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Transparent Interconnection of Lots of Links (TRILL): Adjacency

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7177.
Authors Donald E. Eastlake 3rd , Radia Perlman , Anoop Ghanwani , Howard Yang , Vishwas Manral
Last updated 2015-10-14 (Latest revision 2014-01-10)
Replaces draft-eastlake-trill-rfc6327bis
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Jon Hudson
Shepherd write-up Show Last changed 2013-10-25
IESG IESG state Became RFC 7177 (Proposed Standard)
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Ted Lemon
Send notices to (None)
IANA IANA review state Version Changed - Review Needed
IANA action state No IANA Actions
TRILL Working Group                                      Donald Eastlake
INTERNET-DRAFT                                                    Huawei
Obsoletes: 6327                                            Radia Perlman
Updates: 6325                                                 Intel Labs
Intended status: Proposed Standard                        Anoop Ghanwani
                                                             Howard Yang
                                                          Vishwas Manral
Expires: July 9, 2014                                   January 10, 2014

                            TRILL: Adjacency


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

Status of This Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Distribution of this document is unlimited. Comments should be sent
   to the TRILL working group mailing list.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

D. Eastlake, et al                                              [Page 1]
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   The list of current Internet-Drafts can be accessed at The list of Internet-Draft
   Shadow Directories can be accessed at

D. Eastlake, et al                                              [Page 2]
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Table of Contents

      1. Introduction............................................4
      1.1 Content and Precedence.................................5
      1.2 Terminology and Acronyms...............................5

      2. The TRILL Hello Environment and Purposes................6
      2.1 RBridge Interconnection by Ethernet....................6
      2.2 Handling Native Frames.................................7
      2.3 Zero or Minimal Configuration..........................8
      2.4 MTU Robustness.........................................8
      2.5 Purposes of the TRILL Hello Protocol...................8

      3. Adjacency State Machinery..............................10
      3.1 TRILL Hellos, Ports, and VLANs........................10
      3.2 Adjacency Table Entries and States....................11
      3.3 Adjacency and Hello Events............................12
      3.4 Adjacency State Diagram and Table.....................15
      3.5 Multiple Parallel Links...............................16
      3.6 Insufficient Space in Adjacency Table.................17

      4. LAN Ports and DRB State................................18
      4.1 Port Table Entries and DRB Election State.............18
      4.2 DRB Election Events...................................19
      4.2.1 DRB Election Details................................20
      4.2.2 Change in DRB.......................................20
      4.2.3 Change in Designated VLAN...........................21
      4.3 State Table and Diagram...............................21

      5. MTU Matching...........................................23
      6. BFD Enabled and BFD Session Bootstrap..................25
      7. Pseudonodes............................................27

      8. More TRILL Hello Details...............................28
      8.1 Contents of TRILL Hellos..............................28
      8.2 Transmitting TRILL Hellos.............................29
      8.2.1 TRILL Neighbor TLVs.................................29
      8.3 Receiving TRILL Hellos................................30

      9. Multiple Ports on the Same Broadcast Link..............32

      10. Security Considerations...............................33
      11. IANA Considerations...................................33

      Appendix A: Changes from [RFC6327]........................34
      Appendix B: Changes to [RFC6325]..........................35
      Appendix Z: Change History................................36
      Normative References......................................37
      Informative References....................................37
      Authors' Addresses........................................40

D. Eastlake, et al                                              [Page 3]
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1. Introduction

   The IETF TRILL (TRansparent Interconnection of Lots of Links)
   protocol [RFC6325] 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 [IS-IS]
   (Intermediate System to Intermediate System) link state routing
   [RFC1195] [RFC6326bis] and a header in TRILL data packets that
   includes a hop count.  The design supports data labeling by VLANs
   (Virtual Local Area Networks) and fine grained labels [RFCfgl] 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 specification 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
   used on point-to-point 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 with applicability and the most relevant section of
   this document as shown:

      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 (Bi-directional Forwarding Detection)
                            for adjacency
       X           7     Creation and use of pseudonodes [IS-IS]

                        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-

   For other aspects of the TRILL protocol, see other documents such as
   [RFC6325], [RFC6439], [RFCclear], and [ESADI].

   This document obsoletes [RFC6327]. See Appendix A for a summary of
   changes from [RFC6327]. This document updates [RFC6325] as described

D. Eastlake, et al                                              [Page 4]
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   in Appendix B.

1.1 Content and Precedence

   In case of conflict between this document and [RFC6325], this
   document prevails.

   Section 2 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.

   Section 3 describes the adjacency state machine, its states, and its
   relevant events.

   Section 4 describes the Designated RBridge (DRB) election state
   machine for RBridge LAN ports, its states, and its relevant events.

   Section 5 describes MTU testing and matching on a TRILL link.

   Section 6 discusses one-hop BFD session bootstrapping in connection
   with adjacency.

   Section 7 discusses pseudonode creation and use on LAN links.

   Section 8 provides more details on the reception and transmission of
   TRILL Hellos.

   Section 9 discusses the case of multiple ports from one RBridge on
   the same link.

1.2 Terminology and Acronyms

   This document uses the acronyms defined in [RFC6325] supplemented by
   the following additional acronyms:

      BFD - Bi-directional Forwarding Detection [RFCbfd].

      SNPA - Subnetwork Point of Attachment [IS-IS].

      TRILL switch - an alternative name for an RBridge.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

D. Eastlake, et al                                              [Page 5]
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2. The TRILL Hello Environment and Purposes

   [IS-IS] 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 [IS-IS] Sections 8.2 and 8.4, 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 2.1 through 2.4 followed by a summation, in Section 2.5,
   of the purposes of the TRILL Hello protocol.

2.1 RBridge Interconnection by Ethernet

   TRILL supports the interconnection of RBridges by multi-access LAN
   links such as Ethernet.  Because this includes general bridged LANs
   [802.1Q], the links between RBridges may contain devices or services
   that can restrict VLAN connectivity, such as [802.1Q] bridges or
   carrier Ethernet services.  In addition, RBridge Ethernet ports, like
   [802.1Q] ports, can be configured to restrict input/output on a VLAN

   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 [RFC6361] generally do not have any
   Outer.VLAN labeling so the Designated VLAN for such links has no

D. Eastlake, et al                                              [Page 6]
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2.2 Handling Native Frames

   This section discusses the handling of "native" frames as defined in
   Section 1.4 of [RFC6325]. 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
   Section 4.9.1 of [RFC6325]).

   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 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 (Media Access Control)
   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 even, for multi-destination frames,
   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.

   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

D. Eastlake, et al                                              [Page 7]
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   end station service.  This is the Designated RBridge and appointed
   forwarder mechanism initially specified in the TRILL base protocol
   [RFC6325], discussed in Section 2.5 below, and further specified both
   in Section 4 below and [RFC6439].

2.3 Zero or Minimal Configuration

   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 [802.1Q] bridges, such as non-default VLAN 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.

2.4 MTU Robustness

   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 [FCoE], 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.

2.5 Purposes of the TRILL Hello Protocol

   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:

   a) To determine which RBridge neighbors have acceptable connectivity
      to be reported as part of the topology (Section 3)

D. Eastlake, et al                                              [Page 8]
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   b) To elect a unique Designated RBridge on broadcast (LAN) links
      (Section 4)

   c) To determine the MTU with which it is possible to safely
      communicate with each RBridge neighbor (Section 5)

   In Layer 3 IS-IS, all three of these functions are combined.  Hellos
   may be padded to the maximum length (see [RFC3719], Section 6) so
   that a router neighbor is not even 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 3,
   4, and 5.

   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
   that backbone to be a single link with hundreds of neighbors.  Thus,
   the TRILL LAN Hello uses a different Neighbor TLV [RFC6326bis] that
   lists neighbors seen for a range of MAC (SNPA) addresses.

D. Eastlake, et al                                              [Page 9]
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3. Adjacency State Machinery

   Each RBridge port has associated with it a port state, as discussed
   in Section 4, 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.

3.1 TRILL Hellos, Ports, and VLANs

   The determination of adjacencies on links is made using TRILL Hellos
   (see Section 8), an optional MTU test (see Section 5), and optionally
   BFD (see Section 6) and/or other connectivity tests.  If the MAC
   (SNPA) address 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 Section 4) 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 [RFC6361].)

   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 DRB,
      -  Desired Designated VLAN,
      -  Port ID, and,
      -  if included, BFD Enabled TLV [RFC6213] and PORT-TRILL-VER sub-
         TLV [RFC6326bis].

   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.

   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

D. Eastlake, et al                                             [Page 10]
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      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
      [RFC6325], Section 4.4.3.

   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.

3.2 Adjacency Table Entries and States

   Each adjacency on broadcast links and the one possible adjacency on
   each point-to-point link is in one of four states. An RBridge
   participates in LSP synchronization at a port as long as it has one
   or more adjacencies out that port that are in the 2-way or Report

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

         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 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 MTU meeting the minimum campus
         requirement or BFD confirming connectivity, has not yet

         There is 2-way connectivity to the neighbor (on the Designated
         VLAN if an Ethernet link) and 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 Section 7).

   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

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   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  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 Section 6).

3.3 Adjacency and Hello Events

   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.

      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-

D. Eastlake, et al                                             [Page 12]
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          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
             lists 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

      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

      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 Section 4.2.1.  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 Section 4.2.

   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

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   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 Section 3.6).  The Designated VLAN referred to is the Designated
   VLAN dictated by the DRB determined without taking the received TRILL
   LAN Hello into account (see Section 4) for a broadcast LAN and the
   local Desired Designated VLAN for a port configured as point-to-

      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

      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 five 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
         Section 4.2.

      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 Section 4.2.3.

   An event A4 resulting in the adjacency transitioning to the Down
   state may also result in an event D3 as described in Section 4.2.

   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.

D. Eastlake, et al                                             [Page 14]
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   See further TRILL Hello receipt details in Section 7.

3.4 Adjacency State Diagram and Table

   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

   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 above.

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

D. Eastlake, et al                                             [Page 15]
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                            |     Down      |<--------+
                            +---------------+         |
                              |     |  ^  |           |
                         A2,A3|     |A8|  |A1         |
                              |     +--+  |           |
                              |           +-----------|---+
                              V                       |   |
                            +----------------+ A4,A8  |   |
                     +----->|      Detect    |------->|   |
                     |      +----------------+        |   |
                     |        |  |         ^          |   |
                     |      A1|  |A2,A3,A5 |          |   |
                     |        |  +---------+          |   |
                     |        |                       |   |
                     |        |          +------------|---+
                     |        |          |            |
                     |        V          V            |
                     |A3,A5 +----------------+ A4,A8  |
                     |<-----|     2-Way      |------->|
                     |      +----------------+        |
                     |       |   ^ |        ^         |
                     |     A6|   | |A1,A2,A7|         |
                     |       |   | +--------+         |
                     |       |   |                    |
                     |       |   |A7                  |
                     |       V   |                    |
                     |A3,A5 +-------------+ A4,A8     |
                     |<-----|   Report    |---------->|
                              |         ^
                              |A1,A2,A6 |

                     Figure 1. Adjacency State Diagram

3.5 Multiple Parallel Links

   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 [802.1AX]
   appear to TRILL as a single link over which only a single TRILL
   adjacency can be established.)

   Any such links that have pseudonodes (see Section 7) are
   distinguished in the topology; such adjacencies, if they are in the
   Report state, appear in LSPs as per Section 7.  However, there can be
   multiple parallel adjacencies without pseudonodes because they are

D. Eastlake, et al                                             [Page 16]
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   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 Section
   4.5.2 (point 3) and Appendix C of [RFC6325].

3.6 Insufficient Space in Adjacency Table

   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.  It is RECOMMENDED, for
   ports configured as point-to-point and which can thus only have zero
   or one adjacency not in the Down state, to reserved space for one
   adjacency so that this condition cannot occur.

   When there is adjacency table space exhaustion, the DRB election
   priority (see Section 4.2.1) of the new entry that would be created
   is compared with that 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.

D. Eastlake, et al                                             [Page 17]
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4. LAN Ports and DRB State

   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.

   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 five 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  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 this port, used in TRILL
         Hellos sent on the port if the link is Ethernet.

      o  A table of zero or more adjacencies (see Section 3).

4.1 Port Table Entries and DRB Election State

   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:

D. Eastlake, et al                                             [Page 18]
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         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

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

         The port is the DRB and can receive and transmit TRILL Data

      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.

4.2 DRB Election Events

   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. Expiration of the suspension timer while the port is in the
          Suspended state or the enablement of the port.

      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

      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. Receipt of a TRILL LAN Hello with 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.

D. Eastlake, et al                                             [Page 19]
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   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.

4.2.1 DRB Election Details

   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 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.

   Resort 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, that 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, 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.

4.2.2 Change in DRB

   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.

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   If a change in the DRB on the link changes the Designated VLAN on an
   Ethernet link, the actions specified in Section 4.2.3 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
   Section 4.4.3 of [RFC6325].

4.2.3 Change in Designated VLAN

   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, in case 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
         Section 3.3).

   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 Section 4.4.3 of [RFC6325].

4.3 State Table and Diagram

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

D. Eastlake, et al                                             [Page 21]
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             | Event | Down   | Suspend |  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   | Suspend | Suspend | Suspend |
             |  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.

   The diagram below presents the same information as in the state

                 |  Down       |<--------------+
                 +-+---+-------+     ^         |
                   |   |   ^         |         |
                 D1|   |D5 |         |         |
                   |   +---+         |D5       |
                   |                 |         |
                   |        +--------+----+    |
                   |        |  Suspended  |<---|---+
                   |        +-+-----+-----+    |   |
                   |        D1|  ^  |   ^      |   |
                   |          |  |  |D4 |      |   |
                   |          |  |  +---+      |   |
                   |          |  |             |   |
                   |          |  |D4           |   |
                   V          V  |             |   |
                 +---------------+-+ D5        |   |
                 |          DRB    |---------->|   |
                 +--------+--+-----+           |   |
                     ^    |  |  ^              |   |
                     |  D2|  |D3|              |   |
                     |    |  +--+              |   |
                     |    |         D4         |   |
                     |D3  |  +-----------------|---+
                     |    V  |                 |
                +----+-------+-+ D5            |
                |   Not DRB    |-------------->|
                     |    ^
                     |D2  |

                       Figure 2. Port State Diagram

D. Eastlake, et al                                             [Page 22]
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5. MTU Matching

   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, which
   might be impossible if the PDU exceeded the MTU of an adjacency that
   was part of the campus topology.

   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 [IS-IS] 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 Section 5 of [RFCclear].)

   Although MTU testing is optional, it is mandatory for an RBridge to
   respond to an MTU-probe PDU with an MTU-ack PDU [RFC6325]
   [RFC6326bis].  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 re-confirming 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

   RB1 can test the MTU size to RB2 as described in Section 4.3.2 of
   [RFC6325].  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 it tests MTU size, RB1 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

   Most types of TRILL IS-IS packets, such as LSPs, can make use of the

D. Eastlake, et al                                             [Page 23]
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   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.

D. Eastlake, et al                                             [Page 24]
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6. BFD Enabled and BFD Session Bootstrap

   When the adjacency between RBridges reaches the 2-Way state, TRILL
   Hellos will already have been exchanged. If an RBridge supports BFD
   [RFCbfd] it will have learned whether the other RBridge has BFD
   enabled by whether or not a BFD Enabled TLV [RFC6213] was included in
   its Hellos. In addition, TRILL Hellos include a nickname of the
   sending RBridge [RFC6326bis] 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 bytes)
         | Length=3*n    |                   (1 bytes)
         | RESV  |         MTID=0        |   (2 bytes)
         | NLPID=0xC0    |                   (1 bytes)
         | possible additional               (3*(n-1) bytes)
         | topology/NLPID pairs

                    Figure 3. BFD Enabled TLV Example/Format

         Type = 148 for BFD Enabled [RFC6213].

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

         MTID is a topology ID [RFC5120] that will be zero unless multi-
         topology is being supported [MT].

         NLPID is a Network Layer Protocol ID [RFC6328] 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 or (b) the
   adjacency was already in 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.

   In such a BFD session, BFD is encapsulated as specified in [RFCbfd].

D. Eastlake, et al                                             [Page 25]
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   The egress nickname to be used will have been learned from received
   Hellos. On a point-to-point link, the Any-RBridge nickname [RFCclear]
   can also be used as egress since support of that nickname is required
   by support of RBridge Channel [RFCchannel] 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, connectivity appearing 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 test (including MTU if
   enabled) have succeeded, as specified in Section 3.

   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.

D. Eastlake, et al                                             [Page 26]
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7. Pseudonodes

   This section only applies to broadcast links as there is no DRB and
   there cannot be a pseudonode [IS-IS] 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.

   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 DRB, then if RB1
   creates a pseudonode that is used, there are 3 LSPs: for, say, RB1.25
   (the pseudonode), 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.  Whereas 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.

D. Eastlake, et al                                             [Page 27]
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8. More TRILL Hello Details

   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 use of Level 2 IS-IS,
   [RFC6325] specifies TRILL using a single Level 1 Area using the fixed
   Area Address zero (see Section 4.2 of [RFC6326bis]).

   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 [IS-IS]) 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 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

8.1 Contents of TRILL Hellos

   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 [RFC6326bis].

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      LAN  P2P  Numb  Content Item
      ---  ---  ----  ------------

       M    M     1    Area Addresses TLV with area zero only
       M    M     1    MT Port Capabilities TLV containing a VLAN-FLAGs
       O    O    0-n   Other MT Port Capability TLVs
       M    -    0-n   TRILL Neighbor TLV (see Section 8.2.1)
       -    M     1    Three-Way Handshake TLV
       O    O    0-n   Protocols Supported TLV that MUST list the TRILL
                       NLPID (0xC0) [RFC6328]
       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.

8.2 Transmitting TRILL Hellos

   TRILL Hellos are sent with the same timing as Layer 3 IS-IS Hellos
   [IS-IS]; however, no Hellos are sent if a port is in the Suspended or
   Down states or if the port is disabled.

   TRILL Hello PDUs SHOULD NOT be padded and MUST NOT be sent exceeding
   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 Section 8.1 for mandatory and some optional Hello TLV contents.

8.2.1 TRILL Neighbor TLVs

   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

D. Eastlake, et al                                             [Page 29]
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   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 Section of [RFC6325], 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
   TLV's 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 Section 6).

   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.

8.3 Receiving TRILL Hellos

   Assuming a packet has the L2-IS-IS Ethertype and, if received on
   Ethernet, the All-IS-IS-RBridges multicast address, 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

D. Eastlake, et al                                             [Page 30]
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         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 [RFC6326bis], it is discarded.

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

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

   If none of the rules in the list above causes the packet to be
   discarded and it 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 Section 3.3.

D. Eastlake, et al                                             [Page 31]
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9. Multiple Ports on the Same Broadcast Link

   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.

   The DRB election is port-based (see Section 4) 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 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. Section 4.4.4 of [RFC6325]
   describes when it is safe to do so.

D. Eastlake, et al                                             [Page 32]
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10. Security Considerations

   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 Section 1.
   It does not change the security considerations of the TRILL base
   protocol that are in Section 6 of [RFC6325].

   See [RFCbfd] for security considerations for BFD whose use in
   connection with TRILL adjacency is discussed in this document,
   particularly Section 6.

11. IANA Considerations

   This document requires no IANA Actions. RFC Editor: Please remove
   this section before publication.

D. Eastlake, et al                                             [Page 33]
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Appendix A: Changes from [RFC6327]

   This document has the following changes from [RFC6327]. It obsoletes

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

   2. Incorporates the updates to [RFC6327] from [RFCclear].

   3. The bulk of [RFC6327] 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 [RFC6361] or IP [TrillIP].
      This replacement document generalizes [RFC6327] to cover such link

   4. Includes a specification of one-hop BFD session establishment in
      connection with adjacency.

   5. Numerous editorial changes.

D. Eastlake, et al                                             [Page 34]
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Appendix B: Changes to [RFC6325]

   Section 2 of this document replaces Section 4.4.1 of [RFC6325] and
   Section 8 of this document replaces Section 4.4.2 of [RFC6325] except
   for Section  The changes in [RFC6325] made by this document

      -  Prohibiting the sending of TRILL Hellos out a port while it is
         in the Suspended state and the specification of the Suspended
         state. ([RFC6325] specifies Hellos to be sent with the same
         timing as [IS-IS].)

      -  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  Section 4.4.2 of

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

D. Eastlake, et al                                             [Page 35]
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Appendix Z: Change History

   RFC Editor Note: Please delete this section before publication.

From -00 to -01

   Improvements to Section 6 related to the rare case of transient
   nickname conflict.

   Numerous editorial improvements and typos fixes.

   Changes to Acknowledgements and author list.

From -01 to -02

   Editorial improvements.

   One editorial correction: deleting "or received in TRILL Hellos" from
   Section 6. (If a new RBridge joins a campus on a link with an MTU
   lower than the campus MTU (Sz), there might be a problem where LSPs
   wouldn't get through the link and, since LSPs carry the
   originatingLSPBufferSize, this might never get fixed. However, this
   has been fixed in draft-ietf-isis-rfc6326bis by requiring
   originatingLSPBufferSize be in LSP fragment zero and restricting the
   size of LSP fragment zero to 1470 bytes so it will get through. This
   snippet of text is left over from when it was thought to fix this in
   a different way by including originatingLSPBufferSize in Hellos.)

   Add one Acknowledgement.

From -02 to -03

   Editorial improvements resulting from GENART review. Add GENART
   reviewer to acknowledgements.

From -03 to -04

   Clarify what this document changes in RFC 6325 and add Appendix B.

   Minor editorial fixes.

D. Eastlake, et al                                             [Page 36]
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Normative References

   [IS-IS] - ISO/IEC 10589:2002, Second Edition, "Intermediate System to
         Intermediate System Intra-Domain Routing Exchange Protocol for
         use in Conjunction with the Protocol for Providing the
         Connectionless-mode Network Service (ISO 8473)", 2002.

   [RFC1195] - Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
         dual environments", RFC 1195, December 1990.

   [RFC2119] - Bradner, S., "Key words for use in RFCs to Indicate
         Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5120] - Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
         Topology (MT) Routing in Intermediate System to Intermediate
         Systems (IS-ISs)", RFC 5120, February 2008.

   [RFC6213] - Hopps, C. and L. Ginsberg, "IS-IS BFD-Enabled TLV", RFC
         6213, April 2011.

   [RFC6325] - Perlman, R., D. Eastlake, D. Dutt, S. Gai, and A.
         Ghanwani, "RBridges: Base Protocol Specification", RFC 6325,
         July 2011.

   [RFC6328] - Eastlake 3rd, D., "IANA Considerations for Network Layer
         Protocol Identifiers", BCP 164, RFC 6328, July 2011.

   [RFCbfd] - Manral, V., D. Eastlake, D. Ward, A. Banerjee, "TRILL
         (Transparent Interconnetion of Lots of Links): Bidirectional
         Forwarding Detection (BFD) Support", draft-ietf-trill-rbridge-
         bfd, in RFC Editor's queue.

   [RFCchannel] - D. Eastlake, V. Manral, Y. Li, S. Aldrin, D. Ward,
         "TRILL: RBridge Channel Support", draft-ietf-trill-rbridge-
         channel-08.txt, in RFC Editor's queue.

   [RFCclear] - Eastlake, D., M. Zhang, A. Ghanwani, V. Manral, A.
         Banerjee, "TRILL: Clarifications, Corrections, and Updates "
         draft-ietf-trill-clear-correct, in RFC Editor's queue.

   [RFC6326bis] - Eastlake, D., Banerjee, A., Dutt, D., Perlman, R., and
         A. Ghanwani, "TRILL Use of IS-IS", draft-ietf-isis-rfc6326bis,
         work in progress.

Informative References

   [802.1AX] - "IEEE Standard for Local and metropolitan area networks /
         Link Aggregation", 802.1AX-2008, 1 January 2008.

D. Eastlake, et al                                             [Page 37]
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   [802.1Q] - "IEEE Standard for Local and metropolitan area networks /
         Media Access Control (MAC) Bridges and Virtual Bridge Local
         Area Networks", 802.1Q-2011, 31 August 2011.

   [FCoE] - From discussion of "FCoE Max Size" generated
         from T11/09-251v1, 04/27/2009, "FCoE frame or FCoE PDU".

   [MT] - D. Eastlake, M. Zhang, A. Banerjee, V. Manral, "TRILL: Multi-
         Topology", draft-eastlake-trill-multi-topology, work in

   [RFC3719] - Parker, J., Ed., "Recommendations for Interoperable
         Networks using Intermediate System to Intermediate System (IS-
         IS)", February 2004.

   [RFC6327] - Eastlake 3rd, D., Perlman, R., Ghanwani, A., Dutt, D.,
         and V. Manral, "Routing Bridges (RBridges): Adjacency", RFC
         6327, July 2011.

   [RFC6361] - Carlson, J. and D. Eastlake 3rd, "PPP Transparent
         Interconnection of Lots of Links (TRILL) Protocol Control
         Protocol", RFC 6361, August 2011.

   [RFC6439] - Perlman, R., Eastlake, D., Li, Y., Banerjee, A., and F.
         Hu, "Routing Bridges (RBridges): Appointed Forwarders", RFC
         6439, November 2011.

   [RFCfgl] - D. Eastlake, M. Zhang, P. Agarwal, R. Perlman, D. Dutt,
         "TRILL (Transparent Interconnection of Lots of Links): Fine-
         Grained Labeling", draft-ietf-trill-fine-labeling, in RFC
         Editor's queue.

   [ESADI] - Zhai, H., F. Hu, R. Perlman, D. Eastlake, O. Stokes, "
         TRILL (Transparent Interconnection of Lots of Links): ESADI
         (End Station Address Distribution Information) Protocol",
         draft-ietf-trill-esadi, work in progress.

   [TrillIP] - M. Wasserman, D. Eastlake, D. Zhang, "Transparent
         Interconnection of Lots of Links (TRILL) over IP", draft-mrw-
         trill-over-ip, work in progress.

D. Eastlake, et al                                             [Page 38]
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   The suggestions and comments of the following are gratefully

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

   The authors of [RFC6327] included Dinesh Dutt and the
   acknowledgements section of [RFC6327] includes the following listed
   in alphabetic order: Jari Arkko, Ayan Banerjee, Les Ginsberg, Sujay
   Gupta, David Harrington, Pete McCann, Erik Nordmark, and Mike Shand.

   The document was prepared in raw nroff. All macros used were defined
   within the source file.

D. Eastlake, et al                                             [Page 39]
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Authors' Addresses

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

      Phone: +1-508-333-2270

      Radia Perlman
      Intel Labs
      2200 Mission College Blvd.
      Santa Clara, CA 95054-1549 USA

      Phone: +1-408-765-8080

      Anoop Ghanwani
      350 Holger Way
      San Jose, CA 95134 USA

      Phone: +1-408-571-3500

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


      Vishwas Manral
      Hewlett Packard Co.
      19111 Pruneridge Ave,
      Cupertino, CA 95014 USA

      Phone: +1-408-447-1497

D. Eastlake, et al                                             [Page 40]
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D. Eastlake, et al                                             [Page 41]