Service (ISO 8473),&quot;">1</a>] has proved to
   be quite durable.  However, a number of original design choices have
   been modified.  This document addresses differences between the
   protocol described in ISO 10589 and the protocol that can be observed
   on the wire today.  A companion document discusses differences
   between the protocol described in <a href="./rfc1195">RFC 1195</a> [<a href="#ref-2" title="&quot;OSI IS-IS for IP and Dual Environment&quot;">2</a>] for routing IP traffic
   and current practice.

   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and "MAY" in
   this document are to be interpreted as described in <a href="./rfc2119">RFC 2119</a> [<a href="#ref-3" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">3</a>].

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Constants That Are Variable</span>

   Some parameters that were defined as constant in ISO 10589 are
   modified in practice.  These include the following

         (1)  MaxAge - the lifetime of a Link State PDU (LSP)

         (2)  ISISHoldingMultiplier - a parameter used to describe the
              generation of hello packets

         (3)  ReceiveLSPBufferSize - discussed in a later section

<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>.  MaxAge</span>

   Each LSP contains a RemainingLifetime field which is initially set to
   the MaxAge value on the generating IS.  The value stored in this
   field is decremented to mark the passage of time and the number of
   times it has been forwarded.  When the value of a foreign LSP becomes
   0, an IS initiates a purging process which will flush the LSP from
   the network.  This ensures that corrupted or otherwise invalid LSPs
   do not remain in the network indefinitely.  The rate at which LSPs
   are regenerated by the originating IS is determined by the value of
   maximumLSPGenerationInterval.









<span class="grey">Parker                       Informational                      [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc3719">RFC 3719</a>           Interoperable Networks using IS-IS      February 2004</span>


   MaxAge is defined in ISO 10589 as an Architectural constant of 20
   minutes, and it is recommended that maximumLSPGenerationInterval be
   set to 15 minutes.  These times have proven to be too short in some
   networks, as they result in a steady flow of LSP updates even when
   nothing is changing.  To reduce the rate of generation, some
   implementations allow these times to be set by the network operator.

   The relation between MaxAge and maximumLSPGenerationInterval is
   discussed in <a href="#section-7.3.21">section 7.3.21</a> of ISO 10589.  If MaxAge is smaller than
   maximumLSPGenerationInterval, then an LSP will expire before it is
   replaced.  Further, as RemainingLifetime is decremented each time it
   is forwarded, an LSP far from its origin appears older and is removed
   sooner.  To make sure that an LSP survives long enough to be
   replaced, MaxAge should exceed maximumLSPGenerationInterval by at
   least ZeroAgeLifetime + minimumLSPTransmissionInterval.  The first
   term, ZeroAgeLifetime, is an estimate of how long it takes to flood
   an LSP through the network.  The second term,
   minimumLSPTransmissionInterval, takes into account how long a router
   might delay before sending an LSP.  The original recommendation was
   that MaxAge be at least 5 minutes larger than
   maximumLSPGenerationInterval, and that recommendation is still valid
   today.

   An implementation MAY use a value of MaxAge that is greater than 1200
   seconds.  MaxAge SHOULD exceed maximumLSPGenerationInterval by at
   least 300 seconds.  An implementation SHOULD NOT use its value of
   MaxAge to discard LSPs from peers, as discussed below.

   An implementation is not required to coordinate the RemainingLifetime
   it assigns to LSPs to the RemainingLifetime values it accepts, and
   MUST ignore the following sentence from <a href="#section-7.3.16.3">section 7.3.16.3</a>. of ISO
   10589.

         "If the value of Remaining Lifetime [of the received LSP] is
         greater than MaxAge, the LSP shall be processed as if there
         were a checksum error."

<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a>.  ISISHoldingMultiplier</span>

   An IS sends IS to IS Hello Protocol Data Units (IIHs) on a periodic
   basis over active circuits, allowing other attached routers to
   monitor their aliveness.  The IIH includes a two byte field called
   the Holding Time which defines the time to live of an adjacency.  If
   an IS does not receive a hello from an adjacent IS within this
   holding time, the adjacent IS is assumed to be no longer operational,
   and the adjacency is removed.





<span class="grey">Parker                       Informational                      [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc3719">RFC 3719</a>           Interoperable Networks using IS-IS      February 2004</span>


   ISO 10589 defines ISISHoldingMultiplier to be 10, and states that the
   value of Holding Time should be ISISHoldingMultiplier multiplied by
   iSISHelloTimer for ordinary systems, and dRISISHelloTimer for a DIS.
   This implies that the neighbor must lose 10 IIHs before an adjacency
   times out.

   In practice, a value of 10 for the ISISHoldingMultiplier has proven
   to be too large.  DECnet PhaseV defined two related values.  The
   variable holdingMultiplier, with a default value of 3, was used for
   point-to-point IIHs, while the variable ISISHoldingMultiplier, with a
   default value of 10, was used for LAN IIHs.  Most implementations
   today set the default ISISHoldingMultiplier to 3 for both circuit
   types.

   Note that adjacent systems may use different values for Holding Time
   and will form an adjacency with non-symmetric hold times.

   An implementation MAY allow ISISHoldingMultiplier to be configurable.
   Values lower than 3 are unstable, and may cause adjacencies to flap.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Variables That Are Constant</span>

   Some values that were defined as variables in ISO 10589 do not vary
   in practice.  These include

         (1)  ID Length - the length of the SystemID

         (2)  maximumAreaAddresses

         (3)  Protocol Version

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  ID Length</span>

   The ID Length is a field carried in all PDUs.  The ID Length defines
   the length of the System ID, and is allowed to take values from 0 to
   8.  A value of 0 is interpreted to define a length of 6 bytes.  As
   suggested in B.1.1.3 of [<a href="#ref-1" title="&quot;Intermediate system to Intermediate system routeing information exchange protocol for use in conjunction with the Protocol for providing the Connectionless-mode Network Service (ISO 8473),&quot;">1</a>], it is easy to use an Ethernet MAC
   address to generate a unique 6 byte System ID.  Since the SystemID
   only has significance within the IGP Domain, 6 bytes has proved to be
   easy to use and ample in practice.  There are also new IS-IS Traffic
   Engineering TLVs which assume a 6 byte System ID.  Choices for the ID
   length other than 6 are difficult to support today.  Implementations
   may interoperate without being able to deal with System IDs of any
   length other than 6.

   An implementation MUST use an ID Length of 6, and MUST check the ID
   Length defined in the IS-IS PDUs it receives.  If a router encounters
   a PDU with an ID Length different from 0 or 6, <a href="#section-7.3.15">section 7.3.15</a>.a.2



<span class="grey">Parker                       Informational                      [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc3719">RFC 3719</a>           Interoperable Networks using IS-IS      February 2004</span>


   dictates that it MUST discard the PDU, and SHOULD generate an
   appropriate notification.  ISO 10589 defines the notification
   iDFieldLengthMismatch, while the IS-IS MIB [<a href="#ref-7" title="&quot;Management Information Base for IS-IS&quot;">7</a>] defines the
   notification isisIDLenMismatch.

<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>.  maximumAreaAddresses</span>

   The value of maximumAreaAddresses is defined to be an integer between
   1 and 254, and defines the number of synonymous Area Addresses that
   can be in use in an L1 area.  This value is advertised in the header
   of each IS-IS PDU.

   Most deployed networks use one Area Address for an L1 area.  When
   merging or splitting areas, a second address is required for seamless
   transition.  The third area address was originally required to
   support DECnet PhaseIV addresses as well as OSI addresses during a
   transition.

   ISO 10589 requires that all Intermediate Systems in an area or domain
   use a consistent value for maximumAreaAddresses.  Common practice is
   for an implementation to use the value 3.  Therefore an
   implementation that only supports 3 can expect to interoperate
   successfully with other conformant systems.

   ISO 10589 specifies that an advertised value of 0 is treated as
   equivalent to 3, and that checking the value for consistency may be
   omitted if an implementation only supports the value 3.

   An implementation SHOULD use the value 3, and it SHOULD check the
   value advertised in IS-IS PDUs it receives.  If a router receives a
   PDU with maximumAreaAddresses that is not 0 or 3, it MUST discard the
   PDU, as described in <a href="#section-7.3.15">section 7.3.15</a>.a.3, and it SHOULD generate an
   appropriate notification.  ISO 10589 defines the notification
   maximumAreaAddressMismatch, while the IS-IS MIB [<a href="#ref-7" title="&quot;Management Information Base for IS-IS&quot;">7</a>] defines the
   notification isisMaxAreaAddressesMismatch.

<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>.  Protocol Version</span>

   IS-IS PDUs include two one-byte fields in the headers:
   "Version/Protocol ID Extension" and "Version".

   An implementation SHOULD set both fields to 1, and it SHOULD check
   the values of these fields in IS-IS PDUs it receives.  If a router
   receives a PDU with a value other than 1 for either field, it MUST
   drop the packet, and SHOULD generate the isisVersionSkew
   notification.





<span class="grey">Parker                       Informational                      [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc3719">RFC 3719</a>           Interoperable Networks using IS-IS      February 2004</span>


<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Alternative Metrics</span>

   <a href="#section-7.2.2">Section 7.2.2</a>, ISO 10589 describes four metrics: Default Metric,
   Delay Metric, Expense Metric, and Error Metric.  None but the Default
   Metric are used in deployed networks, and most implementations only
   consider the Default Metric.  In ISO 10589, the most significant bit
   of the 8 bit metrics was the field S (Supported), used to define if
   the metric was meaningful.

         If this IS does not support this metric it shall set bit S to 1
         to indicate that the metric is unsupported.

   The Supported bit was always 0 for the Default Metric, which must
   always be supported.  However, <a href="./rfc2966">RFC 2966</a> [<a href="#ref-5" title="&quot;Domain-wide Prefix Distribution with Two-Level IS-IS&quot;">5</a>] uses this bit in the
   Default Metric to mark L1 routes that have been leaked from L1 to L2
   and back down into L1 again.

   Implementations MUST generate the Default Metric when using narrow
   metrics, and SHOULD ignore the other three metrics when using narrow
   metrics.  Implementations MUST assume that the Default Metric is
   supported, even if the S bit is set.  <a href="./rfc2966">RFC 2966</a> describes restrictions
   on leaking such routes learned from L1 into L2.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  ReceiveLSPBufferSize</span>

   Since IS-IS does not allow segmentation of protocol PDUs, Link State
   PDUs (LSPs) must be propagated without modification on all IS-IS
   enabled links throughout the area/domain.  Thus it is essential to
   configure a maximum size that all routers can forward, receive, and
   store.

   This affects three aspects, which we discuss in turn:

         (1)  The largest LSP we can receive (ReceiveLSPBufferSize)

         (2)  The size of the largest LSP we can generate
              (originatingL1LSPBufferSize and
              originatingL2LSPBufferSize)

         (3)  Available Link MTU for supported Circuits (MTU).  Note
              this often differs from the MTU available to IP clients.

   ISO 10589 defines the architectural constant ReceiveLSPBufferSize
   with value 1492 bytes, and two private management parameters,
   originatingL1LSPBufferSize for level 1 PDUs and
   originatingL2LSPBufferSize for level 2 PDUs.  The originating buffer





<span class="grey">Parker                       Informational                      [Page 6]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-7" ></span>
<span class="grey"><a href="./rfc3719">RFC 3719</a>           Interoperable Networks using IS-IS      February 2004</span>


   size parameters define the maximum size of an LSP that a router can
   generate.  ISO 10589 directs the implementor to treat a PDU larger
   than ReceiveLSPBufferSize as an error.

   It is crucial that
            originatingL1LSPBufferSize &lt;= ReceiveLSPBufferSize
            originatingL2LSPBufferSize &lt;= ReceiveLSPBufferSize
   and that for all L1 links in the area
            originatingL1LSPBufferSize &lt;= MTU
   and for all L2 links in the domain
            originatingL2LSPBufferSize &lt;= MTU

   The original thought was that operators could decrease the
   originating Buffer size when dealing with smaller MTUs, but would not
   need to increase ReceiveLSPBufferSize beyond 1492.

   With the definition of new information to be advertised in LSPs, such
   as the Traffic Engineering TLVs, the limited space of the LSP
   database which may be generated by each router (256 * 1492 bytes at
   each level) has become an issue.  Given that modern networks with
   MTUs larger than 1492 on all links are not uncommon, one method which
   can be used to expand the LSP database size is to allow values of
   ReceiveLSPBufferSize greater than 1492.

   Allowing ReceiveLSPBUfferSize to become a configurable parameter
   rather than an architectural constant must be done with care: if any
   system in the network does not support values larger than 1492 or one
   or more link MTUs used by IS-IS anywhere in the area/domain is
   smaller than the largest LSP which may be generated by any router,
   then full propagation of all LSPs may not be possible, resulting in
   routing loops and black holes.

   The steps below are recommended when changing ReceiveLSPBufferSize.

      (1)  Set the ReceiveLSPBufferSize to a consistent value throughout
           the network.

      (2)  The implementation MUST not enable IS-IS on circuits which do
           not support an MTU at least as large as the originating
           BufferSize at the appropriate level.

      (3)  Include an originatingLSPBufferSize TLV when generating LSPs,
           introduced in <a href="#section-9.8">section 9.8</a> of ISO 10589:2002 [<a href="#ref-1" title="&quot;Intermediate system to Intermediate system routeing information exchange protocol for use in conjunction with the Protocol for providing the Connectionless-mode Network Service (ISO 8473),&quot;">1</a>].

      (4)  When receiving LSPs, check for an originatingLSPBufferSize
           TLV, and report the receipt of values larger than the local
           value of ReceiveLSPBufferSize through the defined
           Notifications and Alarms.



<span class="grey">Parker                       Informational                      [Page 7]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-8" ></span>
<span class="grey"><a href="./rfc3719">RFC 3719</a>           Interoperable Networks using IS-IS      February 2004</span>


      (5)  Report the receipt of a PDU larger than the local
           ReceiveLSPBufferSize through the defined Notifications and
           Alarms.

      (6)  Do not discard large PDUs by default.  Storing and processing
           them as normal PDUs may help maintain coherence in a
           misconfigured network.

   Steps 1 and 2 are enough by themselves, but the consequences of
   mismatch are serious enough and difficult enough to detect, that
   steps 3-6 are recommended to help track down and correct problems.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Padding Hello PDUs</span>

   To prevent the establishment of adjacencies between systems which may
   not be able to successfully receive and propagate IS-IS PDUs due to
   inconsistent settings for originatingLSPBufferSize and
   ReceiveLSPBufferSize, section 8.2.3 of [<a href="#ref-1" title="&quot;Intermediate system to Intermediate system routeing information exchange protocol for use in conjunction with the Protocol for providing the Connectionless-mode Network Service (ISO 8473),&quot;">1</a>] requires padding on
   point-to-point links.

   On point-to-point links, the initial IIH is to be padded to the
   maximum of

      (1)  Link MTU

      (2)  originatingL1LSPBufferSize if the link is to be used for L1
           traffic

      (3)  originatingL2LSPBufferSize if the link is to be used for L2
           traffic

   In <a href="#section-6.7.2">section 6.7.2</a> e) ISO 10589 assumes

         Provision that failure to deliver a specific subnetwork SDU
         will result in the timely disconnection of the subnetwork
         connection in both directions and that this failure will be
         reported to both systems

   With this service provided by the link layer, the requirement that
   only the initial IIH be padded was sufficient to check the
   consistency of the MTU on the two sides.  If the PDU was too big to
   be received, the link would be reset.  However, link layer protocols
   in use on point-to-point circuits today often lack this service, and
   the initial padded PDU might be silently dropped without resetting
   the circuit.  Therefore, the requirement that only the initial IIH be
   padded does not provide the guarantees anticipated in ISO 10589.





<span class="grey">Parker                       Informational                      [Page 8]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-9" ></span>
<span class="grey"><a href="./rfc3719">RFC 3719</a>           Interoperable Networks using IS-IS      February 2004</span>


   If an implementation is using padding to detect problems, point-to-
   point IIH PDUs SHOULD be padded until the sender declares an
   adjacency on the link to be in state Up.  If the implementation
   implements <a href="./rfc3373">RFC 3373</a> [<a href="#ref-4" title="&quot; Three-Way Handshake for Intermediate System to Intermediate System (IS-IS) Point-to-Point Adjacencies&quot;">4</a>], "Three-Way Handshake for IS-IS Point-to-
   Point Adjacencies" then this is when the three-way state is Up: if
   the implementation use the "classic" algorithm described in ISO
   10589, this is when adjacencyState is Up.  Transmission of padded IIH
   PDUs SHOULD be resumed whenever the adjacency is torn down, and
   SHOULD continue until the sender declares the adjacency to be in
   state Up again.

   If an implementation is using padding, and originatingL1LSPBUfferSize
   or originatingL2LSPBUfferSize is modified, adjacencies SHOULD be
   brought down and reestablished so the protection provided by padding
   IIH PDUs is performed consistent with the modified values.

   Some implementations choose not to pad.  Padding does not solve all
   problems of misconfigured systems.  In particular, it does not
   provide a transitive relation.  Assume that A, B, and C all pad IIH
   PDUs, that A and B can establish an adjacency, and that B and C can
   establish an adjacency.  We still cannot conclude that A and C could
   establish an adjacency, if they were neighbors.

   The presence or absence of padding TLVs MUST NOT be one of the
   acceptance tests applied to a received IIH regardless of the state of
   the adjacency.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Zero Checksum</span>

   A checksum of 0 is impossible if the checksum is computed according
   to the rules of ISO 8473 [<a href="#ref-8" title="&quot;Information technology - Protocol for providing the connectionless-mode network service&quot;">8</a>].

   ISO 10589, <a href="#section-7.3.14.2">section 7.3.14.2</a>(i), states:

         A Link State PDU received with a zero checksum shall be treated
         as if the Remaining Lifetime were zero.  The age, if not zero,
         shall be overwritten with zero.

   That is, ISO 10589 directs the receiver to purge the LSP.  This has
   proved to be disruptive in practice.  An implementation SHOULD treat
   all LSPs with a zero checksum and a non-zero remaining lifetime as if
   they had as checksum error.  Such packets SHOULD be discarded.









<span class="grey">Parker                       Informational                      [Page 9]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-10" ></span>
<span class="grey"><a href="./rfc3719">RFC 3719</a>           Interoperable Networks using IS-IS      February 2004</span>


<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Purging Corrupted PDUs</span>

   While ISO 10589 requires in <a href="#section-7.3.14.2">section 7.3.14.2</a> e) that any LSP received
   with an invalid PDU checksum should be purged, this has been found to
   be disruptive.  Most implementations today follow the revised
   specification, and simply drop the LSP.

   In ISO 10589:2002 [<a href="#ref-1" title="&quot;Intermediate system to Intermediate system routeing information exchange protocol for use in conjunction with the Protocol for providing the Connectionless-mode Network Service (ISO 8473),&quot;">1</a>], Section 7.3.14.2, it states:

      (e)  An Intermediate system receiving a Link State PDU with an
           incorrect LSP Checksum or with an invalid PDU syntax SHOULD

           1) generate a corruptedLSPReceived circuit event,

           2) discard the PDU.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Checking System ID in Received point-to-point IIH PDUs</span>

   In <a href="#section-8.2.4.2">section 8.2.4.2</a>, ISO 10589 does not explicitly require comparison
   of the source ID of a received IIH with the neighbourSystemID
   associated with an existing adjacency on a point-to-point link.

   To address this omission, implementations receiving an IIH PDU on a
   point to point circuit with an established adjacency SHOULD check the
   Source ID field and compare that with the neighbourSystemID of the
   adjacency.  If these differ, an implementation SHOULD delete the
   adjacency.

   Given that IIH PDUs as specified in ISO 10589 do not include a
   check-sum, it is possible that a corrupted IIH may falsely indicate a
   change in the neighbor's System ID.  The required subnetwork
   guarantees for point-to-point links, as described in 6.7.2 g) 1)
   assume that undetected corrupted PDUs are very rare (one event per
   four years).  A link with frequent errors that produce corrupted data
   could lead to flapping an adjacency.  Inclusion of an optional
   checksum TLV as specified in "Optional Checksums in IS-IS" [<a href="#ref-6" title="&quot;Optional Checksums in Intermediate System to Intermediate System (ISIS)&quot;">6</a>], may
   be used to detect such corruption.  Hello packets carrying this TLV
   that are corrupted PDUs SHOULD be silently dropped, rather than
   dropping the adjacency.

   Some implementations have chosen to discard received IIHs where the
   source ID differs from the neighbourSystemID.  This may prevent
   needless flapping caused by undetected PDU corruption.  If an actual
   administrative change to the neighbor's system ID has occurred, using
   this strategy may require the existing adjacency to timeout before an
   adjacency with the new neighbor can be established.  This is





<span class="grey">Parker                       Informational                     [Page 10]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-11" ></span>
<span class="grey"><a href="./rfc3719">RFC 3719</a>           Interoperable Networks using IS-IS      February 2004</span>


   expedited if the neighbor resets the circuit as anticipated in 10589
   after a System ID change, or resets the 3-way adjacency state, as
   anticipated in <a href="./rfc3373">RFC 3373</a>.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  Doppelganger LSPs</span>

   When an Intermediate System shuts down, it may leave old LSPs in the
   network.  In the normal course of events, a rebooting system flushes
   out these old LSPs by reissuing those fragments with a higher
   sequence number, or by purging fragments that it is not currently
   generating.

   In the case where a received LSP or SNP entry and an LSP in the local
   database have the same LSP ID, same sequence number, non-zero
   remaining lifetimes, but different non-zero checksums, the rules
   defined in [<a href="#ref-1" title="&quot;Intermediate system to Intermediate system routeing information exchange protocol for use in conjunction with the Protocol for providing the Connectionless-mode Network Service (ISO 8473),&quot;">1</a>] cannot determine which of the two is "newer".  In this
   case, an implementation may opt to perform an additional test as a
   tie breaker by comparing the checksums.  Implementations that elect
   to use this method MUST consider the LSP/SNP entry with the higher
   checksum as newer.  When comparing the checksums the checksum field
   is treated as a 16 bit unsigned integer in network byte order (i.e.,
   most significant byte first).

   The choice of higher checksum, rather than lower, while arbitrary,
   aligns with existing implementations and ensures compatibility.

   Note that a purged LSP (i.e., an LSP with remaining lifetime set to
   0) is always considered newer than a non-purged copy of the same LSP.

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  Generating a Complete Set of CSNPs</span>

   There are a number of cases in which a complete set of CSNPs must be
   generated.  The DIS on a LAN, two IS's peering over a P2P link, and
   an IS helping another IS perform graceful restart must generate a
   complete set of CSNPs to assure consistent LSP Databases throughout.
   Section 7.3.15.3 of [<a href="#ref-1" title="&quot;Intermediate system to Intermediate system routeing information exchange protocol for use in conjunction with the Protocol for providing the Connectionless-mode Network Service (ISO 8473),&quot;">1</a>] defines a complete set of CSNPs to be:

         "A complete set of CSNPs is a set whose Start LSPID and End
         LSPID ranges cover the complete possible range of LSPIDs.
         (i.e., there is no possible LSPID value which does not appear
         within the range of one of the CSNPs in the set). "

   Strict adherence to this definition is required to ensure the
   reliability of the update process.  Deviation can lead to subtle and
   hard to detect defects.  It is not sufficient to send a set of CSNPs
   which merely cover the range of LSPIDs which are in the local
   database.  The set of CSNPs must cover the complete possible range of
   LSPIDs.



<span class="grey">Parker                       Informational                     [Page 11]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
<span class="grey"><a href="./rfc3719">RFC 3719</a>           Interoperable Networks using IS-IS      February 2004</span>


   Consider the following example:

   If the current Level 1 LSP database on a router consists of the
   following non pseudo-node LSPs:

      From system 1111.1111.1111 LSPs numbered 0-89(59H)
      From system 2222.2222.2222 LSPs numbered 0-89(59H)

   If the maximum size of a CSNP is 1492 bytes, then 90 CSNP entries can
   fit into a single CSNP PDU.  The following set of CSNP start/end
   LSPIDs constitute a correctly formatted complete set:

      Start LSPID              End LSPID
      0000.0000.0000.00-00     1111.1111.1111.00-59
      1111.1111.1111.00-5A     FFFF.FFFF.FFFF.FF-FF

   The following are examples of incomplete sets of CSNPS:

      Start LSPID              End LSPID
      0000.0000.0000.00-00     1111.1111.1111.00-59
      1111.1111.1111.00-5A     2222.2222.2222.00-59

   The sequence above has a gap after the second entry.

      Start LSPID              End LSPID
      0000.0000.0000.00-00     1111.1111.1111.00-59
      2222.2222.2222.00-00     FFFF.FFFF.FFFF.FF-FF

   The sequence above has a gap between the first and second entry.

   Although it is legal to send a CSNP which contains no actual LSP
   entry TLVs, it should never be necessary to do so in order to conform
   to the specification.

<span class="h2"><a class="selflink" id="section-12" href="#section-12">12</a>.  Overload Bit</span>

   To deal with transient problems that prevent an IS from storing all
   the LSPs it receives, ISO 10589 defines an LSP Database Overload
   condition in <a href="#section-7.3.19">section 7.3.19</a>.  When an IS is in Database Overload
   condition, it sets a flag called the Overload Bit in the non-
   pseudonode LSP number Zero that it generates.  <a href="#section-7.2.8.1">Section 7.2.8.1</a> of ISO
   10589 instructs other systems not to use the overloaded IS as a
   transit router.  Since the overloaded IS does not have complete
   information, it may not be able to compute the right routes, and
   routing loops could develop.






<span class="grey">Parker                       Informational                     [Page 12]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-13" ></span>
<span class="grey"><a href="./rfc3719">RFC 3719</a>           Interoperable Networks using IS-IS      February 2004</span>


   An overloaded router might become the DIS.  An implementation SHOULD
   not set the Overload bit in PseudoNode LSPs that it generates, and
   Overload bits seen in PseudoNode LSPs SHOULD be ignored.

<span class="h2"><a class="selflink" id="section-13" href="#section-13">13</a>.  Security Considerations</span>

   The clarifications in this document do not raise any new security
   concerns, as there is no change in the underlying protocol described
   in ISO 10589 [<a href="#ref-1" title="&quot;Intermediate system to Intermediate system routeing information exchange protocol for use in conjunction with the Protocol for providing the Connectionless-mode Network Service (ISO 8473),&quot;">1</a>].

<span class="h2"><a class="selflink" id="section-14" href="#section-14">14</a>.  References</span>

<span class="h3"><a class="selflink" id="section-14.1" href="#section-14.1">14.1</a>.  Normative References</span>

   [<a id="ref-1">1</a>]  ISO, "Intermediate system to Intermediate system routeing
        information exchange protocol for use in conjunction with the
        Protocol for providing the Connectionless-mode Network Service
        (ISO 8473)," ISO/IEC 10589:2002.

   [<a id="ref-2">2</a>]  Callon, R., "OSI IS-IS for IP and Dual Environment", <a href="./rfc1195">RFC 1195</a>,
        December 1990.

   [<a id="ref-3">3</a>]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.

   [<a id="ref-4">4</a>]  Katz, D. and Saluja, R., " Three-Way Handshake for Intermediate
        System to Intermediate System (IS-IS) Point-to-Point
        Adjacencies", <a href="./rfc3373">RFC 3373</a>, September 2002.

   [<a id="ref-5">5</a>]  Li, T., Przygienda, T. and H. Smit, "Domain-wide Prefix
        Distribution with Two-Level IS-IS", <a href="./rfc2966">RFC 2966</a>, October 2000.

   [<a id="ref-6">6</a>]  Koodli, R. and R. Ravikanth, "Optional Checksums in Intermediate
        System to Intermediate System (ISIS)", <a href="./rfc3358">RFC 3358</a>, August 2002.

<span class="h3"><a class="selflink" id="section-14.2" href="#section-14.2">14.2</a>.  Informative References</span>

   [<a id="ref-7">7</a>]  Parker, J., <a style="text-decoration: none" href='https://www.google.com/search?sitesearch=datatracker.ietf.org%2Fdoc%2Fhtml%2F&amp;q=inurl:draft-+%22Management+Information+Base+for+IS-IS%22'>"Management Information Base for IS-IS"</a>, Work in
        Progress, January 2004.

   [<a id="ref-8">8</a>]  ITU, "Information technology - Protocol for providing the
        connectionless-mode network service", ISO/IEC 8473-1, 1998.









<span class="grey">Parker                       Informational                     [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc3719">RFC 3719</a>           Interoperable Networks using IS-IS      February 2004</span>


<span class="h2"><a class="selflink" id="section-15" href="#section-15">15</a>.  Acknowledgments</span>

   This document is the work of many people, and is the distillation of
   over a thousand mail messages.  Thanks to Vishwas Manral, who pushed
   to create such a document.  Thanks to Danny McPherson, the original
   editor, for kicking things off.  Thanks to Mike Shand, for his work
   in creating the protocol, and his uncanny ability to remember what
   everything is for.  Thanks to Micah Bartell and Philip Christian, who
   showed us how to document difference without displaying discord.
   Thanks to Les Ginsberg, Neal Castagnoli, Jeff Learman, and Dave Katz,
   who spent many hours educating the editor.  Thanks to Radia Perlman,
   who is always ready to explain anything.  Thanks to Satish Dattatri,
   who was tenacious in seeing things written up correctly.  Thanks to
   Russ White, whose writing improved the treatment of every topic he
   touched.  Thanks to Shankar Vemulapalli, who read several drafts with
   close attention.  Thanks to Don Goodspeed, for his close reading of
   the text.  Thanks to Aravind Ravikumar, who pointed out that we
   should check Source ID on point-to-point IIH packets.  Thanks to
   Michael Coyle for identifying the quotation from Jan L.A. van de
   Snepscheut.  Thanks for Alex Zinin's ministrations behind the scenes.
   Thanks to Tony Li and Tony Przygienda, who kept us on track as the
   discussions veered into the weeds.  And thanks to all those who have
   contributed, but whose names I have carelessly left from this list.

<span class="h2"><a class="selflink" id="section-16" href="#section-16">16</a>.  Author's Address</span>

   Jeff Parker
   Axiowave Networks
   200 Nickerson Road
   Marlborough, Mass 01752
   USA

   EMail: jparker@axiowave.com


















<span class="grey">Parker                       Informational                     [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc3719">RFC 3719</a>           Interoperable Networks using IS-IS      February 2004</span>


<span class="h2"><a class="selflink" id="section-17" href="#section-17">17</a>.  Full Copyright Statement</span>

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
   REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
   INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
   IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed
   to pertain to the implementation or use of the technology
   described in this document or the extent to which any license
   under such rights might or might not be available; nor does it
   represent that it has made any independent effort to identify any
   such rights.  Information on the procedures with respect to
   rights in RFC documents can be found in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and <a href="https://www.rfc-editor.org/bcp/bcp79">BCP 79</a>.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use
   of such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository
   at <a href="http://www.ietf.org/ipr">http://www.ietf.org/ipr</a>.

   The IETF invites any interested party to bring to its attention
   any copyrights, patents or patent applications, or other
   proprietary rights that may cover technology that may be required
   to implement this standard.  Please address the information to the
   IETF at ietf-ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.









Parker                       Informational                     [Page 15]
</pre>