<pre>Internet Engineering Task Force (IETF)                   D. Eastlake 3rd
Request for Comments: 7177                                        Huawei
Obsoletes: <a href="./rfc6327">6327</a>                                               R. Perlman
Updates: <a href="./rfc6325">6325</a>                                                 Intel Labs
Category: Standards Track                                    A. Ghanwani
ISSN: 2070-1721                                                     Dell
                                                                 H. Yang
                                                                   Cisco
                                                               V. Manral
                                                             Ionos Corp.
                                                                May 2014


    <span class="h1">Transparent Interconnection of Lots of Links (TRILL): Adjacency</span>

Abstract

   The IETF Transparent Interconnection of Lots of Links (TRILL)
   protocol supports arbitrary link technologies between TRILL switches,
   including point-to-point links and multi-access Local Area Network
   (LAN) links that can have multiple TRILL switches and end stations
   attached.  TRILL uses Intermediate System to Intermediate System
   (IS-IS) routing.  This document specifies the establishment,
   reporting, and termination of IS-IS adjacencies between TRILL
   switches, also known as RBridges (Routing Bridges).  It also concerns
   four other link-local aspects of TRILL: Designated RBridge (DRB)
   selection, MTU (Maximum Transmission Unit) testing, pseudonode
   creation, and BFD (Bidirectional Forwarding Detection) session
   bootstrapping in connection with adjacency.  State diagrams are
   included where appropriate.  This document obsoletes <a href="./rfc6327">RFC 6327</a> and
   updates <a href="./rfc6325">RFC 6325</a>.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in <a href="./rfc5741#section-2">Section&nbsp;2 of RFC 5741</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="http://www.rfc-editor.org/info/rfc7177">http://www.rfc-editor.org/info/rfc7177</a>.






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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.





































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<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


Table of Contents

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-4">4</a>
      <a href="#section-1.1">1.1</a>. Content and Precedence .....................................<a href="#page-5">5</a>
      <a href="#section-1.2">1.2</a>. Terminology and Acronyms ...................................<a href="#page-5">5</a>
   <a href="#section-2">2</a>. The TRILL Hello Environment and Purposes ........................<a href="#page-6">6</a>
      <a href="#section-2.1">2.1</a>. RBridge Interconnection by Ethernet ........................<a href="#page-6">6</a>
      <a href="#section-2.2">2.2</a>. Handling Native Frames .....................................<a href="#page-7">7</a>
      <a href="#section-2.3">2.3</a>. Zero or Minimal Configuration ..............................<a href="#page-8">8</a>
      <a href="#section-2.4">2.4</a>. MTU Robustness .............................................<a href="#page-8">8</a>
      <a href="#section-2.5">2.5</a>. Purposes of the TRILL Hello Protocol .......................<a href="#page-9">9</a>
   <a href="#section-3">3</a>. Adjacency State Machinery ......................................<a href="#page-10">10</a>
      <a href="#section-3.1">3.1</a>. TRILL Hellos, Ports, and VLANs ............................<a href="#page-10">10</a>
      <a href="#section-3.2">3.2</a>. Adjacency Table Entries and States ........................<a href="#page-11">11</a>
      <a href="#section-3.3">3.3</a>. Adjacency and Hello Events ................................<a href="#page-12">12</a>
      <a href="#section-3.4">3.4</a>. Adjacency State Diagram and Table .........................<a href="#page-15">15</a>
      <a href="#section-3.5">3.5</a>. Multiple Parallel Links ...................................<a href="#page-17">17</a>
      <a href="#section-3.6">3.6</a>. Insufficient Space in Adjacency Table .....................<a href="#page-17">17</a>
   <a href="#section-4">4</a>. LAN Ports and DRB State ........................................<a href="#page-17">17</a>
      <a href="#section-4.1">4.1</a>. Port Table Entries and DRB Election State .................<a href="#page-18">18</a>
      <a href="#section-4.2">4.2</a>. DRB Election Events .......................................<a href="#page-19">19</a>
           <a href="#section-4.2.1">4.2.1</a>. DRB Election Details ...............................<a href="#page-20">20</a>
           <a href="#section-4.2.2">4.2.2</a>. Change in DRB ......................................<a href="#page-20">20</a>
           <a href="#section-4.2.3">4.2.3</a>. Change in Designated VLAN ..........................<a href="#page-21">21</a>
      <a href="#section-4.3">4.3</a>. Port State Table and Diagram ..............................<a href="#page-21">21</a>
   <a href="#section-5">5</a>. MTU Matching ...................................................<a href="#page-22">22</a>
   <a href="#section-6">6</a>. BFD-Enabled TLV and BFD Session Bootstrapping ..................<a href="#page-24">24</a>
   <a href="#section-7">7</a>. Pseudonodes ....................................................<a href="#page-25">25</a>
   <a href="#section-8">8</a>. More TRILL Hello Details .......................................<a href="#page-26">26</a>
      <a href="#section-8.1">8.1</a>. Contents of TRILL Hellos ..................................<a href="#page-27">27</a>
      <a href="#section-8.2">8.2</a>. Transmitting TRILL Hellos .................................<a href="#page-27">27</a>
           <a href="#section-8.2.1">8.2.1</a>. TRILL Neighbor TLVs ................................<a href="#page-28">28</a>
      <a href="#section-8.3">8.3</a>. Receiving TRILL Hellos ....................................<a href="#page-29">29</a>
   <a href="#section-9">9</a>. Multiple Ports on the Same Broadcast Link ......................<a href="#page-29">29</a>
   <a href="#section-10">10</a>. IANA Considerations ...........................................<a href="#page-30">30</a>
   <a href="#section-11">11</a>. Security Considerations .......................................<a href="#page-30">30</a>
   <a href="#appendix-A">Appendix A</a>. Changes from <a href="./rfc6327">RFC 6327</a> .................................<a href="#page-31">31</a>
   <a href="#appendix-B">Appendix B</a>. Changes to <a href="./rfc6325">RFC 6325</a> ...................................<a href="#page-31">31</a>
   Normative References...............................................<a href="#page-32">32</a>
   Informative References.............................................<a href="#page-33">33</a>
   Acknowledgements...................................................<a href="#page-34">34</a>










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

   The IETF Transparent Interconnection of Lots of Links (TRILL)
   protocol [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] provides optimal pair-wise data frame forwarding
   without configuration, safe forwarding even during transient loops,
   and support for transmission of both unicast and multicast traffic
   taking advantage of multiple paths in multi-hop networks with
   arbitrary topology.  End stations are connected to TRILL switches
   with Ethernet, but TRILL switches can be interconnected with
   arbitrary link technology.  TRILL accomplishes this by using [<a href="#ref-IS-IS" title="&quot;Information technology -- Telecommunications and information exchange between systems -- Intermediate System to Intermediate System intra-domain routeing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode network service (ISO 8473)&quot;">IS-IS</a>]
   (Intermediate System to Intermediate System) link-state routing
   [<a href="./rfc1195" title="&quot;Use of OSI IS-IS for routing in TCP/IP and dual environments&quot;">RFC1195</a>] [<a href="./rfc7176" title="&quot;Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS&quot;">RFC7176</a>] and a header in TRILL Data packets that includes
   a hop count.  The design supports data labeling by Virtual Local Area
   Networks (VLANs) and fine-grained labels [<a href="./rfc7172" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Fine-Grained Labeling&quot;">RFC7172</a>] as well as
   optimization of the distribution of multi-destination frames based on
   data label and multicast groups.  Devices that implement TRILL are
   called TRILL switches or RBridges (Routing Bridges).

   This document provides detailed specifications for five of the link-
   local aspects of the TRILL protocol used on broadcast links (also
   called LAN or multi-access links) and for the three of these aspects
   that are also used on point-to-point (P2P) links.  It includes state
   diagrams and implementation details where appropriate.  Alternative
   implementations that interoperate on the wire are permitted.

   The scope of this document is limited to the following aspects of the
   TRILL protocol; their applicability, along with the most relevant
   section of this document, are as shown here:

     LAN  P2P  Section  Link-Local Aspect
     ---  ---  -------  ----------------------------------------------

      X    X      3     Adjacency formation and dissolution
      X           4     DRB (Designated RBridge) election
      X    X      5     MTU (Maximum Transmission Unit) matching
      X    X      6     1-hop BFD (Bidirectional Forwarding Detection)
                           for adjacency
      X           7     Creation and use of pseudonodes [<a href="#ref-IS-IS" title="&quot;Information technology -- Telecommunications and information exchange between systems -- Intermediate System to Intermediate System intra-domain routeing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode network service (ISO 8473)&quot;">IS-IS</a>]

                      Table 1: LAN/P2P Applicability

   There is no DRB (also known as DIS (Designated Intermediate System))
   election and no pseudonode creation on links configured as point-to-
   point.

   For other aspects of the TRILL base protocol, see [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>],
   [<a href="./rfc6439" title="&quot;Routing Bridges (RBridges): Appointed Forwarders&quot;">RFC6439</a>], [<a href="./rfc7180" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Clarifications, Corrections, and Updates&quot;">RFC7180</a>], and [<a href="#ref-ESADI" title="&quot;TRILL: ESADI (End Station Address Distribution Information) Protocol&quot;">ESADI</a>].




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   This document obsoletes [<a href="./rfc6327" title="&quot;Routing Bridges (RBridges): Adjacency&quot;">RFC6327</a>].  See <a href="#appendix-A">Appendix A</a> for a summary of
   changes from [<a href="./rfc6327" title="&quot;Routing Bridges (RBridges): Adjacency&quot;">RFC6327</a>].  This document updates [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] as described
   in <a href="#appendix-B">Appendix B</a>.

<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  Content and Precedence</span>

   In cases of conflict between this document and [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>], this
   document prevails.

   <a href="#section-2">Section 2</a> below explains the rationale for the differences between
   the TRILL Hello protocol and the Layer 3 IS-IS Hello protocol in
   light of the environment for which the TRILL protocol is designed.
   It also describes the purposes of the TRILL Hello protocol.

   <a href="#section-3">Section 3</a> describes the adjacency state machine, its states, and its
   relevant events.

   <a href="#section-4">Section 4</a> describes the Designated RBridge (DRB) election state
   machine for RBridge LAN ports, its states, and its relevant events.

   <a href="#section-5">Section 5</a> describes MTU testing and matching on a TRILL link.

   <a href="#section-6">Section 6</a> discusses one-hop BFD session bootstrapping in connection
   with adjacency.

   <a href="#section-7">Section 7</a> discusses pseudonode creation and use on LAN links.

   <a href="#section-8">Section 8</a> provides more details on the reception and transmission of
   TRILL Hellos.

   <a href="#section-9">Section 9</a> discusses the case of multiple ports from one RBridge on
   the same link.

<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>.  Terminology and Acronyms</span>

   This document uses the acronyms defined in [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>], supplemented by
   the following additional acronyms:

      BFD - Bidirectional Forwarding Detection [<a href="./rfc7175" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Bidirectional Forwarding Detection (BFD) Support&quot;">RFC7175</a>].

      SNPA - Subnetwork Point of Attachment [<a href="#ref-IS-IS" title="&quot;Information technology -- Telecommunications and information exchange between systems -- Intermediate System to Intermediate System intra-domain routeing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode network service (ISO 8473)&quot;">IS-IS</a>].

      TRILL switch - an alternative name for an RBridge.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].




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<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  The TRILL Hello Environment and Purposes</span>

   [<a id="ref-IS-IS">IS-IS</a>] has subnetwork-independent functions and subnetwork-dependent
   functions.  Currently, Layer 3 use of IS-IS supports two types of
   subnetworks: (1) point-to-point link subnetworks between routers and
   (2) general broadcast (LAN) subnetworks.  Because of the differences
   between the environment of Layer 3 routers and the environment of
   TRILL RBridges, instead of the subnetwork-dependent functions used at
   Layer 3, which are specified in Sections <a href="#section-8.2">8.2</a> and <a href="#section-8.4">8.4</a> of [<a href="#ref-IS-IS" title="&quot;Information technology -- Telecommunications and information exchange between systems -- Intermediate System to Intermediate System intra-domain routeing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode network service (ISO 8473)&quot;">IS-IS</a>], the
   TRILL protocol uses modified subnetwork-dependent functions for
   point-to-point subnetworks and broadcast (LAN) subnetworks.  The
   differences between the TRILL and Layer 3 environments are described
   in Sections <a href="#section-2.1">2.1</a> through <a href="#section-2.4">2.4</a> followed by a summation, in <a href="#section-2.5">Section 2.5</a>,
   of the purposes of the TRILL Hello protocol.

<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>.  RBridge Interconnection by Ethernet</span>

   TRILL supports the interconnection of RBridges by multi-access LAN
   links such as Ethernet.  Because this includes general bridged LANs
   [<a href="#ref-802.1Q" title="&quot;IEEE Standard for Local and metropolitan area networks-- Media Access Control (MAC) Bridges and Virtual Bridged Local Area Networks&quot;">802.1Q</a>], the links between RBridges may contain devices or services
   that can restrict VLAN connectivity, such as [<a href="#ref-802.1Q" title="&quot;IEEE Standard for Local and metropolitan area networks-- Media Access Control (MAC) Bridges and Virtual Bridged Local Area Networks&quot;">802.1Q</a>] bridges or
   carrier Ethernet services.  In addition, RBridge Ethernet ports, like
   [<a href="#ref-802.1Q" title="&quot;IEEE Standard for Local and metropolitan area networks-- Media Access Control (MAC) Bridges and Virtual Bridged Local Area Networks&quot;">802.1Q</a>] ports, can be configured to restrict input/output on a VLAN
   basis.

   For this reason, TRILL Data and TRILL IS-IS packets are sent on
   Ethernet links in a Designated VLAN that is assumed to provide
   connectivity between all RBridges on the link.  The Designated VLAN
   is dictated for a LAN link by the elected Designated RBridge on that
   link (DRB, equivalent to the Designated Intermediate System at
   Layer 3).  On an RBridge Ethernet port configured as point-to-point,
   TRILL Data and IS-IS packets are sent in that port's Desired
   Designated VLAN, regardless of the state of any other ports on the
   link.  Connectivity on an Ethernet link configured as point-to-point
   generally depends on both ends being configured with the same Desired
   Designated VLAN.  Because TRILL Data packets flow between RBridges on
   an Ethernet link only in the link's Designated VLAN, adjacency for
   routing calculations is based only on connectivity characteristics in
   that VLAN.

   (Non-Ethernet links, such as PPP [<a href="./rfc6361" title="&quot;PPP Transparent Interconnection of Lots of Links (TRILL) Protocol Control Protocol&quot;">RFC6361</a>] generally do not have any
   Outer.VLAN labeling, so the Designated VLAN for such links has no
   effect.)








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<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


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

   This section discusses the handling of "native" frames as defined in
   <a href="./rfc6325#section-1.4">Section&nbsp;1.4 of [RFC6325]</a>.  As such, this section is not applicable to
   point-to-point links between TRILL switches or any link where all the
   TRILL switch ports on the link have been configured as "trunk ports"
   by setting the end-station service disable bit for the port (see
   <a href="./rfc6325#section-4.9.1">Section&nbsp;4.9.1 of [RFC6325]</a>).

   Layer 3 data packets, such as IP packets, are already "tamed" when
   they are originated by an end station: they include a hop count and
   Layer 3 source and destination address fields.  Furthermore, for
   ordinary data packets, there is no requirement to preserve any outer
   Layer 2 addressing, and, if the packets are unicast, they are
   explicitly addressed to their first-hop router.

   In contrast, TRILL switches must accept, transport, and deliver
   "untamed" native frames: native frames that lack a hop count field
   usable by TRILL and have Layer 2 MAC (Media Access Control) addresses
   that indicate their source and destination.  These Layer 2 addresses
   must be preserved for delivery to the native frame's Layer 2
   destination.  One resulting difference is that RBridge ports
   providing native frame service must receive in promiscuous MAC
   address mode, while Layer 3 router ports typically receive in a
   selective MAC address mode.

   TRILL handles these requirements by having, on the link where an end
   station originates a native frame, one RBridge "ingress" such a
   locally originated native frame by adding a TRILL Header that
   includes a hop count, thus converting it to a TRILL Data packet.
   This augmented packet is then routed to one RBridge on the link
   having the destination end station for the frame (or one RBridge on
   each such link if it is a multi-destination frame).  Such final
   RBridges perform an "egress" function, removing the TRILL Header and
   delivering the original frame to its destination(s).  (For the
   purposes of TRILL, a Layer 3 router is an end station.)

   Care must be taken to avoid a loop that would involve egressing a
   native frame and then re-ingressing it because, while it is in native
   form, it would not be protected by a hop count and could loop
   forever.  Such a loop could, for multi-destination frames, even
   involve multiplication of the number of frames each time around and
   would likely saturate all links involved within milliseconds.  For
   TRILL, safety against such loops for a link is more important than
   transient loss of data connectivity on that link.






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<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


   The primary TRILL defense mechanism against such loops, which is
   mandatory, is to assure that, as far as practically possible, there
   is only a single RBridge that is in charge of ingressing and
   egressing native frames from and to a link where TRILL is offering
   end-station service.  This is the Designated RBridge and Appointed
   Forwarder mechanism initially specified in the TRILL base protocol
   [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>], discussed in <a href="#section-2.5">Section 2.5</a> below, and further specified in
   both <a href="#section-4">Section 4</a> below and [<a href="./rfc6439" title="&quot;Routing Bridges (RBridges): Appointed Forwarders&quot;">RFC6439</a>].

<span class="h3"><a class="selflink" id="section-2.3" href="#section-2.3">2.3</a>.  Zero or Minimal Configuration</span>

   TRILL provides connectivity and least-cost paths with zero
   configuration.  For additional services, it strives to require only
   minimal configuration; however, services that require configuration
   when offered by [<a href="#ref-802.1Q" title="&quot;IEEE Standard for Local and metropolitan area networks-- Media Access Control (MAC) Bridges and Virtual Bridged Local Area Networks&quot;">802.1Q</a>] bridges, such as non-default VLANs or
   priority, will require configuration.  This differs from Layer 3
   routing where routers typically need to be configured as to the
   subnetworks connected to each port, etc., to provide service.

<span class="h3"><a class="selflink" id="section-2.4" href="#section-2.4">2.4</a>.  MTU Robustness</span>

   TRILL IS-IS needs to be robust against links with reasonably
   restricted MTUs, including links that accommodate only the classic
   Ethernet frame size, despite the addition of reasonable headers such
   as VLAN tags.  Such robustness is particularly required for TRILL
   Hellos to assure correct adjacency and the election of a unique DRB
   on LAN links.

   TRILL will also be used inside data centers where it is common for
   all or most of the links and switches to support frames substantially
   larger than the classic Ethernet maximum size.  For example, they may
   have an MTU adequate to comfortably handle Fiber Channel over
   Ethernet frames, for which T11 recommends a 2,500-byte MTU [<a href="#ref-FCoE" title="&quot;FCoE Max Size&quot;">FCoE</a>], or
   even 9K byte jumbo frames.  It would be beneficial for a TRILL campus
   with such a large MTU to be able to safely make use of it.

   These needs are met by a mandatory maximum on the size of TRILL
   Hellos and by the optional use of MTU testing as described below.













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<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


<span class="h3"><a class="selflink" id="section-2.5" href="#section-2.5">2.5</a>.  Purposes of the TRILL Hello Protocol</span>

   There are three purposes for the TRILL Hello protocol.  They are
   listed below, along with a reference to the section of this document
   in which each is discussed:

   1. To determine which RBridge neighbors have acceptable connectivity
      to be reported as part of the topology (<a href="#section-3">Section 3</a>)

   2. To elect a unique Designated RBridge on broadcast (LAN) links
      (<a href="#section-4">Section 4</a>)

   3. To determine the MTU with which it is possible to safely
      communicate with each RBridge neighbor (<a href="#section-5">Section 5</a>)

   In Layer 3 IS-IS, all three of these functions are combined.  Hellos
   may be padded to the maximum length (see <a href="./rfc3719#section-6">[RFC3719], Section&nbsp;6</a>) so
   that a router neighbor is not discovered if it is impossible to
   communicate with it using maximum-sized Layer 3 IS-IS packets.  Also,
   even if Hellos from a neighbor R2 are received by R1, if connectivity
   to R2 is not 2-way (i.e., R2 does not list R1 in R2's Hello), then R1
   does not consider R2 as a Designated Intermediate System (Designated
   Router) candidate.  Because of this logic, it is possible at Layer 3
   for multiple Designated Routers to be elected on a LAN, with each
   representing the LAN as a pseudonode.  It appears to the topology as
   if the LAN is now two or more separate LANs.  Although this is
   surprising, this does not cause problems for Layer 3.

   In contrast, this behavior is not acceptable for TRILL, since in
   TRILL it is important that all RBridges on a link know about each
   other, and on broadcast (LAN) links that they choose a single RBridge
   to be the DRB to control the native frame ingress and egress.
   Otherwise, multiple RBridges might ingress/egress the same native
   frame, forming loops that are not protected by the hop count in the
   TRILL Header as discussed above.

   The TRILL Hello protocol is best understood by focusing separately on
   each of these three functions listed above, which we do in
   Sections <a href="#section-3">3</a>, <a href="#section-4">4</a>, and <a href="#section-5">5</a>.

   One other issue with TRILL LAN Hellos is to ensure that subsets of
   the information can appear in any single message, and be processable,
   in the spirit of IS-IS Link State PDUs (LSPs) and Complete Sequence
   Number PDUs (CSNPs).  LAN TRILL Hello packets, even though they are
   not padded, can become very large.  An example where this might be
   the case is when some sort of backbone technology interconnects
   hundreds of TRILL sites over what would appear to TRILL to be a giant
   Ethernet, where the RBridges connected to that cloud will perceive



<span class="grey">Eastlake, et al.             Standards Track                    [Page 9]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-10" ></span>
<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


   that backbone to be a single link with hundreds of neighbors.  Thus,
   the TRILL LAN Hello uses a different Neighbor TLV [<a href="./rfc7176" title="&quot;Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS&quot;">RFC7176</a>] that
   lists neighbors seen for a range of MAC (SNPA) addresses.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Adjacency State Machinery</span>

   Each RBridge port has associated with it a port state, as discussed
   in <a href="#section-4">Section 4</a>, and a table of zero or more adjacencies (if the port is
   configured as point-to-point, zero, or one) as discussed in this
   section.  The states such adjacencies can have, the events that cause
   adjacency state changes, the actions associated with those state
   changes, a state table, and a state diagram are given below.

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  TRILL Hellos, Ports, and VLANs</span>

   The determination of adjacencies on links is made using TRILL Hellos
   (see <a href="#section-8">Section 8</a>), an optional MTU test (see <a href="#section-5">Section 5</a>), and,
   optionally, BFD (see <a href="#section-6">Section 6</a>) and/or other connectivity tests.  If
   the MAC (SNPA) addresses of more than one RBridge port on a broadcast
   link are the same, all but one of such ports are put in the Suspended
   state (see <a href="#section-4">Section 4</a>) and do not participate in the link, except to
   monitor whether they should stay suspended.  If the two ports on a
   point-to-point link have MAC (SNPA) addresses, it does not affect
   TRILL operation if they are the same.  (PPP ports, for example, do
   not have MAC addresses [<a href="./rfc6361" title="&quot;PPP Transparent Interconnection of Lots of Links (TRILL) Protocol Control Protocol&quot;">RFC6361</a>].)

   The following items MUST be the same for all TRILL Hellos issued by
   an RBridge on a particular Ethernet port, regardless of the VLAN in
   which the Hello is sent:

   - Source MAC address,

   - Priority to be the DRB,

   - Desired Designated VLAN,

   - Port ID, and,

   - if included, BFD-Enabled TLV [<a href="./rfc6213" title="&quot;IS-IS BFD-Enabled TLV&quot;">RFC6213</a>] and PORT-TRILL-VER
     sub-TLV [<a href="./rfc7176" title="&quot;Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS&quot;">RFC7176</a>].

   Of course, the priority, Desired Designated VLAN, and possibly the
   inclusion or value of the PORT-TRILL-VER sub-TLV, and/or BFD-Enabled
   TLV can change on occasion, but then the new value(s) must similarly
   be used in all TRILL Hellos on the LAN port, regardless of VLAN.






<span class="grey">Eastlake, et al.             Standards Track                   [Page 10]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-11" ></span>
<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


   On broadcast links:

      Because bridges acting as glue on an Ethernet broadcast link might
      be configured in such a way that some VLANs are partitioned, it is
      necessary for RBridges to transmit Hellos on Ethernet links with
      multiple VLAN tags.  The conceptually simplest solution may have
      been to have RBridges transmit up to 4,094 times as many Hellos,
      one with each legal VLAN ID enabled at each port, but this would
      obviously have deleterious performance implications.  So, the
      TRILL protocol specifies that if RB1 knows it is not the DRB, it
      transmits its Hellos on only a limited set of VLANs.  Only an
      RBridge that believes itself to be the DRB on a broadcast Ethernet
      link "sprays" its TRILL Hellos on all of its enabled VLANs at the
      port.  And in both cases, an RBridge can be configured to send
      Hellos on only a subset of those VLANs.  The details are given in
      <a href="./rfc6325#section-4.4.3">[RFC6325], Section&nbsp;4.4.3</a>.

   On point-to-point links:

      If the link technology is VLAN sensitive, such as Ethernet, an
      RBridge sends TRILL Hellos only in the Desired Designated VLAN for
      which it is configured.

<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>.  Adjacency Table Entries and States</span>

   Every adjacency is in one of four states, whether it is one of the
   adjacencies on a broadcast link or the one possible adjacency on a
   point-to-point link.  An RBridge participates in LSP synchronization
   at a port as long as it has one or more adjacencies out of that port
   that are in the 2-Way or Report state.

   Down: This is a virtual state for convenience in creating state
      diagrams and tables.  It indicates that the adjacency is
      nonexistent, and there is no entry in the adjacency table for it.

   Detect: A neighbor RBridge has been detected through receipt of a
      TRILL Hello, but either 2-way connectivity has not been confirmed
      or the detection was on an Ethernet link in a VLAN other than the
      Designated VLAN.

   2-Way: 2-way connectivity to the neighbor has been found and, if the
      link is Ethernet, it was found on the Designated VLAN, but some
      enabled test, such as the link MTU meeting the minimum campus
      requirement or BFD confirming link connectivity, has not yet
      succeeded.






<span class="grey">Eastlake, et al.             Standards Track                   [Page 11]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


   Report: There is 2-way connectivity to the neighbor (on the
      Designated VLAN if an Ethernet link); all enabled tests have
      succeeded, including, if enabled, MTU and/or BFD testing.  This
      state will cause adjacency to be reported in an LSP (with
      appropriate provision for a pseudonode, if any, as described in
      <a href="#section-7">Section 7</a>).

   For an adjacency in any of the three non-Down states (Detect, 2-Way,
   or Report), there will be an adjacency table entry.  That entry will
   give the state of the adjacency and will also include the information
   listed below.

   o  The address, if any, of the neighbor, the Port ID, and the System
      ID in the received Hellos.  Together, these three quantities
      uniquely identify the adjacency on a broadcast link.

   o  One or more Hello holding timers.  For a point-to-point adjacency,
      there is a single Hello holding timer.  For a broadcast LAN
      adjacency, there are exactly two Hello holding timers: a
      Designated VLAN holding timer and a non-Designated VLAN holding
      timer.  Each timer consists of a 16-bit unsigned integer number
      of seconds.

   o  If the adjacency is on a broadcast link, the 7-bit unsigned
      priority of the neighbor to be the DRB.

   o  The 5 bytes of data from the PORT-TRILL-VER received in the most
      recent TRILL Hello from the neighbor RBridge.

   o  The VLAN that the neighbor RBridge wants to be the Designated VLAN
      on the link, called the Desired Designated VLAN.

   o  For an adjacency table at an RBridge that supports BFD, a flag
      indicating whether the last received TRILL Hello from the neighbor
      RBridge contained a BFD-Enabled TLV (see <a href="#section-6">Section 6</a>).

<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>.  Adjacency and Hello Events</span>

   The following events can change the state of an adjacency:

   A0. Receive a TRILL Hello for a broadcast LAN adjacency whose source
       MAC address (SNPA) is equal to that of the port on which it is
       received.  This is a special event that cannot occur on a port
       configured as point-to-point and is handled as described
       immediately after this list of events.  It does not appear in the
       state transition table or diagram.





<span class="grey">Eastlake, et al.             Standards Track                   [Page 12]</span></pre>
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<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


   A1. Receive a TRILL Hello (other than an A0 event) such that:

       - If received on an Ethernet port, it was received in the
         Designated VLAN.

       - If received for a broadcast LAN adjacency, it contains a TRILL
         Neighbor TLV that explicitly lists the receiving port's (SNPA)
         address.

       - If received for a point-to-point adjacency, it contains a
         Three-Way Handshake TLV with the receiver's System ID and
         Extended Circuit ID.

   A2. Event A2 is not possible for a port configured as point-to-point.
       Receive a TRILL Hello (other than an A0 event) such that either

       - The port is Ethernet and the Hello was not on the Designated
         VLAN (any TRILL Neighbor TLV in such a Hello is ignored), or

       - The Hello does not contain a TRILL Neighbor TLV covering an
         address range that includes the receiver's (SNPA) address.

   A3. Receive a TRILL Hello (other than an A0 event) such that:

       - If received on an Ethernet port, it was received in the
         Designated VLAN.

       - If received for a broadcast LAN adjacency, it contains one or
         more TRILL Neighbor TLVs covering an address range that
         includes the receiver's (SNPA) address and none of which list
         the receiver.

       - If received for a point-to-point adjacency, it contains a
         Three-Way Handshake TLV with either the System ID or Extended
         Circuit ID or both not equal to that of the receiver.

   A4. Either

       (1) the Hello holding timer expires on a point-to-point
           adjacency, or

       (2) on a broadcast LAN adjacency,

           (2a) both Hello timers expire simultaneously or

           (2b) one Hello timer expires when the other Hello timer is
                already in the expired state.




<span class="grey">Eastlake, et al.             Standards Track                   [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


   A5. For a broadcast LAN adjacency, the Designated VLAN Hello holding
       timer expires, but the non-Designated VLAN Hello holding timer
       still has time left until it expires.  This event cannot occur
       for a point-to-point adjacency.

   A6. MTU if enabled, BFD if enabled, and all other enabled
       connectivity tests successful.

   A7. MTU if enabled, BFD if enabled, and all other enabled
       connectivity tests were successful but one or more now fail.

   A8. The RBridge port goes operationally down.

   For the special A0 event, the Hello is examined to determine if it
   has a higher priority than the port on which it is received such that
   the sending port should be the DRB as described in <a href="#section-4.2.1">Section 4.2.1</a>.  If
   the Hello is of lower priority than the receiving port, it is
   discarded with no further action.  If it is of higher priority than
   the receiving port, then any adjacencies for the receiving port are
   discarded (transitioned to the Down state), and the port is suspended
   as described in <a href="#section-4.2">Section 4.2</a>.

   The receipt of a TRILL Hello that is not an event A0 causes the
   following actions (except where the Hello would have created a new
   adjacency table entry but both the adjacency table is full and the
   Hello is too low priority to displace an existing entry as described
   in <a href="#section-3.6">Section 3.6</a>).  The Designated VLAN referred to is the Designated
   VLAN dictated by the DRB determined without taking the received TRILL
   LAN Hello into account (see <a href="#section-4">Section 4</a>) for a broadcast LAN and the
   local Desired Designated VLAN for a port configured as point-to-
   point.

   o  If the receipt of a Hello creates a new adjacency table entry, the
      neighbor RBridge MAC (SNPA) address (if any), Port ID, and System
      ID are set from the Hello.

   o  For a point-to-point adjacency, the Hello holding timer is set
      from the Holding Time field of the Hello.  For a broadcast link
      adjacency, the appropriate Hello holding timer for that adjacency,
      depending on whether or not the Hello was received in the
      Designated VLAN, is set to the Holding Time field of the Hello and
      if the receipt of the LAN Hello is creating a new adjacency table
      entry, the other timer is set to expired.

   o  For a broadcast link adjacency, the priority of the neighbor
      RBridge to be the DRB is set to the priority field of the LAN
      Hello.




<span class="grey">Eastlake, et al.             Standards Track                   [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


   o  For a broadcast link adjacency, the VLAN that the neighbor RBridge
      wants to be the Designated VLAN on the link is set from the Hello.

   o  The 5 bytes of PORT-TRILL-VER data are set from that sub-TLV in
      the Hello or set to zero if that sub-TLV does not occur in the
      Hello.

   o  For a broadcast link, if the creation of a new adjacency table
      entry or the priority update above changes the results of the DRB
      election on the link, the appropriate RBridge port event (D2 or
      D3) occurs, after the above actions, as described in <a href="#section-4.2">Section 4.2</a>.

   o  For a broadcast link adjacency, if there is no change in the DRB,
      but the neighbor Hello is from the DRB and has a changed
      Designated VLAN from the previous Hello received from the DRB, the
      result is a change in Designated VLAN for the link as specified in
      <a href="#section-4.2.3">Section 4.2.3</a>.

   An event A4 resulting in the adjacency transitioning to the Down
   state may also result in an event D3 as described in <a href="#section-4.2">Section 4.2</a>.

   Concerning events A6 and A7, if none of MTU, BFD, or other testing is
   enabled, A6 is considered to occur immediately upon the adjacency
   entering the 2-Way state, and A7 cannot occur.

   See further TRILL Hello receipt details in <a href="#section-8">Section 8</a>.

<span class="h3"><a class="selflink" id="section-3.4" href="#section-3.4">3.4</a>.  Adjacency State Diagram and Table</span>

   The table below shows the transitions between the states defined
   above, based on the events defined above:

               | Event |  Down  | Detect | 2-Way  | Report |
               +-------+--------+--------+--------+--------+
               |  A1   | 2-Way  | 2-Way  | 2-Way  | Report |
               |  A2   | Detect | Detect | 2-Way  | Report |
               |  A3   | Detect | Detect | Detect | Detect |
               |  A4   |  N/A   | Down   | Down   | Down   |
               |  A5   |  N/A   | Detect | Detect | Detect |
               |  A6   |  N/A   |  N/A   | Report | Report |
               |  A7   |  N/A   |  N/A   | 2-Way  | 2-Way  |
               |  A8   | Down   | Down   | Down   | Down   |

                      Table 2: Adjacency State Table







<span class="grey">Eastlake, et al.             Standards Track                   [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


   "N/A" indicates that the event to the left is not applicable in the
   state at the top of the column.  These events affect only a single
   adjacency.  The special A0 event transitions all adjacencies to Down,
   as explained immediately after the list of adjacency events in
   <a href="#section-3.3">Section 3.3</a>.

   The diagram below presents the same information as that in the state
   table:

                         +---------------+
                         |     Down      |&lt;--------+
                         +---------------+         |
                           |     |  ^  |           |
                      A2,A3|     |A8|  |A1         |
                           |     +--+  |           |
                           |           +-----------|---+
                           V                       |   |
                         +----------------+ A4,A8  |   |
                  +-----&gt;|      Detect    |-------&gt;|   |
                  |      +----------------+        |   |
                  |        |  |         ^          |   |
                  |      A1|  |A2,A3,A5 |          |   |
                  |        |  +---------+          |   |
                  |        |                       |   |
                  |        |          +------------|---+
                  |        |          |            |
                  |        V          V            |
                  |A3,A5 +----------------+ A4,A8  |
                  |&lt;-----|     2-Way      |-------&gt;|
                  |      +----------------+        |
                  |       |   ^ |        ^         |
                  |     A6|   | |A1,A2,A7|         |
                  |       |   | +--------+         |
                  |       |   |                    |
                  |       |   |A7                  |
                  |       V   |                    |
                  |A3,A5 +-------------+ A4,A8     |
                  |&lt;-----|   Report    |----------&gt;|
                         +-------------+
                           |         ^
                           |A1,A2,A6 |
                           +---------+

                     Figure 1: Adjacency State Diagram







<span class="grey">Eastlake, et al.             Standards Track                   [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


<span class="h3"><a class="selflink" id="section-3.5" href="#section-3.5">3.5</a>.  Multiple Parallel Links</span>

   There can be multiple parallel adjacencies between neighbor RBridges
   that are visible to TRILL.  (Multiple low-level links that have been
   bonded together by technologies such as link aggregation [<a href="#ref-802.1AX" title="&quot;IEEE Standard for Local and metropolitan area networks-- Link Aggregation&quot;">802.1AX</a>]
   appear to TRILL as a single link over which only a single TRILL
   adjacency can be established.)

   Any such links that have pseudonodes (see <a href="#section-7">Section 7</a>) are
   distinguished in the topology; such adjacencies, if they are in the
   Report state, appear in LSPs as per <a href="#section-7">Section 7</a>.  However, there can be
   multiple parallel adjacencies without pseudonodes because they are
   point-to-point adjacencies or LAN adjacencies for which a pseudonode
   is not being created.  Such parallel, non-pseudonode adjacencies in
   the Report state appear in LSPs as a single adjacency.  The cost of
   such an adjacency MAY be adjusted downwards to account for the
   parallel paths.  Multipathing across such parallel connections can be
   freely done for unicast TRILL Data traffic on a per-flow basis but is
   restricted for multi-destination traffic, as described in
   <a href="#section-4.5.2">Section 4.5.2</a> (point 3) of [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] and <a href="./rfc6325#appendix-C">Appendix&nbsp;C of [RFC6325]</a>.

<span class="h3"><a class="selflink" id="section-3.6" href="#section-3.6">3.6</a>.  Insufficient Space in Adjacency Table</span>

   If the receipt of a TRILL Hello would create a new adjacency table
   entry (that is, would transition an adjacency out of the Down state),
   there may be no space for the new entry.  For ports that are
   configured as point-to-point and can thus only have zero or one
   adjacency not in the Down state, it is RECOMMENDED that space be
   reserved for one adjacency so that this condition cannot occur.

   When there is adjacency table space exhaustion, the DRB election
   priority (see <a href="#section-4.2.1">Section 4.2.1</a>) of the new entry that would be created
   is compared with the DRB election priority for the existing entries.
   If the new entry is higher priority than the lowest priority existing
   entry, it replaces the lowest priority existing entry, which is
   transitioned to the Down state.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  LAN Ports and DRB State</span>

   This section specifies the DRB election process in TRILL at a
   broadcast (LAN) link port.  Since there is no such election when a
   port is configured as point-to-point, this section does not apply in
   that case.








<span class="grey">Eastlake, et al.             Standards Track                   [Page 17]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


   The information at an RBridge associated with each of its broadcast
   LAN ports includes the following:

   o  Enablement bit, which defaults to enabled.

   o  The 5 bytes of PORT-TRILL-VER sub-TLV data used in TRILL Hellos
      sent on the port.

   o  SNPA address (commonly a 48-bit MAC address) of the port.

   o  Port ID, used in TRILL Hellos sent on the port.

   o  The Holding Time, used in TRILL Hellos sent on the port.

   o  The priority to be the DRB, used in TRILL LAN Hellos sent on the
      port.

   o  BFD support.  If the port supports BFD, a BFD Enabled flag that
      controls whether or not a BFD-Enabled TLV is included in TRILL
      Hellos sent on the port.

   o  The DRB state of the port, determined as specified below.

   o  A 16-bit unsigned Suspension Timer, measured in seconds.

   o  The Desired Designated VLAN.  The VLAN this RBridge wants to be
      the Designated VLAN for the link out of this port, used in TRILL
      Hellos sent on the port if the link is Ethernet.

   o  A table of zero or more adjacencies (see <a href="#section-3">Section 3</a>).

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Port Table Entries and DRB Election State</span>

   The TRILL equivalent of the DIS (Designated Intermediate System) on a
   broadcast link is the DRB or Designated RBridge.  The DRB election
   state machinery is described below.

   Each RBridge port that is not configured as point-to-point is in one
   of the following four DRB states:

   Down: The port is operationally down.  It might be administratively
      disabled or down at the link layer.  In this state, there will be
      no adjacencies for the port, and no TRILL Hellos or other TRILL
      IS-IS PDUs or TRILL Data packets are accepted or transmitted.

   Suspended: Operation of the port is suspended because there is a
      higher priority port on the link with the same MAC (SNPA) address.
      This is the same as the Down state, with the exception that TRILL



<span class="grey">Eastlake, et al.             Standards Track                   [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


      Hellos are accepted for the sole purpose of determining whether to
      change the value of the Suspension Timer for the port as described
      below.

   DRB: The port is the DRB and can receive and transmit TRILL Data
      packets.

   Not DRB: The port is deferring to another port on the link, which it
      believes is the DRB, but can still receive and transmit TRILL Data
      packets.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  DRB Election Events</span>

   The following events can change the DRB state of a port.  Note that
   this is only applicable to broadcast links.  There is no DRB state or
   election at a port configured to be point-to-point.

   D1. The port becomes enabled or the Suspension Timer expires while
       the port is in the Suspended state.

   D2. The adjacency table for the port changes, and there are now
       entries for one or more other RBridge ports on the link that
       appear to be higher priority to be the DRB than the local port.

   D3. The port is not Down or Suspended, and the adjacency table for
       the port changes, so there are now no entries for other RBridge
       ports on the link that appear to be higher priority to be the DRB
       than the local port.

   D4. A TRILL LAN Hello is received that has the same MAC address
       (SNPA) as the receiving port and higher priority to be the DRB as
       described for event A0.

   D5. The port becomes operationally down.

   Event D1 is considered to occur on RBridge boot if the port is
   administratively and link-layer enabled.

   Event D4 causes the port to enter the Suspended state and all
   adjacencies for the port to be discarded (transitioned to the Down
   state).  If the port was in some state other than Suspended, the
   Suspension Timer is set to the Holding Time in the Hello that causes
   event D4.  If it was in the Suspended state, the Suspension Timer is
   set to the maximum of its current value and the Holding Time in the
   Hello that causes event D4.






<span class="grey">Eastlake, et al.             Standards Track                   [Page 19]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


<span class="h4"><a class="selflink" id="section-4.2.1" href="#section-4.2.1">4.2.1</a>.  DRB Election Details</span>

   Events D2 and D3 constitute losing and winning the DRB election at
   the port, respectively.

   The candidates for election are the local RBridge and all RBridges
   with which there is an adjacency on the port in an adjacency state
   other than the Down state.  The winner is the RBridge with highest
   priority to be the DRB, as determined from the 7-bit priority field
   in that RBridge's Hellos received and the local port's priority to be
   the DRB field, with MAC (SNPA) address as a tiebreaker, Port ID as a
   secondary tiebreaker, and System ID as a tertiary tiebreaker.  These
   fields are compared as unsigned integers, with the larger magnitude
   being considered higher priority.

   Resorting to the secondary and tertiary tiebreakers should only be
   necessary in rare circumstances when multiple ports have the same
   priority and MAC (SNPA) address and some of them are not yet
   suspended.  For example, RB1, which has low priority to be the DRB on
   the link, could receive Hellos from two other ports on the link that
   have the same MAC address as each other and are higher priority to be
   the DRB.  One of these two ports with the same MAC address will be
   suspended and cease sending Hellos, and the Hello from it received by
   RB1 will eventually time out.  But, in the meantime, RB1 can use the
   tiebreakers to determine which port is the DRB and thus which port's
   Hello to believe for such purposes as setting the Designated VLAN on
   the link.

<span class="h4"><a class="selflink" id="section-4.2.2" href="#section-4.2.2">4.2.2</a>.  Change in DRB</span>

   Events D2 and D3 result from a change in the apparent DRB on the
   link.  Unnecessary DRB changes should be avoided, especially on links
   offering native frame service, as a DRB change will generally cause a
   transient interruption to native frame service.

   If a change in the DRB on the link changes the Designated VLAN on an
   Ethernet link, the actions specified in <a href="#section-4.2.3">Section 4.2.3</a> are taken.

   If an RBridge changes in either direction between being the DRB and
   not being the DRB at a port, this will generally change the VLANs on
   which that RBridge sends Hellos through that port, as specified in
   <a href="./rfc6325#section-4.4.3">Section&nbsp;4.4.3 of [RFC6325]</a>.









<span class="grey">Eastlake, et al.             Standards Track                   [Page 20]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-21" ></span>
<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


<span class="h4"><a class="selflink" id="section-4.2.3" href="#section-4.2.3">4.2.3</a>.  Change in Designated VLAN</span>

   Unnecessary changes in the Designated VLAN on an Ethernet link should
   be avoided because a change in the Designated VLAN can cause a
   transient interruption to adjacency and thus to TRILL Data forwarding
   on the link.  When practical, all RBridge ports on a link should be
   configured with the same Desired Designated VLAN so that if the
   winner of the DRB election changes for any reason, the Designated
   VLAN will remain the same.

   If an RBridge detects a change in Designated VLAN on an Ethernet
   link, then, for all adjacency table entries for a port to that link,
   the RBridge takes the following steps, in the order given.

   o  The non-Designated VLAN Hello holding timer is set to the maximum
      of its time to expiration and the current time to expiration of
      the Designated VLAN Hello holding timer.

   o  The Designated VLAN Hello holding timer is then set to expired (if
      necessary), and an event A5 occurs for the adjacency (see
      <a href="#section-3.3">Section 3.3</a>).

   If the Designated VLAN for a link changes, this will generally change
   the VLANs on which Hellos are sent by an RBridge port on that link as
   specified in <a href="./rfc6325#section-4.4.3">Section&nbsp;4.4.3 of [RFC6325]</a>.

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  Port State Table and Diagram</span>

   The table below shows the transitions between the DRB states defined
   above, based on the events defined above:

         | Event | Down   | Suspended |    DRB    |  Not DRB  |
         +-------+--------+-----------+-----------+-----------+
         |  D1   | DRB    | DRB       |  N/A      |  N/A      |
         |  D2   |  N/A   |  N/A      | Not DRB   | Not DRB   |
         |  D3   |  N/A   |  N/A      | DRB       | DRB       |
         |  D4   |  N/A   | Suspended | Suspended | Suspended |
         |  D5   | Down   | Down      | Down      | Down      |

                         Table 3: Port State Table

   "N/A" indicates that the event to the left is not applicable in the
   state at the top of the column.








<span class="grey">Eastlake, et al.             Standards Track                   [Page 21]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-22" ></span>
<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


   The diagram below presents the same information as in the state
   table:

              +-------------+
              |  Down       |&lt;--------------+
              +-+---+-------+     ^         |
                |   |   ^         |         |
              D1|   |D5 |         |         |
                |   +---+         |D5       |
                |                 |         |
                |        +--------+----+    |
                |        |  Suspended  |&lt;---|---+
                |        +-+-----+-----+    |   |
                |        D1|  ^  |   ^      |   |
                |          |  |  |D4 |      |   |
                |          |  |  +---+      |   |
                |          |  |             |   |
                |          |  |D4           |   |
                V          V  |             |   |
              +---------------+-+ D5        |   |
              |          DRB    |----------&gt;|   |
              +--------+--+-----+           |   |
                  ^    |  |  ^              |   |
                  |  D2|  |D3|              |   |
                  |    |  +--+              |   |
                  |    |         D4         |   |
                  |D3  |  +-----------------|---+
                  |    V  |                 |
             +----+-------+-+ D5            |
             |   Not DRB    |--------------&gt;|
             +----+---------+
                  |    ^
                  |D2  |
                  +----+

                       Figure 2: Port State Diagram

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

   The purpose of MTU testing is to ensure that the links used in the
   campus topology can pass TRILL IS-IS packets, particularly LSP PDUs,
   at the TRILL campus MTU.  The LSP PDUs generated at a TRILL switch
   could, as part of the flooding process, be sent over any adjacency in
   the campus.  To assure correct operation of IS-IS, an LSP PDU must be
   able to reach every RBridge in the IS-IS reachable campus; this might
   be impossible if the PDU exceeded the MTU of an adjacency that was
   part of the campus topology.




<span class="grey">Eastlake, et al.             Standards Track                   [Page 22]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-23" ></span>
<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


   An RBridge, RB1, determines the desired campus link MTU by
   calculating the minimum of its originatingL1LSPBufferSize and the
   originatingL1LSPBufferSize of other RBridges in the campus, as
   advertised in the link-state database, but not less than 1,470 bytes.
   Although originatingL1LSPBufferSize in Layer 3 [<a href="#ref-IS-IS" title="&quot;Information technology -- Telecommunications and information exchange between systems -- Intermediate System to Intermediate System intra-domain routeing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode network service (ISO 8473)&quot;">IS-IS</a>] is limited to
   the range 512 to 1,492 bytes inclusive, in TRILL it is limited to the
   range 1,470 to 65,535 bytes inclusive.  (See <a href="./rfc7180#section-5">Section&nbsp;5 of [RFC7180]</a>.)

   Although MTU testing is optional, it is mandatory for an RBridge to
   respond to an MTU-probe PDU with an MTU-ack PDU [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] [<a href="./rfc7176" title="&quot;Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS&quot;">RFC7176</a>].
   The use of multicast or unicast for MTU-probe and MTU-ack is an
   implementation choice.  However, the burden on the link is generally
   minimized by the following: (1) multicasting MTU-probes when a
   response from all other RBridges on the link is desired, such as when
   initializing or reconfirming MTU, (2) unicasting MTU-probes when a
   response from a single RBridge is desired, such as one that has just
   been detected on the link, and (3) unicasting all MTU-ack packets.

   RB1 can test the MTU size to RB2 as described in <a href="./rfc6325#section-4.3.2">Section&nbsp;4.3.2 of
   [RFC6325]</a>.  For this purpose, MTU testing is only done in the
   Designated VLAN.  An adjacency that fails the MTU test at the campus
   MTU will not enter the Report state, or, if the adjacency is in that
   state, it leaves that state.  Thus, an adjacency failing the MTU test
   at the campus minimum MTU will not be reported by the RBridge
   performing the test.  Since inclusion in least-cost route computation
   requires the adjacency to be reported by both ends, as long as the
   RBridge at either end of the adjacency notices the MTU failure, it
   will not be so used.

   If RB1 tests MTU size, it reports the largest size for which the MTU
   test succeeds or a flag indicating that it fails at the campus MTU.
   This report always appears with the neighbor in RB1's TRILL Neighbor
   TLV.  RB1 MAY also report this with the adjacency in an Extended
   Reachability TLV in RB1's LSP.  RB1 MAY choose to test MTU sizes
   greater than the desired campus MTU as well as the desired
   campus MTU.

   Most types of TRILL IS-IS packets, such as LSPs, can make use of the
   campus MTU.  The exceptions are TRILL Hellos, which must be kept
   small for loop safety, and the MTU PDUs, whose size must be adjusted
   appropriately for the tests being performed.










<span class="grey">Eastlake, et al.             Standards Track                   [Page 23]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-24" ></span>
<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  BFD-Enabled TLV and BFD Session Bootstrapping</span>

   When the adjacency between RBridges reaches the 2-Way state, TRILL
   Hellos will already have been exchanged.  If an RBridge supports BFD
   [<a href="./rfc7175" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Bidirectional Forwarding Detection (BFD) Support&quot;">RFC7175</a>], it will have learned whether the other RBridge has BFD
   enabled by whether or not a BFD-Enabled TLV [<a href="./rfc6213" title="&quot;IS-IS BFD-Enabled TLV&quot;">RFC6213</a>] was included in
   its Hellos.  In addition, TRILL Hellos include a nickname of the
   sending RBridge [<a href="./rfc7176" title="&quot;Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS&quot;">RFC7176</a>] so that information will be available to
   the receiving RBridge.

   The BFD-Enabled TLVs in TRILL Hellos will look like the following:

         +-+-+-+-+-+-+-+-+
         | Type=148      |                   (1 byte)
         +-+-+-+-+-+-+-+-+
         | Length=3*n    |                   (1 byte)
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         | RESV  |        MT ID=0        |   (2 bytes)
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         | NLPID=0xC0    |                   (1 byte)
         +-+-+-+-+-+-+-+-+-+-+-...
         | possible additional               (3*(n-1) bytes)
         | topology/NLPID pairs
         +-+-+-+-+-+-+-...

                 Figure 3: BFD-Enabled TLV Example/Format

         Type = 148 for BFD Enabled [<a href="./rfc6213" title="&quot;IS-IS BFD-Enabled TLV&quot;">RFC6213</a>].

         Length will be 3 times the number of topology and protocol
         pairs in the TLV.

         MT ID is a topology ID [<a href="./rfc5120" title="&quot;M-ISIS: Multi Topology (MT) Routing in Intermediate System to Intermediate Systems (IS-ISs)&quot;">RFC5120</a>] that will be zero unless
         multi-topology is being supported [<a href="#ref-MT" title="&quot;TRILL: Multi-Topology&quot;">MT</a>].

         NLPID is a Network Layer Protocol ID [<a href="./rfc6328" title="&quot;IANA Considerations for Network Layer Protocol Identifiers&quot;">RFC6328</a>] and will be 0xC0
         for TRILL, but additional topology and protocol pairs could
         conceivably be listed.

   An RBridge port initiates a one-hop BFD session with another RBridge
   if the following conditions are met: (1) it has BFD enabled, (2) it
   has an adjacency to another RBridge in the 2-Way or Report state, and
   (3) the Hellos it receives indicate that the other RBridge also has
   BFD enabled.  Either (a) BFD was enabled on both RBridge ports when
   the adjacency changed to the 2-Way or Report state, (b) the adjacency
   was already in the 2-Way or Report state and BFD was enabled on one
   RBridge port when BFD had been enabled on the other, or (c) BFD was
   simultaneously enabled on both RBridge ports.



<span class="grey">Eastlake, et al.             Standards Track                   [Page 24]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-25" ></span>
<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


   In such a BFD session, BFD is encapsulated as specified in [<a href="./rfc7175" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Bidirectional Forwarding Detection (BFD) Support&quot;">RFC7175</a>].
   The egress nickname to be used will have been learned from received
   Hellos.  On a point-to-point link, the Any-RBridge nickname [<a href="./rfc7180" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Clarifications, Corrections, and Updates&quot;">RFC7180</a>]
   can also be used as egress, since support of that nickname is
   required by support of RBridge Channel [<a href="./rfc7178" title="&quot;Transparent Interconnection of Lots of Links (TRILL): RBridge Channel Support&quot;">RFC7178</a>] and support of
   RBridge Channel is required for support of BFD over TRILL.

   The rare case of transient nickname conflict (due to the network
   operator configuring a conflict, new connectivity to a previously
   isolated RBridge, or the like) can cause transient failure of an
   ongoing BFD session.  This can be avoided in the one-hop point-to-
   point case by using the Any-RBridge egress nickname.  In cases where
   Any-RBridge cannot be used as the egress nickname and a transient
   nickname conflict is detected for the intended destination of a BFD
   session, initiation of the session SHOULD be delayed until the
   conflict is resolved.

   If a one-hop BFD session is initiated when the adjacency is in the
   2-Way state, the adjacency MUST NOT advance to the Report state until
   BFD and any other enabled connectivity tests (including MTU, if
   enabled) have succeeded, as specified in <a href="#section-3">Section 3</a>.

   If a one-hop BFD session is established when the adjacency is in the
   Report state, due to enablement at the RBridges, then, to minimize
   unnecessary topology changes, the adjacency MUST remain in the Report
   state unless and until the BFD session (or some other enabled
   connectivity test) fails.

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

   This section only applies to broadcast links, as there is no DRB and
   there cannot be a pseudonode [<a href="#ref-IS-IS" title="&quot;Information technology -- Telecommunications and information exchange between systems -- Intermediate System to Intermediate System intra-domain routeing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode network service (ISO 8473)&quot;">IS-IS</a>] for a link configured as point-
   to-point.  The Designated RBridge (DRB), determined as described
   above, controls whether a pseudonode will be used on a link.

   If the DRB sets the bypass pseudonode bit in its TRILL LAN Hellos,
   the RBridges on the link (including the DRB) just directly report all
   their adjacencies on the LAN that are in the Report state.  If the
   DRB does not set the bypass pseudonode bit in its TRILL Hellos, then
   (1) the DRB reports in its LSP its adjacency to the pseudonode, (2)
   the DRB sends LSPs on behalf of the pseudonode in which it reports
   adjacency to all other RBridges on the link where it sees that
   adjacency in the Report state, and (3) all other RBridges on the link
   report their adjacency to the pseudonode if they see their adjacency
   to the DRB as being in the Report state and do not report any other
   adjacencies on the link.  Setting the bypass pseudonode bit has no
   effect on how LSPs are flooded on a link.  It only affects what LSPs
   are generated.



<span class="grey">Eastlake, et al.             Standards Track                   [Page 25]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-26" ></span>
<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


   It is anticipated that many links between RBridges will actually be
   point-to-point even in cases where the link technology supports
   operation as a multi-access broadcast link, in which case using a
   pseudonode merely adds to the complexity.  For example, if RB1 and
   RB2 are the only RBridges on the link, and RB1 is the DRB, then if
   RB1 creates a pseudonode -- for example, RB1.25 -- that is used,
   there are then 3 LSPs: RB1.25, RB1, and RB2, where RB1.25 reports
   connectivity to RB1 and RB2, and RB1 and RB2 each just say they are
   connected to RB1.25.  However, if DRB RB1 sets the bypass pseudonode
   bit in its Hellos, then there will be only 2 LSPs: RB1 and RB2, each
   reporting connectivity to each other.

   A DRB SHOULD set the bypass pseudonode bit in its Hellos if it has
   not seen at least two simultaneous adjacencies in the Report state
   since it last rebooted or was reset by network management.

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  More TRILL Hello Details</span>

   This section provides further details on the receipt, transmission,
   and content of TRILL Hellos.  Unless otherwise stated, it applies to
   both LAN and point-to-point Hellos.

   TRILL Hellos, like all TRILL IS-IS packets, are primarily
   distinguished from Layer 3 IS-IS packets on Ethernet by being sent to
   the All-IS-IS-RBridges multicast address (01-80-C2-00-00-41).  TRILL
   IS-IS packets on Ethernet also have the L2-IS-IS Ethertype (0x22F4)
   and are Ethertype encoded.

   Although future extensions to TRILL may include the use of Level 2
   IS-IS, [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] specifies TRILL using a single Level 1 Area using
   the fixed Area Address zero (see <a href="./rfc7176#section-4.2">Section&nbsp;4.2 of [RFC7176]</a>).

   IS-IS Layer 3 routers are frequently connected to other Layer 3
   routers that are part of a different routing domain.  In that case,
   the externalDomain flag (see [<a href="#ref-IS-IS" title="&quot;Information technology -- Telecommunications and information exchange between systems -- Intermediate System to Intermediate System intra-domain routeing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode network service (ISO 8473)&quot;">IS-IS</a>]) is normally set for the port
   through which such a connection is made.  The setting of this flag to
   "true" causes no IS-IS PDUs to be sent out of the port and any IS-IS
   PDUs received to be discarded, including Hellos.  RBridges operate in
   a different environment where all neighbor RBridges merge into a
   single campus.  For loop safety, RBridges do not implement the
   externalDomain flag or implement it with the fixed value "false".
   They send and can receive TRILL Hellos on every port that is not
   disabled.








<span class="grey">Eastlake, et al.             Standards Track                   [Page 26]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-27" ></span>
<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>.  Contents of TRILL Hellos</span>

   The table below lists mandatory (M) and optional (O) content TLVs for
   TRILL Hellos that are particularly relevant to this document,
   distinguishing between TRILL LAN Hellos and TRILL P2P Hellos.  A "-"
   indicates that an occurrence would be ignored.  There are additional
   TLVs and sub-TLVs that can occur in TRILL Hellos [<a href="./rfc7176" title="&quot;Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS&quot;">RFC7176</a>].

     LAN  P2P  Number  Content Item
     ---  ---  ------  ----------------------------------------------

      M    M     1     Area Addresses TLV with Area Address zero only
      M    M     1     MT Port Capabilities TLV containing a
                       VLAN-FLAGs sub-TLV
      O    O    0-n    Other MT Port Capability TLVs
      M    -    0-n    TRILL Neighbor TLV (see <a href="#section-8.2.1">Section 8.2.1</a>)
      -    M     1     Three-Way Handshake TLV
      O    O    0-n    Protocols Supported TLV -- MUST list the
                       TRILL NLPID (0xC0) [<a href="./rfc6328" title="&quot;IANA Considerations for Network Layer Protocol Identifiers&quot;">RFC6328</a>]
      O    O    0-1    BFD-Enabled TLV
      O    O    0-1    Authentication TLV
      -    -    0-n    Padding TLV -- SHOULD NOT be included

                       Table 4: TRILL Hello Contents

   A TRILL Hello MAY also contain any TLV permitted in a Layer 3 IS-IS
   Hello.  As with all IS-IS PDUs, TLVs that are unsupported/unknown in
   TRILL Hellos are ignored.

<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>.  Transmitting TRILL Hellos</span>

   TRILL Hellos are sent with the same timing as Layer 3 IS-IS Hellos
   [<a href="#ref-IS-IS" title="&quot;Information technology -- Telecommunications and information exchange between systems -- Intermediate System to Intermediate System intra-domain routeing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode network service (ISO 8473)&quot;">IS-IS</a>]; however, no Hellos are sent if a port is in the Suspended or
   Down state or if the port is disabled.

   TRILL Hello PDUs SHOULD NOT be padded and MUST NOT be sent if they
   exceed 1,470 bytes; however, a received TRILL Hello longer than
   1,470 bytes is processed normally.

   TRILL Hello PDU headers MUST conform to the following:

   o  Maximum Area Addresses equal to 1.

   o  Circuit Type equal to 1.

   See <a href="#section-8.1">Section 8.1</a> for mandatory Hello TLV contents and some optional
   Hello TLV contents.




<span class="grey">Eastlake, et al.             Standards Track                   [Page 27]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-28" ></span>
<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


<span class="h4"><a class="selflink" id="section-8.2.1" href="#section-8.2.1">8.2.1</a>.  TRILL Neighbor TLVs</span>

   A TRILL Neighbor TLV SHOULD NOT be included in TRILL point-to-point
   Hellos, as it MUST be ignored in that context and wastes space.

   TRILL Neighbor TLVs sent in a LAN Hello on an Ethernet link MUST show
   the neighbor information, as sensed by the transmitting RBridge, for
   the VLAN on which the Hello is sent.  Since implementations
   conformant to this document maintain such information on a per-VLAN
   basis only for the Designated VLAN, such implementations only send
   the TRILL Neighbor TLV in TRILL LAN Hellos in the Designated VLAN.

   It is RECOMMENDED that, if there is sufficient room, a TRILL Neighbor
   TLV or TLVs, as described in <a href="./rfc6325#section-4.4.2.1">Section&nbsp;4.4.2.1 of [RFC6325]</a>, covering
   the entire range of MAC addresses and listing all adjacencies with a
   non-zero Designated VLAN Hello Holding Time, or an empty list of
   neighbors if there are no such adjacencies, be in TRILL Hellos sent
   on the Designated VLAN.  If this is not possible, then TRILL Neighbor
   TLVs covering sub-ranges of MAC addresses should be sent so that the
   entire range is covered reasonably promptly.  Delays in sending TRILL
   Neighbor TLVs will delay the advancement of adjacencies to the Report
   state and the discovery of some link failures.  Rapid (for example,
   sub-second) detection of link or node failures is best addressed with
   a protocol designed for that purpose, such as BFD (see <a href="#section-6">Section 6</a>).

   To ensure that any RBridge RB2 can definitively determine whether RB1
   can hear RB2, RB1's neighbor list MUST eventually cover every
   possible range of IDs, that is, within a period that depends on RB1's
   policy and not necessarily within any specific period such as its
   Holding Time.  In other words, if X1 is the smallest ID reported in
   one of RB1's neighbor lists, and the "smallest" flag is not set, then
   X1 MUST appear in a different neighbor list as well, as the largest
   ID reported in that fragment.  Or lists may overlap, as long as there
   is no gap, such that some range, say, between Xi and Xj, would never
   appear in any list.
















<span class="grey">Eastlake, et al.             Standards Track                   [Page 28]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-29" ></span>
<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


<span class="h3"><a class="selflink" id="section-8.3" href="#section-8.3">8.3</a>.  Receiving TRILL Hellos</span>

   Assuming that a packet is labeled as TRILL IS-IS -- for example, on
   Ethernet it has the L2-IS-IS Ethertype and the All-IS-IS-RBridges
   destination multicast address or is so marked by the appropriate code
   point on other link types such as PPP [<a href="./rfc6361" title="&quot;PPP Transparent Interconnection of Lots of Links (TRILL) Protocol Control Protocol&quot;">RFC6361</a>] or a pseudowire
   [<a href="./rfc7173" title="&quot;Transparent Interconnection of Lots of Links (TRILL) Transport Using Pseudowires&quot;">RFC7173</a>] -- it will be examined to see if it appears to be an IS-IS
   PDU.  If so, and it appears to be a TRILL Hello PDU, the following
   tests are performed:

   o  The type of Hello PDU (LAN or P2P) is compared with the port
      configuration.  If a LAN Hello is received on a port configured to
      be point-to-point or a P2P Hello is received on a port not
      configured to be point-to-point, it is discarded.

   o  If the Circuit Type field is not 1, the PDU is discarded.

   o  If the PDU does not contain an Area Address TLV or it contains an
      Area Address TLV that is not the single Area Address zero, it is
      discarded.

   o  If the Hello includes a Protocols Supported TLV that does not list
      the TRILL NLPID (0xC0), it is discarded.  It is acceptable if
      there is no Protocols Supported TLV present.

   o  If the Hello does not contain an MT Port Capabilities TLV
      containing a VLAN-FLAGS sub-TLV [<a href="./rfc7176" title="&quot;Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS&quot;">RFC7176</a>], it is discarded.

   o  If the maximumAreaAddresses field of the PDU is not 1, it is
      discarded.

   o  If IS-IS authentication is in use on the link and either the PDU
      has no Authentication TLV or validation of the PDU's
      Authentication TLV fails, it is discarded.

   If none of the rules in the list above cause the packet to be
   discarded and the packet is parseable, it is assumed to be a
   well-formed TRILL Hello received on the link.  It is treated as an
   event A0, A1, A2, or A3, based on the criteria listed in <a href="#section-3.3">Section 3.3</a>.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Multiple Ports on the Same Broadcast Link</span>

   It is possible for an RBridge RB1 to have multiple ports on the same
   broadcast (LAN) link that are not in the Suspended state.  It is
   important for RB1 to recognize which of its ports are on the same
   link.  RB1 can detect this condition based on receiving TRILL Hello
   messages with the same LAN ID on multiple ports.




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<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


   The DRB election is port-based (see <a href="#section-4">Section 4</a>), and only the Hellos
   from the elected port can perform certain functions such as dictating
   the Designated VLAN or whether a pseudonode will be used; however,
   the election also designates the RBridge with that port as the DRB
   for the link.  An RBridge may choose to load split some tasks among
   its ports on the link if it has more than one.  <a href="./rfc6325#section-4.4.4">Section&nbsp;4.4.4 of
   [RFC6325]</a> describes when it is safe to do so.

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

   This document serves as a reference for 'Fail' (Failed MTU test),
   value 0, in the "TRILL Neighbor TLV NEIGHBOR RECORD Flags" registry.
   IANA has updated that reference to point to this RFC.

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  Security Considerations</span>

   This memo provides improved documentation of some aspects of the
   TRILL base protocol standard, particularly five aspects of the TRILL
   adjacency establishment and Hello protocol as listed in <a href="#section-1">Section 1</a>.
   It does not change the security considerations of the TRILL base
   protocol as detailed in <a href="./rfc6325#section-6">Section&nbsp;6 of [RFC6325]</a>.

   See [<a href="./rfc7175" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Bidirectional Forwarding Detection (BFD) Support&quot;">RFC7175</a>] for security considerations for BFD, whose use in
   connection with TRILL adjacency is discussed in this document,
   particularly <a href="#section-6">Section 6</a>.


























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<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>.  Changes from <a href="./rfc6327">RFC 6327</a></span>

   This document has the following changes from [<a href="./rfc6327" title="&quot;Routing Bridges (RBridges): Adjacency&quot;">RFC6327</a>].  It obsoletes
   [<a href="./rfc6327" title="&quot;Routing Bridges (RBridges): Adjacency&quot;">RFC6327</a>].

   1. This document extends the TRILL Hello size limit, MTU testing, and
      state machine to point-to-point links.

   2. This document incorporates the updates to [<a href="./rfc6327" title="&quot;Routing Bridges (RBridges): Adjacency&quot;">RFC6327</a>] from
      [<a href="./rfc7180" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Clarifications, Corrections, and Updates&quot;">RFC7180</a>].

   3. The bulk of [<a href="./rfc6327" title="&quot;Routing Bridges (RBridges): Adjacency&quot;">RFC6327</a>] was written from the point of view that
      links between TRILL switches would only be Ethernet.  In fact,
      they could be any technology, such as PPP [<a href="./rfc6361" title="&quot;PPP Transparent Interconnection of Lots of Links (TRILL) Protocol Control Protocol&quot;">RFC6361</a>], pseudowire
      [<a href="./rfc7173" title="&quot;Transparent Interconnection of Lots of Links (TRILL) Transport Using Pseudowires&quot;">RFC7173</a>], or IP [<a href="#ref-TrillIP" title="&quot;Transparent Interconnection of Lots of Links (TRILL) over IP&quot;">TrillIP</a>].  This replacement document generalizes
      [<a href="./rfc6327" title="&quot;Routing Bridges (RBridges): Adjacency&quot;">RFC6327</a>] to cover such link types.

   4. This document includes a specification of one-hop BFD session
      establishment in connection with adjacency.

   5. Numerous editorial changes were incorporated.

<span class="h2"><a class="selflink" id="appendix-B" href="#appendix-B">Appendix B</a>.  Changes to <a href="./rfc6325">RFC 6325</a></span>

   <a href="#section-2">Section 2</a> of this document replaces <a href="./rfc6325#section-4.4.1">Section&nbsp;4.4.1 of [RFC6325]</a>.
   <a href="#section-8">Section 8</a> of this document replaces <a href="./rfc6325#section-4.4.2">Section&nbsp;4.4.2 of [RFC6325]</a>,
   except for <a href="#section-4.4.2.1">Section 4.4.2.1</a>.  The changes in [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] made by this
   document include

   - Prohibiting the sending of TRILL Hellos out of a port while it is
     in the Suspended state, and the specification of the Suspended
     state.  ([<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] specifies that Hellos be sent with the same
     timing as [<a href="#ref-IS-IS" title="&quot;Information technology -- Telecommunications and information exchange between systems -- Intermediate System to Intermediate System intra-domain routeing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode network service (ISO 8473)&quot;">IS-IS</a>].)

   - Permitting the inclusion of the Three-Way-Handshake TLV,
     BFD-Enabled TLV, and other TLVs in TRILL Hellos when these were
     omitted in TRILL Hello contents lists in <a href="./rfc6325#section-4.4.2">Section&nbsp;4.4.2 of
     [RFC6325]</a>.

   - Extending the TRILL Hello protocol to support point-to-point and
     non-Ethernet links.










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<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


Normative References

   [<a id="ref-IS-IS">IS-IS</a>]    ISO/IEC 10589:2002, Second Edition, "Information
              technology -- Telecommunications and information exchange
              between systems -- Intermediate System to Intermediate
              System intra-domain routeing information exchange protocol
              for use in conjunction with the protocol for providing the
              connectionless-mode network service (ISO 8473)", 2002.

   [<a id="ref-RFC1195">RFC1195</a>]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
              dual environments", <a href="./rfc1195">RFC 1195</a>, December 1990.

   [<a id="ref-RFC2119">RFC2119</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-RFC5120">RFC5120</a>]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", <a href="./rfc5120">RFC 5120</a>, February 2008.

   [<a id="ref-RFC6213">RFC6213</a>]  Hopps, C. and L. Ginsberg, "IS-IS BFD-Enabled TLV",
              <a href="./rfc6213">RFC 6213</a>, April 2011.

   [<a id="ref-RFC6325">RFC6325</a>]  Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
              Ghanwani, "Routing Bridges (RBridges): Base Protocol
              Specification", <a href="./rfc6325">RFC 6325</a>, July 2011.

   [<a id="ref-RFC6328">RFC6328</a>]  Eastlake 3rd, D., "IANA Considerations for Network Layer
              Protocol Identifiers", <a href="https://www.rfc-editor.org/bcp/bcp164">BCP 164</a>, <a href="./rfc6328">RFC 6328</a>, July 2011.

   [<a id="ref-RFC7175">RFC7175</a>]  Manral, V., Eastlake 3rd, D., Ward, D., and A. Banerjee,
              "Transparent Interconnection of Lots of Links (TRILL):
              Bidirectional Forwarding Detection (BFD) Support",
              <a href="./rfc7175">RFC 7175</a>, May 2014.

   [<a id="ref-RFC7176">RFC7176</a>]  Eastlake 3rd, D., Senevirathne, T., Ghanwani, A., Dutt,
              D., and A. Banerjee, "Transparent Interconnection of Lots
              of Links (TRILL) Use of IS-IS", <a href="./rfc7176">RFC 7176</a>, May 2014.

   [<a id="ref-RFC7178">RFC7178</a>]  Eastlake 3rd, D., Manral, V., Li, Y., Aldrin, S., and D.
              Ward, "Transparent Interconnection of Lots of Links
              (TRILL): RBridge Channel Support", <a href="./rfc7178">RFC 7178</a>, May 2014.

   [<a id="ref-RFC7180">RFC7180</a>]  Eastlake 3rd, D., Zhang, M., Ghanwani, A., Manral, V., and
              A. Banerjee, "Transparent Interconnection of Lots of Links
              (TRILL): Clarifications, Corrections, and Updates",
              <a href="./rfc7180">RFC 7180</a>, May 2014.





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Informative References

   [<a id="ref-802.1AX">802.1AX</a>]  "IEEE Standard for Local and metropolitan area networks--
              Link Aggregation", 802.1AX-2008, January 2008.

   [<a id="ref-802.1Q">802.1Q</a>]   "IEEE Standard for Local and metropolitan area networks--
              Media Access Control (MAC) Bridges and Virtual Bridged
              Local Area Networks", 802.1Q-2011, August 2011.

   [<a id="ref-ESADI">ESADI</a>]    Zhai, H., Hu, F., Perlman, R., Eastlake 3rd, D., and O.
              Stokes, "TRILL: ESADI (End Station Address Distribution
              Information) Protocol", Work in Progress, April 2014.

   [<a id="ref-FCoE">FCoE</a>]     Excerpt of discussion of "FCoE Max Size" generated from
              T11/09-251v1, 04/27/2009, "FCoE frame or FCoE PDU",
              &lt;www.t11.org&gt;.

   [<a id="ref-MT">MT</a>]       Eastlake, D., Zhang, M., Banerjee, A., and V. Manral,
              "TRILL: Multi-Topology", Work in Progress, September 2013.

   [<a id="ref-RFC3719">RFC3719</a>]  Parker, J., Ed., "Recommendations for Interoperable
              Networks using Intermediate System to Intermediate System
              (IS-IS)", <a href="./rfc3719">RFC 3719</a>, February 2004.

   [<a id="ref-RFC6327">RFC6327</a>]  Eastlake 3rd, D., Perlman, R., Ghanwani, A., Dutt, D., and
              V. Manral, "Routing Bridges (RBridges): Adjacency",
              <a href="./rfc6327">RFC 6327</a>, July 2011.

   [<a id="ref-RFC6361">RFC6361</a>]  Carlson, J. and D. Eastlake 3rd, "PPP Transparent
              Interconnection of Lots of Links (TRILL) Protocol Control
              Protocol", <a href="./rfc6361">RFC 6361</a>, August 2011.

   [<a id="ref-RFC6439">RFC6439</a>]  Perlman, R., Eastlake, D., Li, Y., Banerjee, A., and F.
              Hu, "Routing Bridges (RBridges): Appointed Forwarders",
              <a href="./rfc6439">RFC 6439</a>, November 2011.

   [<a id="ref-RFC7172">RFC7172</a>]  Eastlake 3rd, D., Zhang, M., Agarwal, P., Perlman, R., and
              D. Dutt, "Transparent Interconnection of Lots of Links
              (TRILL): Fine-Grained Labeling", <a href="./rfc7172">RFC 7172</a>, May 2014.

   [<a id="ref-RFC7173">RFC7173</a>]  Yong, L., Eastlake 3rd, D., Aldrin, S., and J. Hudson,
              "Transparent Interconnection of Lots of Links (TRILL)
              Transport Using Pseudowires", <a href="./rfc7173">RFC 7173</a>, May 2014.

   [<a id="ref-TrillIP">TrillIP</a>]  Wasserman, M., Eastlake, D., and D. Zhang, "Transparent
              Interconnection of Lots of Links (TRILL) over IP", Work in
              Progress, March 2014.




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<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


Acknowledgements

   The suggestions and comments of the following are gratefully
   acknowledged:

      Stewart Bryant, Elwyn Davies, Adrian Farrel, Brian Haberman, Jon
      Hudson, Arnel Lim, and Gayle Noble.

   The authors of [<a href="./rfc6327" title="&quot;Routing Bridges (RBridges): Adjacency&quot;">RFC6327</a>] included Dinesh Dutt.  The acknowledgements
   section of [<a href="./rfc6327" title="&quot;Routing Bridges (RBridges): Adjacency&quot;">RFC6327</a>] includes the following, listed in alphabetic
   order: Jari Arkko, Ayan Banerjee, Les Ginsberg, Sujay Gupta, David
   Harrington, Pete McCann, Erik Nordmark, and Mike Shand.

Authors' Addresses

   Donald E. Eastlake 3rd
   Huawei Technologies
   155 Beaver Street
   Milford, MA  01757
   USA

   Phone: +1-508-333-2270
   EMail: d3e3e3@gmail.com


   Radia Perlman
   Intel Labs
   2200 Mission College Blvd.
   Santa Clara, CA  95054
   USA

   Phone: +1-408-765-8080
   EMail: Radia@alum.mit.edu


   Anoop Ghanwani
   Dell
   5450 Great America Parkway
   Santa Clara, CA  95054
   USA

   EMail: anoop@alumni.duke.edu









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<span class="grey"><a href="./rfc7177">RFC 7177</a>                    TRILL: Adjacency                    May 2014</span>


   Howard Yang
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   EMail: howardy@cisco.com


   Vishwas Manral
   Ionos Corp.
   4100 Moorpark Ave.
   San Jose, CA  95117
   USA

   EMail: vishwas@ionosnetworks.com



































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</pre>