<pre>Internet Engineering Task Force (IETF)                   D. Eastlake 3rd
Request for Comments: 7780                                      M. Zhang
Obsoletes: <a href="./rfc7180">7180</a>                                                   Huawei
Updates: <a href="./rfc6325">6325</a>, <a href="./rfc7177">7177</a>, <a href="./rfc7179">7179</a>                                     R. Perlman
Category: Standards Track                                            EMC
ISSN: 2070-1721                                              A. Banerjee
                                                                   Cisco
                                                             A. Ghanwani
                                                                    Dell
                                                                S. Gupta
                                                             IP Infusion
                                                           February 2016


         <span class="h1">Transparent Interconnection of Lots of Links (TRILL):</span>
                <span class="h1">Clarifications, Corrections, and Updates</span>

Abstract

   Since the publication of the TRILL (Transparent Interconnection of
   Lots of Links) base protocol in 2011, active development and
   deployment of TRILL have revealed errata in <a href="./rfc6325">RFC 6325</a> and areas that
   could use clarifications or updates.  <a href="./rfc7177">RFC 7177</a>, <a href="./rfc7357">RFC 7357</a>, and an
   intended replacement of <a href="./rfc6439">RFC 6439</a> provide clarifications and updates
   with respect to adjacency, the TRILL ESADI (End Station Address
   Distribution Information) protocol, and Appointed Forwarders,
   respectively.  This document provides other known clarifications,
   corrections, and updates.  It obsoletes <a href="./rfc7180">RFC 7180</a> (the previous "TRILL
   clarifications, corrections, and updates" RFC), and it updates RFCs
   6325, 7177, and 7179.

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/rfc7780">http://www.rfc-editor.org/info/rfc7780</a>.







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Copyright Notice

   Copyright (c) 2016 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="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


Table of Contents

   <a href="#section-1">1</a>. Introduction (Changed) ..........................................<a href="#page-5">5</a>
      <a href="#section-1.1">1.1</a>. Precedence (Changed) .......................................<a href="#page-5">5</a>
      <a href="#section-1.2">1.2</a>. Changes That Are Not Backward Compatible (Unchanged) .......<a href="#page-6">6</a>
      <a href="#section-1.3">1.3</a>. Terminology and Acronyms (Changed) .........................<a href="#page-6">6</a>
   <a href="#section-2">2</a>. Overloaded and/or Unreachable RBridges (Unchanged) ..............<a href="#page-7">7</a>
      <a href="#section-2.1">2.1</a>. Reachability ...............................................<a href="#page-8">8</a>
      <a href="#section-2.2">2.2</a>. Distribution Trees .........................................<a href="#page-8">8</a>
      <a href="#section-2.3">2.3</a>. Overloaded Receipt of TRILL Data Packets ...................<a href="#page-9">9</a>
           <a href="#section-2.3.1">2.3.1</a>. Known Unicast Receipt ...............................<a href="#page-9">9</a>
           <a href="#section-2.3.2">2.3.2</a>. Multi-Destination Receipt ...........................<a href="#page-9">9</a>
      <a href="#section-2.4">2.4</a>. Overloaded Origination of TRILL Data Packets ...............<a href="#page-9">9</a>
           <a href="#section-2.4.1">2.4.1</a>. Known Unicast Origination ..........................<a href="#page-10">10</a>
           <a href="#section-2.4.2">2.4.2</a>. Multi-Destination Origination ......................<a href="#page-10">10</a>
                  <a href="#section-2.4.2.1">2.4.2.1</a>. An Example Network ........................<a href="#page-10">10</a>
                  <a href="#section-2.4.2.2">2.4.2.2</a>. Indicating OOMF Support ...................<a href="#page-11">11</a>
                  <a href="#section-2.4.2.3">2.4.2.3</a>. Using OOMF Service ........................<a href="#page-11">11</a>
   <a href="#section-3">3</a>. Distribution Trees and RPF Check (Changed) .....................<a href="#page-12">12</a>
      <a href="#section-3.1">3.1</a>. Number of Distribution Trees (Unchanged) ..................<a href="#page-12">12</a>
      <a href="#section-3.2">3.2</a>. Distribution Tree Update Clarification (Unchanged) ........<a href="#page-12">12</a>
      <a href="#section-3.3">3.3</a>. Multicast Pruning Based on IP Address (Unchanged) .........<a href="#page-13">13</a>
      <a href="#section-3.4">3.4</a>. Numbering of Distribution Trees (Unchanged) ...............<a href="#page-13">13</a>
      <a href="#section-3.5">3.5</a>. Link Cost Directionality (Unchanged) ......................<a href="#page-13">13</a>
      <a href="#section-3.6">3.6</a>. Alternative RPF Check (New) ...............................<a href="#page-14">14</a>
           <a href="#section-3.6.1">3.6.1</a>. Example of the Potential Problem ...................<a href="#page-14">14</a>
           <a href="#section-3.6.2">3.6.2</a>. Solution and Discussion ............................<a href="#page-15">15</a>
   <a href="#section-4">4</a>. Nickname Selection (Unchanged) .................................<a href="#page-17">17</a>
   <a href="#section-5">5</a>. MTU (Maximum Transmission Unit) (Unchanged) ....................<a href="#page-18">18</a>
      <a href="#section-5.1">5.1</a>. MTU-Related Errata in <a href="./rfc6325">RFC 6325</a> ............................<a href="#page-19">19</a>
           <a href="#section-5.1.1">5.1.1</a>. MTU PDU Addressing .................................<a href="#page-19">19</a>
           <a href="#section-5.1.2">5.1.2</a>. MTU PDU Processing .................................<a href="#page-20">20</a>
           <a href="#section-5.1.3">5.1.3</a>. MTU Testing ........................................<a href="#page-20">20</a>
      <a href="#section-5.2">5.2</a>. Ethernet MTU Values .......................................<a href="#page-20">20</a>
   <a href="#section-6">6</a>. TRILL Port Modes (Unchanged) ...................................<a href="#page-21">21</a>
   <a href="#section-7">7</a>. The CFI/DEI Bit (Unchanged) ....................................<a href="#page-22">22</a>
   <a href="#section-8">8</a>. Other IS-IS Considerations (Changed) ...........................<a href="#page-23">23</a>
      <a href="#section-8.1">8.1</a>. E-L1FS Support (New) ......................................<a href="#page-24">24</a>
           <a href="#section-8.1.1">8.1.1</a>. Backward Compatibility .............................<a href="#page-24">24</a>
           <a href="#section-8.1.2">8.1.2</a>. E-L1FS Use for Existing (Sub-)TLVs .................<a href="#page-25">25</a>
      <a href="#section-8.2">8.2</a>. Control Packet Priorities (New) ...........................<a href="#page-26">26</a>
      <a href="#section-8.3">8.3</a>. Unknown PDUs (New) ........................................<a href="#page-27">27</a>
      <a href="#section-8.4">8.4</a>. Nickname Flags APPsub-TLV (New) ...........................<a href="#page-27">27</a>
      <a href="#section-8.5">8.5</a>. Graceful Restart (Unchanged) ..............................<a href="#page-29">29</a>
      <a href="#section-8.6">8.6</a>. Purge Originator Identification (New) .....................<a href="#page-29">29</a>
   <a href="#section-9">9</a>. Updates to <a href="./rfc7177">RFC 7177</a> (Adjacency) (Changed) ......................<a href="#page-30">30</a>





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   <a href="#section-10">10</a>. TRILL Header Update (New) .....................................<a href="#page-31">31</a>
      <a href="#section-10.1">10.1</a>. Color Bit ................................................<a href="#page-32">32</a>
      <a href="#section-10.2">10.2</a>. Flags Word Changes (Update to <a href="./rfc7179">RFC 7179</a>) ..................<a href="#page-32">32</a>
           <a href="#section-10.2.1">10.2.1</a>. Extended Hop Count ................................<a href="#page-32">32</a>
                  <a href="#section-10.2.1.1">10.2.1.1</a>. Advertising Support ......................<a href="#page-33">33</a>
                  <a href="#section-10.2.1.2">10.2.1.2</a>. Ingress Behavior .........................<a href="#page-33">33</a>
                  <a href="#section-10.2.1.3">10.2.1.3</a>. Transit Behavior .........................<a href="#page-33">33</a>
                  <a href="#section-10.2.1.4">10.2.1.4</a>. Egress Behavior ..........................<a href="#page-34">34</a>
           <a href="#section-10.2.2">10.2.2</a>. Extended Color Field ..............................<a href="#page-34">34</a>
      <a href="#section-10.3">10.3</a>. Updated Flags Word Summary ...............................<a href="#page-35">35</a>
   <a href="#section-11">11</a>. Appointed Forwarder Status Lost Counter (New) .................<a href="#page-35">35</a>
   <a href="#section-12">12</a>. IANA Considerations (Changed) .................................<a href="#page-37">37</a>
      <a href="#section-12.1">12.1</a>. Previously Completed IANA Actions (Unchanged) ............<a href="#page-37">37</a>
      <a href="#section-12.2">12.2</a>. New IANA Actions (New) ...................................<a href="#page-37">37</a>
           <a href="#section-12.2.1">12.2.1</a>. Reference Updated .................................<a href="#page-37">37</a>
           <a href="#section-12.2.2">12.2.2</a>. The "E" Capability Bit ............................<a href="#page-37">37</a>
           <a href="#section-12.2.3">12.2.3</a>. NickFlags APPsub-TLV Number and Registry ..........<a href="#page-38">38</a>
           <a href="#section-12.2.4">12.2.4</a>. Updated TRILL Extended Header Flags ...............<a href="#page-38">38</a>
           <a href="#section-12.2.5">12.2.5</a>. TRILL-VER Sub-TLV Capability Flags ................<a href="#page-39">39</a>
           <a href="#section-12.2.6">12.2.6</a>. Example Nicknames .................................<a href="#page-39">39</a>
   <a href="#section-13">13</a>. Security Considerations (Changed) .............................<a href="#page-39">39</a>
   <a href="#section-14">14</a>. References ....................................................<a href="#page-40">40</a>
      <a href="#section-14.1">14.1</a>. Normative References .....................................<a href="#page-40">40</a>
      <a href="#section-14.2">14.2</a>. Informative References ...................................<a href="#page-42">42</a>
   <a href="#appendix-A">Appendix A</a>. Life Cycle of a TRILL Switch Port (New) ...............<a href="#page-45">45</a>
   <a href="#appendix-B">Appendix B</a>. Example TRILL PDUs (New) ..............................<a href="#page-48">48</a>
      <a href="#appendix-B.1">B.1</a>. LAN Hello over Ethernet ...................................<a href="#page-48">48</a>
      <a href="#appendix-B.2">B.2</a>. LSP over PPP ..............................................<a href="#page-50">50</a>
      <a href="#appendix-B.3">B.3</a>. TRILL Data over Ethernet ..................................<a href="#page-51">51</a>
      <a href="#appendix-B.4">B.4</a>. TRILL Data over PPP .......................................<a href="#page-52">52</a>
   <a href="#appendix-C">Appendix C</a>. Changes to Previous RFCs (New) ........................<a href="#page-53">53</a>
      <a href="#appendix-C.1">C.1</a>. Changes to Obsoleted <a href="./rfc7180">RFC 7180</a> .............................<a href="#page-53">53</a>
         <a href="#appendix-C.1.1">C.1.1</a>. Changes ..............................................<a href="#page-53">53</a>
         <a href="#appendix-C.1.2">C.1.2</a>. Additions ............................................<a href="#page-53">53</a>
         <a href="#appendix-C.1.3">C.1.3</a>. Deletions ............................................<a href="#page-54">54</a>
      <a href="#appendix-C.2">C.2</a>. Changes to <a href="./rfc6325">RFC 6325</a> .......................................<a href="#page-55">55</a>
      <a href="#appendix-C.3">C.3</a>. Changes to <a href="./rfc7177">RFC 7177</a> .......................................<a href="#page-55">55</a>
      <a href="#appendix-C.4">C.4</a>. Changes to <a href="./rfc7179">RFC 7179</a> .......................................<a href="#page-55">55</a>
   Acknowledgments ...................................................<a href="#page-56">56</a>
   Authors' Addresses ................................................<a href="#page-56">56</a>











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

   Since the TRILL base protocol [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] was published in 2011, active
   development and deployment of TRILL have revealed errors in the
   specification [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] and several areas that could use
   clarifications or updates.

   [<a href="./rfc7177" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Adjacency&quot;">RFC7177</a>], [<a href="./rfc7357" title="&quot;Transparent Interconnection of Lots of Links (TRILL): End Station Address Distribution Information (ESADI) Protocol&quot;">RFC7357</a>], and [<a href="#ref-RFC6439bis">RFC6439bis</a>] provide clarifications and
   updates with respect to adjacency, the TRILL ESADI (End Station
   Address Distribution Information) protocol, and Appointed Forwarders,
   respectively.  This document provides other known clarifications,
   corrections, and updates to [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>], [<a href="./rfc7177" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Adjacency&quot;">RFC7177</a>], and [<a href="./rfc7179" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Header Extension&quot;">RFC7179</a>].
   This document obsoletes [<a href="./rfc7180" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Clarifications, Corrections, and Updates&quot;">RFC7180</a>] (the previous TRILL
   "clarifications, corrections, and updates" document), updates
   [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>], updates [<a href="./rfc7177" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Adjacency&quot;">RFC7177</a>] as described in <a href="#section-9">Section 9</a>, and updates
   [<a href="./rfc7179" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Header Extension&quot;">RFC7179</a>] as described in Sections <a href="#section-10.2">10.2</a> and <a href="#section-10.3">10.3</a>.  The changes to
   these RFCs are summarized in <a href="#appendix-C">Appendix C</a>.

   Sections of this document are annotated as to whether they are "New"
   technical material, material that has been technically "Changed", or
   material that is technically "Unchanged", by the appearance of one of
   these three words in parentheses at the end of the section header.  A
   section with only editorial changes is annotated as "(Unchanged)".
   If no such notation appears, then the first notation encountered on
   going to successively higher-level section headers (those with
   shorter section numbers) applies.  <a href="#appendix-C">Appendix C</a> describes changes,
   summarizes material added, and lists material deleted.

<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  Precedence (Changed)</span>

   In the event of any conflicts between this document and [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>],
   [<a href="./rfc7177" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Adjacency&quot;">RFC7177</a>], or [<a href="./rfc7179" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Header Extension&quot;">RFC7179</a>], this document takes precedence.

   In addition, <a href="./rfc6325#section-1.2">Section&nbsp;1.2 of [RFC6325]</a> ("Normative Content and
   Precedence") is updated to provide a more complete precedence
   ordering of the sections of [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>], as shown below, where sections
   to the left take precedence over sections to their right.  There are
   no known conflicts between these sections; however, Sections <a href="#section-1">1</a> and <a href="#section-2">2</a>
   are less detailed and do not mention every corner case, while
   subsequent sections of [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] are more detailed.  This precedence
   is specified as a fallback in case some conflict is found in the
   future.

                       4 &gt; 3 &gt; 7 &gt; 5 &gt; 2 &gt; 6 &gt; 1







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<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>.  Changes That Are Not Backward Compatible (Unchanged)</span>

   The change made by <a href="#section-3.4">Section 3.4</a> below (unchanged from <a href="./rfc7180#section-3.4">Section&nbsp;3.4 of
   [RFC7180]</a>) is not backward compatible with [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] but has
   nevertheless been adopted to reduce distribution tree changes
   resulting from topology changes.

   Several other changes herein that are fixes to errata for [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>]
   -- [<a href="#ref-Err3002" title="Erratum ID 3002">Err3002</a>], [<a href="#ref-Err3003" title="Erratum ID 3003">Err3003</a>], [<a href="#ref-Err3004" title="Erratum ID 3004">Err3004</a>], [<a href="#ref-Err3052" title="Erratum ID 3052">Err3052</a>], [<a href="#ref-Err3053" title="Erratum ID 3053">Err3053</a>], and
   [<a href="#ref-Err3508" title="Erratum ID 3508">Err3508</a>] -- may not be backward compatible with previous
   implementations that conformed to errors in the specification.

<span class="h3"><a class="selflink" id="section-1.3" href="#section-1.3">1.3</a>.  Terminology and Acronyms (Changed)</span>

   This document uses the acronyms defined in [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>], some of which
   are repeated below for convenience, along with some additional
   acronyms and terms, as follows:

   BFD - Bidirectional Forwarding Detection.

   Campus - A TRILL network consisting of TRILL switches, links, and
      possibly bridges bounded by end stations and IP routers.  For
      TRILL, there is no "academic" implication in the name "campus".

   CFI - Canonical Format Indicator [<a href="#ref-802" title="&quot;IEEE Standard for Local and Metropolitan Area Networks: Overview and Architecture&quot;">802</a>].

   CSNP - Complete Sequence Number PDU.

   DEI - Drop Eligibility Indicator [<a href="#ref-802.1Q-2014">802.1Q-2014</a>].

   FGL - Fine-Grained Labeling [<a href="./rfc7172" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Fine-Grained Labeling&quot;">RFC7172</a>].

   FS-LSP - Flooding Scope LSP.

   OOMF - Overload Originated Multi-destination Frame.

   P2P - Point-to-point.

   PDU - Protocol Data Unit.

   PSNP - Partial Sequence Number PDU.

   RBridge - Routing Bridge, an alternative name for a TRILL switch.

   RPFC - Reverse Path Forwarding Check.

   SNPA - Subnetwork Point of Attachment (for example, Media Access
      Control (MAC) address).



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   ToS - Type of Service.

   TRILL - Transparent Interconnection of Lots of Links or Tunneled
      Routing in the Link Layer.

   TRILL switch - A device implementing the TRILL protocol.  An
      alternative name for an RBridge.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "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>].

   In this document, a "packet" usually refers to a TRILL Data packet or
   TRILL IS-IS packet received from or sent to a TRILL switch, while a
   "frame" usually refers to a native frame being received from or sent
   to an end station.  (The word "frame" also occurs in other contexts,
   such as the "Frame Check Sequence" that is at the end of Ethernet
   transmissions.)

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Overloaded and/or Unreachable RBridges (Unchanged)</span>

   In this section, the term "neighbor" refers only to actual RBridges
   and ignores pseudonodes.

   RBridges may be in overload, as indicated by the [<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>] overload
   flag in their LSPs (Link State PDUs).  This means that either (1)
   they are incapable of holding the entire link-state database and thus
   do not have a view of the entire topology or (2) they have been
   configured to have the overload bit set.  Although networks should be
   engineered to avoid actual link-state overload, it might occur under
   various circumstances -- for example, if a very large campus included
   one or more low-end TRILL switches.

   It is a common operational practice to set the overload bit in an
   [<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>] router (such as a TRILL switch) when performing maintenance
   on that router that might affect its ability to correctly forward
   packets; this will usually leave the router reachable for maintenance
   traffic, but transit traffic will not be routed through it.  (Also,
   in some cases, TRILL provides for setting the overload bit in the
   pseudonode of a link to stop TRILL Data traffic on an access link
   (see <a href="./rfc6325#section-4.9.1">Section&nbsp;4.9.1 of [RFC6325]</a>).)

   [<a id="ref-IS-IS">IS-IS</a>] and TRILL make a reasonable effort to do what they can, even
   if some TRILL switches/routers are in overload.  They can do
   reasonably well if a few scattered nodes are in overload.  However,
   actual least-cost paths are no longer assured if any TRILL switches
   are in overload.



<span class="grey">Eastlake, et al.             Standards Track                    [Page 7]</span></pre>
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<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


   For the effect of overload on the appointment of forwarders, see
   [<a href="#ref-RFC6439bis">RFC6439bis</a>].

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

   Packets are not least-cost routed through an overloaded TRILL switch,
   although they may originate or terminate at an overloaded TRILL
   switch.  In addition, packets will not be least-cost routed over
   links with cost 2**24 - 1 [<a href="./rfc5305" title="&quot;IS-IS Extensions for Traffic Engineering&quot;">RFC5305</a>]; such links are reserved for
   traffic-engineered packets, the handling of which is beyond the scope
   of this document.

   As a result, a portion of the campus may be unreachable for
   least-cost routed TRILL Data because all paths to it would be either
   through a link with cost 2**24 - 1 or through an overloaded RBridge.
   For example, an RBridge (TRILL switch) RB1 is not reachable by TRILL
   Data if all of its neighbors are connected to RB1 by links with cost
   2**24 - 1.  Such RBridges are called "data unreachable".

   The link-state database at an RBridge -- for example, RB1 -- can also
   contain information on TRILL switches that are unreachable by IS-IS
   link-state flooding due to link or RBridge failures.  When such
   failures partition the campus, the TRILL switches adjacent to the
   failure and on the same side of the failure as RB1 will update their
   LSPs to show the lack of connectivity, and RB1 will receive those
   updates.  As a result, RB1 will be aware of the partition.  Nodes on
   the far side of the partition are both IS-IS unreachable and data
   unreachable from RB1.  However, LSPs held by RB1 for TRILL switches
   on the far side of the failure will not be updated and may stay
   around until they time out, which could be tens of minutes or longer.
   (The default in [<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 twenty minutes.)

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

   An RBridge in overload cannot be trusted to correctly calculate
   distribution trees or correctly perform the RPFC (Reverse Path
   Forwarding Check).  Therefore, it cannot be trusted to forward
   multi-destination TRILL Data packets.  It can only appear as a leaf
   node in a TRILL multi-destination distribution tree.  Furthermore, if
   all the immediate neighbors of an RBridge are overloaded, then it is
   omitted from all trees in the campus and is unreachable by
   multi-destination packets.

   When an RBridge determines what nicknames to use as the roots of the
   distribution trees it calculates, it MUST ignore all nicknames held
   by TRILL switches that are in overload or are data unreachable.  When
   calculating RPFCs for multi-destination packets, an RBridge such as
   RB1 MAY, to avoid calculating unnecessary RPFC state information,



<span class="grey">Eastlake, et al.             Standards Track                    [Page 8]</span></pre>
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<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


   ignore any trees that cannot reach RB1, even if other RBridges list
   those trees as trees that other TRILL switches might use.  (However,
   see <a href="#section-3">Section 3</a>.)

<span class="h3"><a class="selflink" id="section-2.3" href="#section-2.3">2.3</a>.  Overloaded Receipt of TRILL Data Packets</span>

   The receipt of TRILL Data packets by overloaded RBridge RB2 is
   discussed in the subsections below.  In all cases, the normal
   Hop Count decrement is performed, and the TRILL Data packets are
   discarded if the result is less than one or if the Egress Nickname is
   illegal.

<span class="h4"><a class="selflink" id="section-2.3.1" href="#section-2.3.1">2.3.1</a>.  Known Unicast Receipt</span>

   RB2 will not usually receive unicast TRILL Data packets unless it is
   the egress, in which case it egresses and delivers the data normally.
   If RB2 receives a unicast TRILL Data packet for which it is not the
   egress, perhaps because a neighbor does not yet know it is in
   overload, RB2 MUST NOT discard the packet because the egress is an
   unknown nickname, as it might not know about all nicknames due to its
   overloaded condition.  If any neighbor other than the neighbor from
   which it received the packet is not overloaded, it MUST attempt to
   forward the packet to one of those neighbors selected at random
   [<a href="./rfc4086" title="&quot;Randomness Requirements for Security&quot;">RFC4086</a>].  If there is no such neighbor, the packet is discarded.

<span class="h4"><a class="selflink" id="section-2.3.2" href="#section-2.3.2">2.3.2</a>.  Multi-Destination Receipt</span>

   If RB2 in overload receives a multi-destination TRILL Data packet,
   RB2 MUST NOT apply an RPFC because, due to overload, it might not do
   so correctly.  RB2 egresses and delivers the frame locally where it
   is Appointed Forwarder for the frame's VLAN (or, if the packet is
   FGL, for the VLAN that FGL maps to at the port), subject to any
   multicast pruning.  But because, as stated above, RB2 can only be the
   leaf of a distribution tree, it MUST NOT forward a multi-destination
   TRILL Data packet (except as an egressed native frame where RB2 is
   Appointed Forwarder).

<span class="h3"><a class="selflink" id="section-2.4" href="#section-2.4">2.4</a>.  Overloaded Origination of TRILL Data Packets</span>

   Overloaded origination of unicast TRILL Data packets with known
   egress and of multi-destination packets is discussed in the
   subsections below.









<span class="grey">Eastlake, et al.             Standards Track                    [Page 9]</span></pre>
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<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


<span class="h4"><a class="selflink" id="section-2.4.1" href="#section-2.4.1">2.4.1</a>.  Known Unicast Origination</span>

   When RB2, an overloaded RBridge, ingresses or creates a known
   destination unicast data packet, it delivers it locally if the
   destination is local.  Otherwise, RB2 unicasts it to any neighbor
   TRILL switch that is not overloaded.  It MAY use what routing
   information it has to help select the neighbor.

<span class="h4"><a class="selflink" id="section-2.4.2" href="#section-2.4.2">2.4.2</a>.  Multi-Destination Origination</span>

   Overloaded RBridge RB2 ingressing or creating a multi-destination
   data packet presents a more complex scenario than that of the known
   unicast case, as discussed below.

<span class="h5"><a class="selflink" id="section-2.4.2.1" href="#section-2.4.2.1">2.4.2.1</a>.  An Example Network</span>

   For example, consider the network diagram below in which, for
   simplicity, end stations and any bridges are not shown.  There is one
   distribution tree of which RB4 is the root, as represented by double
   lines.  Only RBridge RB2 is overloaded.

            +-----+    +-----+     +-----+     +-----+
            | RB7 +====+ RB5 +=====+ RB3 +=====+ RB1 |
            +-----+    +--+--+     +-++--+     +--+--+
                          |          ||           |
                      +---+---+      ||           |
               +------+RB2(ov)|======++           |
               |      +-------+      ||           |
               |                     ||           |
            +--+--+    +-----+   ++==++=++     +--+--+
            | RB8 +====+ RB6 +===++ RB4 ++=====+ RB9 |
            +-----+    +-----+   ++=====++     +-----+

   Since RB2 is overloaded, it does not know what the distribution tree
   or trees are for the network.  Thus, there is no way it can provide
   normal TRILL Data service for multi-destination native frames.  So,
   RB2 tunnels the frame in a TRILL Data packet to a neighbor that is
   not overloaded if it has such a neighbor that has signaled that it is
   willing to offer this service.  RBridges indicate this in their
   Hellos as described below.  This service is called the OOMF (Overload
   Originated Multi-destination Frame) service.

   - The multi-destination frame MUST NOT be locally distributed in
     native form at RB2, because this would cause the frame to be
     delivered twice.  Instead, it is tunneling to a neighbor as
     described in this section.  For example, if RB2 locally distributed
     a multicast native frame and then tunneled it to RB5, RB2 would get
     a copy of the frame when RB3 transmitted it as a TRILL Data packet



<span class="grey">Eastlake, et al.             Standards Track                   [Page 10]</span></pre>
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<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


     on the multi-access RB2-RB3-RB4 link.  Since RB2 would, in general,
     not be able to tell that this was a frame it had tunneled for
     distribution, RB2 would decapsulate it and locally distribute it a
     second time.

   - On the other hand, if there is no neighbor of RB2 offering RB2 the
     OOMF service, RB2 cannot tunnel the frame to a neighbor.  In this
     case, RB2 MUST locally distribute the frame where it is Appointed
     Forwarder for the frame's VLAN and optionally subject to multicast
     pruning.

<span class="h5"><a class="selflink" id="section-2.4.2.2" href="#section-2.4.2.2">2.4.2.2</a>.  Indicating OOMF Support</span>

   An RBridge RB3 indicates its willingness to offer the OOMF service to
   RB2 in the TRILL Neighbor TLV in RB3's TRILL Hellos by setting a bit
   associated with the SNPA (Subnetwork Point of Attachment, also known
   as MAC address) of RB2 on the link (see the IANA Considerations
   section).  Overloaded RBridge RB2 can only distribute
   multi-destination TRILL Data packets to the campus if a neighbor of
   RB2 not in overload offers RB2 the OOMF service.  If RB2 does not
   have OOMF service available to it, RB2 can still receive
   multi-destination packets from non-overloaded neighbors, and if RB2
   should originate or ingress such a frame, it distributes it locally
   in native form.

<span class="h5"><a class="selflink" id="section-2.4.2.3" href="#section-2.4.2.3">2.4.2.3</a>.  Using OOMF Service</span>

   If RB2 sees this OOMF (Overload Originated Multi-destination Frame)
   service advertised for it by any of its neighbors on any link to
   which RB2 connects, it selects one such neighbor by a means that is
   beyond the scope of this document.  Assuming that RB2 selects RB3 to
   handle multi-destination packets it originates, RB2 MUST advertise in
   its LSP that it might use any of the distribution trees that RB3
   advertises so that the RPFC will work in the rest of the campus.
   Thus, notwithstanding its overloaded state, RB2 MUST retain this
   information from RB3 LSPs, which it will receive, as it is directly
   connected to RB3.

   RB2 then encapsulates such frames as TRILL Data packets to RB3 as
   follows: "M" bit = 0; Hop Count = 2; Ingress Nickname = a nickname
   held by RB2; and, since RB2 cannot tell what distribution tree RB3
   will use, Egress Nickname = a special nickname indicating an OOMF
   packet (see the IANA Considerations section).  RB2 then unicasts this
   TRILL Data packet to RB3.  (Implementation of Item 4 in <a href="#section-4">Section 4</a>
   below provides reasonable assurance that, notwithstanding its
   overloaded state, the ingress nickname used by RB2 will be unique
   within at least the portion of the campus that is IS-IS reachable
   from RB2.)



<span class="grey">Eastlake, et al.             Standards Track                   [Page 11]</span></pre>
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<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


   On receipt of such a packet, RB3 does the following:

   - changes the Egress Nickname field to designate a distribution tree
     that RB3 normally uses,

   - sets the "M" bit to one,

   - changes the Hop Count to the value it would normally use if it were
     the ingress, and

   - forwards the TRILL Data packet on that tree.

   RB3 MAY rate-limit the number of packets for which it is providing
   this service by discarding some such packets from RB2.  The provision
   of even limited bandwidth for OOMFs by RB3, perhaps via the slow
   path, may be important to the bootstrapping of services at RB2 or at
   end stations connected to RB2, such as supporting DHCP and ARP/ND
   (Address Resolution Protocol / Neighbor Discovery).  (Everyone
   sometimes needs a little OOMF (pronounced "oomph") to get off the
   ground.)

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Distribution Trees and RPF Check (Changed)</span>

   Two corrections, a clarification, and two updates related to
   distribution trees appear in the subsections below, along with an
   alternative, stronger RPF (Reverse Path Forwarding) check.  See also
   <a href="#section-2.2">Section 2.2</a>.

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  Number of Distribution Trees (Unchanged)</span>

   In <a href="./rfc6325#section-4.5.2">[RFC6325], Section&nbsp;4.5.2</a>, page 56, point 2, fourth paragraph, the
   parenthetical "(up to the maximum of {j,k})" is incorrect [<a href="#ref-Err3052" title="Erratum ID 3052">Err3052</a>].
   It should read "(up to k if j is zero or the minimum of (j, k) if j
   is non-zero)".

<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>.  Distribution Tree Update Clarification (Unchanged)</span>

   When a link-state database change causes a change in the distribution
   tree(s), several possible types of change can occur.  If a tree root
   remains a tree root but the tree changes, then local forwarding and
   RPFC entries for that tree should be updated as soon as practical.
   Similarly, if a new nickname becomes a tree root, forwarding and RPFC
   entries for the new tree should be installed as soon as practical.
   However, if a nickname ceases to be a tree root and there is
   sufficient room in local tables, the forwarding and RPFC entries for
   the former tree MAY be retained so that any multi-destination TRILL
   Data packets already in flight on that tree have a higher probability
   of being delivered.



<span class="grey">Eastlake, et al.             Standards Track                   [Page 12]</span></pre>
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<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>.  Multicast Pruning Based on IP Address (Unchanged)</span>

   The TRILL base protocol specification [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] provides for, and
   recommends the pruning of, multi-destination packet distribution
   trees based on the location of IP multicast routers and listeners;
   however, multicast listening is identified by derived MAC addresses
   as communicated in the Group MAC Address sub-TLV [<a href="./rfc7176" title="&quot;Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS&quot;">RFC7176</a>].

   TRILL switches MAY communicate multicast listeners and prune
   distribution trees based on the actual IPv4 or IPv6 multicast
   addresses involved.  Additional Group Address sub-TLVs are provided
   in [<a href="./rfc7176" title="&quot;Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS&quot;">RFC7176</a>] to carry this information.  A TRILL switch that is only
   capable of pruning based on derived MAC addresses SHOULD calculate
   and use such derived MAC addresses from the multicast listener IPv4
   or IPv6 address information it receives.

<span class="h3"><a class="selflink" id="section-3.4" href="#section-3.4">3.4</a>.  Numbering of Distribution Trees (Unchanged)</span>

   <a href="./rfc6325#section-4.5.1">Section&nbsp;4.5.1 of [RFC6325]</a> specifies that, when building distribution
   tree number j, node (RBridge) N that has multiple possible parents in
   the tree is attached to possible parent number j mod p.  Trees are
   numbered starting with 1, but possible parents are numbered starting
   with 0.  As a result, if there are two trees and two possible
   parents, then in tree 1 parent 1 will be selected, and in tree 2
   parent 0 will be selected.

   This is changed so that the selected parent MUST be (j-1) mod p.  As
   a result, in the case above, tree 1 will select parent 0, and tree 2
   will select parent 1.  This change is not backward compatible with
   [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>].  If all RBridges in a campus do not determine distribution
   trees in the same way, then for most topologies, the RPFC will drop
   many multi-destination packets before they have been properly
   delivered.

<span class="h3"><a class="selflink" id="section-3.5" href="#section-3.5">3.5</a>.  Link Cost Directionality (Unchanged)</span>

   Distribution tree construction, like other least-cost aspects of
   TRILL, works even if link costs are asymmetric, so the cost of the
   hop from RB1 to RB2 is different from the cost of the hop from RB2 to
   RB1.  However, it is essential that all RBridges calculate the same
   distribution trees, and thus all must use either the cost away from
   the tree root or the cost towards the tree root.  The text in
   <a href="./rfc6325#section-4.5.1">Section&nbsp;4.5.1 of [RFC6325]</a> is incorrect, as documented in [<a href="#ref-Err3508" title="Erratum ID 3508">Err3508</a>].
   The text says:

      In other words, the set of potential parents for N, for the tree
      rooted at R, consists of those that give equally minimal cost
      paths from N to R and ...



<span class="grey">Eastlake, et al.             Standards Track                   [Page 13]</span></pre>
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<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


   but the text should say "from R to N":

      In other words, the set of potential parents for N, for the tree
      rooted at R, consists of those that give equally minimal cost
      paths from R to N and ...

<span class="h3"><a class="selflink" id="section-3.6" href="#section-3.6">3.6</a>.  Alternative RPF Check (New)</span>

   [<a id="ref-RFC6325">RFC6325</a>] mandates a Reverse Path Forwarding (RPF) check on
   multi-destination TRILL Data packets to avoid possible multiplication
   and/or looping of multi-destination traffic during TRILL campus
   topology transients.  This check is logically performed at each TRILL
   switch input port and determines whether it is arriving on the
   expected port based on where the packet started (the ingress
   nickname) and the tree on which it is being distributed.  If not, the
   packet is silently discarded.  This check is fine for point-to-point
   links; however, there are rare circumstances involving multi-access
   ("broadcast") links where a packet can be duplicated despite this
   RPF check and other checks performed by TRILL.

   <a href="#section-3.6.1">Section 3.6.1</a> gives an example of the potential problem, and
   <a href="#section-3.6.2">Section 3.6.2</a> specifies a solution.  This solution is an alternative,
   stronger RPF check that TRILL switches can implement in place of the
   RPF check discussed in [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>].

<span class="h4"><a class="selflink" id="section-3.6.1" href="#section-3.6.1">3.6.1</a>.  Example of the Potential Problem</span>

   Consider this network:

            F--A--B--C--o--D
                        |
                        E

   All the links except the link between C, D, and E are point-to-point
   links.  C, D, and E are connected over a broadcast link represented
   by the pseudonode "o".  For example, they could be connected by a
   bridged LAN.  (Bridged LANs are transparent to TRILL.)

   Although the choice of root is unimportant here, assume that D or F
   is chosen as the root of a distribution tree so that it is obvious
   that the tree looks just like the diagram above.










<span class="grey">Eastlake, et al.             Standards Track                   [Page 14]</span></pre>
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<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


   Now assume that a link comes up from A to the same bridged LAN.  The
   network then looks like this:

               +--------+
               |        |
            F--A--B--C--o--D
                        |
                        E

   Let's say the resulting tree in steady state includes all links
   except the B-C link.  After the network has converged, a packet that
   starts from F will go F-&gt;A.  Then A will send one copy on the A-B
   link and another copy into the bridged LAN from which it will be
   received by C and D.

   Now consider a transition stage where A and D have acted on the new
   LSPs and programmed their forwarding plane, while B and C have not
   yet done so.  This means that B and C both consider the link between
   them to still be part of the tree.  In this case, a packet that
   starts out from F and reaches A will be copied by A into the A-B link
   and to the bridged LAN.  D's RPF check says to accept packets on this
   tree coming from F over its port on the bridged LAN, so it gets
   accepted.  D is also adjacent to A on the tree, so the tree adjacency
   check, a separate check mandated by [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>], also passes.

   However, the packet that gets to B gets sent out by B to C.  C's RPF
   check still has the old state, and it thinks the packet is OK.  C
   sends the packet along the old tree, which sends the packet into the
   bridged LAN.  D receives one more packet, but the tree adjacency
   check passes at D because C is adjacent to D in the new tree as well.
   The RPF check also passes at D because D's port on the bridged LAN is
   OK for receiving packets from F.

   So, during this transient state, D gets duplicates of every
   multi-destination packet ingressed at F (unless the packet gets
   pruned) until B and C act on the new LSPs and program their
   forwarding tables.

<span class="h4"><a class="selflink" id="section-3.6.2" href="#section-3.6.2">3.6.2</a>.  Solution and Discussion</span>

   The problem stems from the RPF check described in [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] depending
   only on the port at which a TRILL Data packet is received, the
   ingress nickname, and the tree being used, that is, a check if
   {ingress nickname, tree, input port} is a valid combination according
   to the receiving TRILL switch's view of the campus topology.  A
   multi-access link actually has multiple adjacencies overlaid on one
   physical link, and to avoid the problem shown in <a href="#section-3.6.1">Section 3.6.1</a>, a
   stronger check is needed that includes the Layer 2 source address of



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<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


   the TRILL Data packet being received.  (TRILL is a Layer 3 protocol,
   and TRILL switches are true routers that logically strip the Layer 2
   header from any arriving TRILL Data packets and add the appropriate
   new Layer 2 header to any outgoing TRILL Data packet to get it to the
   next TRILL switch, so the Layer 2 source address in a TRILL Data
   packet identifies the immediately previous TRILL switch that
   forwarded the packet.)

   What is needed, instead of checking the validity of the triplet
   {ingress nickname, tree, input port}, is to check that the quadruplet
   {ingress nickname, source SNPA, tree, input port} is valid (where
   "source SNPA" (Subnetwork Point of Attachment) is the Outer.MacSA for
   an Ethernet link).  Although it is true that [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] also requires
   a check to ensure that a multi-destination TRILL Data packet is from
   a TRILL switch that is adjacent in the distribution tree being used,
   this check is separate from the RPF check, and these two independent
   checks are not as powerful as the single unified check for a valid
   quadruplet.

                  _______
                 /       \
               RB1 ------ o ----- RB2
                 \_______/

   However, this stronger RPF check is not without cost.  In the simple
   case of a multi-access link where each TRILL switch has only one port
   on the link, it merely increases the size of validity entries by
   adding the source SNPA (Outer.MacSA).  However, assume that some
   TRILL switch RB1 has multiple ports attached to a multi-access link.
   In the figure above, RB1 is shown with three ports on the
   multi-access link.  RB1 is permitted to load split multi-destination
   traffic it is sending into the multi-access link across those ports
   (<a href="./rfc6325#section-4.4.4">Section&nbsp;4.4.4 of [RFC6325]</a>).  Assume that RB2 is another TRILL
   switch on the link and RB2 is adjacent to RB1 in the distribution
   tree.  The number of validity quadruplets at RB2 for ingress
   nicknames whose multi-destination traffic would arrive through RB1 is
   multiplied by the number of ports RB1 has on the access link, because
   RB2 has to accept such traffic from any such ports.  Although such
   instances seem to be very rare in practice, the number of ports an
   RBridge has on a link could in principle be tens or even a hundred or
   more ports, vastly increasing the RPF check state at RB2 when this
   stronger RPF check is used.

   Another potential cost of the stronger RPF check is increased
   transient loss of multi-destination TRILL Data packets during a
   topology change.  For TRILL switch D, the new stronger RPF check is
   (tree-&gt;A, Outer.MacSA=A, ingress=A, arrival port=if1), while the old
   one was (tree-&gt;A, Outer.MacSA=C, ingress=A, arrival port=if1).



<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="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


   Suppose that both A and B have switched to the new tree for multicast
   forwarding but D has not updated its RPF check yet; the multicast
   packet will then be dropped at D's input port, because D still
   expects a packet from "Outer.MacSA=C".  But we do not have this
   packet loss issue if the weaker triplet check (tree-&gt;A, ingress=A,
   arrival port=if1) is used.  Thus, the stronger check can increase the
   RPF check discard of multi-destination packets during topology
   transients.

   Because of these potential costs, implementation of this stronger
   RPF check is optional.  The TRILL base protocol is updated to provide
   that TRILL switches MUST, for multi-destination packets, either
   implement the RPF and other checks as described in [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] or
   implement this stronger RPF check as a substitute for the [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>]
   RPF and tree adjacency checks.  There is no problem with a campus
   having a mixture of TRILL switches, some of which implement one of
   these RPF checks and some of which implement the other.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Nickname Selection (Unchanged)</span>

   Nickname selection is covered by <a href="./rfc6325#section-3.7.3">Section&nbsp;3.7.3 of [RFC6325]</a>.
   However, the following should be noted:

   1. The second sentence in the second bullet item in <a href="./rfc6325#section-3.7.3">Section&nbsp;3.7.3 of
      [RFC6325]</a> on page 25 is erroneous [<a href="#ref-Err3002" title="Erratum ID 3002">Err3002</a>] and is corrected as
      follows:

      o  The occurrence of "IS-IS ID (LAN ID)" is replaced with
         "priority".

      o  The occurrence of "IS-IS System ID" is replaced with "7-byte
         IS-IS ID (LAN ID)".

      The resulting corrected sentence in [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] reads as follows:

         If RB1 chooses nickname x, and RB1 discovers, through receipt
         of an LSP for RB2 at any later time, that RB2 has also chosen
         x, then the RBridge or pseudonode with the numerically higher
         priority keeps the nickname, or if there is a tie in priority,
         the RBridge with the numerically higher 7-byte IS-IS ID
         (LAN ID) keeps the nickname, and the other RBridge MUST select
         a new nickname.

   2. In examining the link-state database for nickname conflicts,
      nicknames held by IS-IS unreachable TRILL switches MUST be
      ignored, but nicknames held by IS-IS reachable TRILL switches
      MUST NOT be ignored even if they are data unreachable.




<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="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


   3. An RBridge may need to select a new nickname, either initially
      because it has none or because of a conflict.  When doing so, the
      RBridge MUST consider as available all nicknames that do not
      appear in its link-state database or that appear to be held by
      IS-IS unreachable TRILL switches; however, it SHOULD give
      preference to selecting new nicknames that do not appear to be
      held by any TRILL switch in the campus, reachable or unreachable,
      so as to minimize conflicts if IS-IS unreachable TRILL switches
      later become reachable.

   4. An RBridge, even after it has acquired a nickname for which there
      appears to be no conflicting claimant, MUST continue to monitor
      for conflicts with the nickname or nicknames it holds.  It does so
      by monitoring any received LSPs that should update its link-state
      database for any occurrence of any of its nicknames held with
      higher priority by some other TRILL switch that is IS-IS reachable
      from it.  If it finds such a conflict, it MUST select a new
      nickname, even when in overloaded state.  (It is possible to
      receive an LSP that should update the link-state database but does
      not do so due to overload.)

   5. In the very unlikely case that an RBridge is unable to obtain a
      nickname because all valid RBridge nicknames (0x0001 through
      0xFFBF inclusive) are in use with higher priority by IS-IS
      reachable TRILL switches, it will be unable to act as an ingress,
      egress, or tree root but will still be able to function as a
      transit TRILL switch.  Although it cannot be a tree root, such an
      RBridge is included in distribution trees computed for the campus
      unless all its neighbors are overloaded.  It would not be possible
      to send a unicast RBridge Channel message specifically to such a
      TRILL switch [<a href="./rfc7178" title="&quot;Transparent Interconnection of Lots of Links (TRILL): RBridge Channel Support&quot;">RFC7178</a>]; however, it will receive unicast RBridge
      Channel messages sent by a neighbor to the Any-RBridge egress
      nickname and will receive appropriate multi-destination RBridge
      Channel messages.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  MTU (Maximum Transmission Unit) (Unchanged)</span>

   MTU values in TRILL are derived from the originatingL1LSPBufferSize
   value communicated in the IS-IS originatingLSPBufferSize TLV [<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 campus-wide value Sz, as described in <a href="./rfc6325#section-4.3.1">Section&nbsp;4.3.1 of [RFC6325]</a>,
   is the minimum value of originatingL1LSPBufferSize for the RBridges
   in a campus, but not less than 1470.  The MTU testing mechanism and
   limiting LSPs to Sz assure that the LSPs can be flooded by IS-IS and
   thus that IS-IS can operate properly.







<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="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


   If an RBridge knows nothing about the MTU of the links or the
   originatingL1LSPBufferSize of other RBridges in a campus, the
   originatingL1LSPBufferSize for that RBridge should default to the
   minimum of the LSP size that its TRILL IS-IS software can handle and
   the minimum MTU of the ports that it might use to receive or transmit
   LSPs.  If an RBridge does have knowledge of link MTUs or other
   RBridge originatingL1LSPBufferSize, then, to avoid the necessity of
   regenerating the local LSPs using a different maximum size, the
   RBridge's originatingL1LSPBufferSize SHOULD be configured to the
   minimum of (1) the smallest value that other RBridges are, or will
   be, announcing as their originatingL1LSPBufferSize and (2) a value
   small enough that the campus will not partition due to a significant
   number of links with limited MTUs.  However, as specified in
   [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>], in no case can originatingL1LSPBufferSize be less than
   1470.  In a well-configured campus, to minimize any LSP regeneration
   due to resizing, all RBridges will be configured with the same
   originatingL1LSPBufferSize.

   <a href="#section-5.1">Section 5.1</a> below corrects errata in [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>], and <a href="#section-5.2">Section 5.2</a>
   clarifies the meaning of various MTU limits for TRILL Ethernet links.

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  MTU-Related Errata in <a href="./rfc6325">RFC 6325</a></span>

   Three MTU-related errata in [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] are corrected in the
   subsections below.

<span class="h4"><a class="selflink" id="section-5.1.1" href="#section-5.1.1">5.1.1</a>.  MTU PDU Addressing</span>

   <a href="./rfc6325#section-4.3.2">Section&nbsp;4.3.2 of [RFC6325]</a> incorrectly states that multi-destination
   MTU-probe and MTU-ack TRILL IS-IS PDUs are sent on Ethernet links
   with the All-RBridges multicast address as the Outer.MacDA [<a href="#ref-Err3004" title="Erratum ID 3004">Err3004</a>].
   As TRILL IS-IS PDUs, when multicast on an Ethernet link, these
   multi-destination MTU-probe and MTU-ack PDUs MUST be sent to the
   All-IS-IS-RBridges multicast address.

















<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="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


<span class="h4"><a class="selflink" id="section-5.1.2" href="#section-5.1.2">5.1.2</a>.  MTU PDU Processing</span>

   As discussed in [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] and (in more detail) [<a href="./rfc7177" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Adjacency&quot;">RFC7177</a>], MTU-probe
   and MTU-ack PDUs MAY be unicast; however, <a href="./rfc6325#section-4.6">Section&nbsp;4.6 of [RFC6325]</a>
   erroneously does not allow for this possibility [<a href="#ref-Err3003" title="Erratum ID 3003">Err3003</a>].  It is
   corrected by replacing Item 1 in <a href="./rfc6325#section-4.6.2">Section&nbsp;4.6.2 of [RFC6325]</a> with the
   following text, to which TRILL switches MUST conform:

      1. If the Ethertype is L2-IS-IS and the Outer.MacDA is either
         All-IS-IS-RBridges or the unicast MAC address of the receiving
         RBridge port, the frame is handled as described in
         <a href="#section-4.6.2.1">Section 4.6.2.1</a>.

   The reference to "<a href="#section-4.6.2.1">Section 4.6.2.1</a>" in the above text is to that
   section in [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>].

<span class="h4"><a class="selflink" id="section-5.1.3" href="#section-5.1.3">5.1.3</a>.  MTU Testing</span>

   The last two sentences of <a href="./rfc6325#section-4.3.2">Section&nbsp;4.3.2 of [RFC6325]</a> contain errors
   [<a href="#ref-Err3053" title="Erratum ID 3053">Err3053</a>].  They currently read as follows:

      If X is not greater than Sz, then RB1 sets the "failed minimum MTU
      test" flag for RB2 in RB1's Hello.  If size X succeeds, and X &gt;
      Sz, then RB1 advertises the largest tested X for each adjacency in
      the TRILL Hellos RB1 sends on that link, and RB1 MAY advertise X
      as an attribute of the link to RB2 in RB1's LSP.

   They should read as follows:

      If X is not greater than or equal to Sz, then RB1 sets the "failed
      minimum MTU test" flag for RB2 in RB1's Hello.  If size X
      succeeds, and X &gt;= Sz, then RB1 advertises the largest tested X
      for each adjacency in the TRILL Hellos RB1 sends on that link,
      and RB1 MAY advertise X as an attribute of the link to RB2 in
      RB1's LSP.

<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>.  Ethernet MTU Values</span>

   originatingL1LSPBufferSize is the maximum permitted size of LSPs
   starting with and including the IS-IS 0x83 "Intradomain Routeing
   Protocol Discriminator" byte.  In Layer 3 IS-IS,
   originatingL1LSPBufferSize defaults to 1492 bytes.  (This is because,
   in its previous life as DECnet Phase V, IS-IS was encoded using the
   SNAP SAP (Subnetwork Access Protocol Service Access Point) [<a href="./rfc7042" title="&quot;IANA Considerations and IETF Protocol and Documentation Usage for IEEE 802 Parameters&quot;">RFC7042</a>]
   format, which takes 8 bytes of overhead and 1492 + 8 = 1500, the
   classic Ethernet maximum.  When standardized by ISO/IEC [<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>] to
   use Logical Link Control (LLC) encoding, this default could have been
   increased by a few bytes but was not.)



<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="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


   In TRILL, originatingL1LSPBufferSize defaults to 1470 bytes.  This
   allows 27 bytes of headroom or safety margin to accommodate legacy
   devices with the classic Ethernet maximum MTU, despite headers such
   as an Outer.VLAN.

   Assuming that the campus-wide minimum link MTU is Sz, RBridges on
   Ethernet links MUST limit most TRILL IS-IS PDUs so that PDUz (the
   length of the PDU starting just after the L2-IS-IS Ethertype and
   ending just before the Ethernet Frame Check Sequence (FCS)) does not
   exceed Sz.  The PDU exceptions are TRILL Hello PDUs, which MUST NOT
   exceed 1470 bytes, and MTU-probe and MTU-ack PDUs that are padded by
   an amount that depends on the size being tested (which may
   exceed Sz).

   Sz does not limit TRILL Data packets.  They are only limited by the
   MTU of the devices and links that they actually pass through;
   however, links that can accommodate IS-IS PDUs up to Sz would
   accommodate, with a generous safety margin, TRILL Data packet
   payloads of (Sz - 24) bytes, starting after the Inner.VLAN and ending
   just before the FCS.

   Most modern Ethernet equipment has ample headroom for frames with
   extensive headers and is sometimes engineered to accommodate 9 KB
   jumbo frames.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  TRILL Port Modes (Unchanged)</span>

   <a href="./rfc6325#section-4.9.1">Section&nbsp;4.9.1 of [RFC6325]</a> specifies four mode bits for RBridge ports
   but may not be completely clear on the effects of all combinations of
   bits in terms of allowed frame types.





















<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="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


   The table below explicitly indicates the effects of all possible
   combinations of the TRILL port mode bits.  "*" in one of the first
   four columns indicates that the bit can be either zero or one.  The
   remaining columns indicate allowed frame types.  The "disable bit"
   normally disables all frames; however, as an implementation choice,
   some or all low-level Layer 2 control messages can still be sent or
   received.  Examples of Layer 2 control messages are those control
   frames for Ethernet identified in <a href="./rfc6325#section-1.4">Section&nbsp;1.4 of [RFC6325]</a> or PPP
   link negotiation messages [<a href="./rfc6361" title="&quot;PPP Transparent Interconnection of Lots of Links (TRILL) Protocol Control Protocol&quot;">RFC6361</a>].

            +-+-+-+-+--------+-------+-------+-------+-------+
            |D| | | |        |       |       |       |       |
            |i| |A| |        |       | TRILL |       |       |
            |s| |c|T|        |Native | Data  |       |       |
            |a| |c|r|        |Ingress|       |       |       |
            |b|P|e|u|        |       |  LSP  |       |       |
            |l|2|s|n|Layer 2 |Native |  SNP  | TRILL |  P2P  |
            |e|P|s|k|Control |Egress |  MTU  | Hello | Hello |
            +-+-+-+-+--------+-------+-------+-------+-------+
            |0|0|0|0|  Yes   |  Yes  |  Yes  |  Yes  |  No   |
            +-+-+-+-+--------+-------+-------+-------+-------+
            |0|0|0|1|  Yes   |  No   |  Yes  |  Yes  |  No   |
            +-+-+-+-+--------+-------+-------+-------+-------+
            |0|0|1|0|  Yes   |  Yes  |  No   |  Yes  |  No   |
            +-+-+-+-+--------+-------+-------+-------+-------+
            |0|0|1|1|  Yes   |  No   |  No   |  Yes  |  No   |
            +-+-+-+-+--------+-------+-------+-------+-------+
            |0|1|0|*|  Yes   |  No   |  Yes  |  No   |  Yes  |
            +-+-+-+-+--------+-------+-------+-------+-------+
            |0|1|1|*|  Yes   |  No   |  No   |  No   |  Yes  |
            +-+-+-+-+--------+-------+-------+-------+-------+
            |1|*|*|*|Optional|  No   |  No   |  No   |  No   |
            +-+-+-+-+--------+-------+-------+-------+-------+

   The formal name of the "access bit" above is the "TRILL traffic
   disable bit".  The formal name of the "trunk bit" is the "end-station
   service disable bit" [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>].

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  The CFI/DEI Bit (Unchanged)</span>

   In May 2011, the IEEE promulgated IEEE Std 802.1Q-2011, which changed
   the meaning of the bit between the priority and VLAN ID bits in the
   payload of C-VLAN tags.  Previously, this bit was called the CFI
   (Canonical Format Indicator) bit [<a href="#ref-802" title="&quot;IEEE Standard for Local and Metropolitan Area Networks: Overview and Architecture&quot;">802</a>] and had a special meaning in
   connection with IEEE 802.5 (Token Ring) frames.  After 802.1Q-2011
   and in subsequent versions of 802.1Q -- the most current of which is





<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="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


   [<a id="ref-802.1Q-2014">802.1Q-2014</a>] -- this bit is now the DEI (Drop Eligibility Indicator)
   bit.  (The corresponding bit in S-VLAN/B-VLAN tags has always been a
   DEI bit.)

   The TRILL base protocol specification [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] assumed, in effect,
   that the link by which end stations are connected to TRILL switches
   and the restricted virtual link provided by the TRILL Data packet are
   IEEE 802.3 Ethernet links on which the CFI bit is always zero.
   Should an end station be attached by some other type of link, such as
   a Token Ring link, [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] implicitly assumed that such frames
   would be canonicalized to 802.3 frames before being ingressed, and
   similarly, on egress, such frames would be converted from 802.3 to
   the appropriate frame type for the link.  Thus, [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] required
   that the CFI bit in the Inner.VLAN, which is shown as the "C" bit in
   <a href="./rfc6325#section-4.1.1">Section&nbsp;4.1.1 of [RFC6325]</a>, always be zero.

   However, for TRILL switches with ports conforming to the change
   incorporated in the IEEE 802.1Q-2011 standard, the bit in the
   Inner.VLAN, now a DEI bit, MUST be set to the DEI value provided by
   the port interface on ingressing a native frame.  Similarly, this bit
   MUST be provided to the port when transiting or egressing a TRILL
   Data packet.  As with the 3-bit Priority field, the DEI bit to use in
   forwarding a transit packet MUST be taken from the Inner.VLAN.  The
   exact effect on the Outer.VLAN DEI and priority bits, and whether or
   not an Outer.VLAN appears at all on the wire for output frames, may
   depend on output port configuration.

   TRILL campuses with a mixture of ports, some compliant with versions
   of 802.1Q from IEEE Std 802.1Q-2011 onward and some compliant with
   pre-802.1Q-2011 standards, especially if they have actual Token Ring
   links, may operate incorrectly and may corrupt data, just as a
   bridged LAN with such mixed ports and links would.

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Other IS-IS Considerations (Changed)</span>

   This section covers Extended Level 1 Flooding Scope (E-L1FS) support,
   control packet priorities, unknown PDUs, the Nickname Flags
   APPsub-TLV, graceful restart, and the Purge Originator
   Identification TLV.












<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="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>.  E-L1FS Support (New)</span>

   TRILL switches MUST support E-L1FS PDUs [<a href="./rfc7356" title="&quot;IS-IS Flooding Scope Link State PDUs (LSPs)&quot;">RFC7356</a>] and MUST include a
   Scope Flooding Support TLV [<a href="./rfc7356" title="&quot;IS-IS Flooding Scope Link State PDUs (LSPs)&quot;">RFC7356</a>] in all TRILL Hellos they send
   indicating support for this scope and any other FS-LSP scopes that
   they support.  This support increases the number of fragments
   available for link-state information by over two orders of magnitude.
   (See <a href="#section-9">Section 9</a> for further information on support of the Scope
   Flooding Support TLV.)

   In addition, TRILL switches MUST advertise their support of E-L1FS
   flooding in a TRILL-VER sub-TLV Capability Flag (see [<a href="./rfc7176" title="&quot;Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS&quot;">RFC7176</a>] and
   <a href="#section-12.2">Section 12.2</a>).  This flag is used by a TRILL switch, say RB1, to
   determine support for E-L1FS by some remote RBx.  The alternative of
   simply looking for an E-L1FS FS-LSP originated by RBx fails because
   (1) RBx might support E-L1FS flooding but is not originating any
   E-L1FS FS-LSPs and (2) even if RBx is originating E-L1FS FS-LSPs
   there might, due to legacy TRILL switches in the campus, be no path
   between RBx and RB1 through TRILL switches supporting E-L1FS
   flooding.  If that were the case, no E-L1FS FS-LSP originated by RBx
   could get to RB1.

   E-L1FS will commonly be used to flood TRILL GENINFO TLVs and enclosed
   TRILL APPsub-TLVs [<a href="./rfc7357" title="&quot;Transparent Interconnection of Lots of Links (TRILL): End Station Address Distribution Information (ESADI) Protocol&quot;">RFC7357</a>].  For robustness, E-L1FS fragment zero
   MUST NOT exceed 1470 bytes in length; however, if such a fragment is
   received that is larger, it is processed normally.  It is anticipated
   that in the future some particularly important TRILL APPsub-TLVs will
   be specified as being flooded in E-L1FS fragment zero.  TRILL GENINFO
   TLVs MUST NOT be sent in LSPs; however, if one is received in an LSP,
   it is processed normally.

<span class="h4"><a class="selflink" id="section-8.1.1" href="#section-8.1.1">8.1.1</a>.  Backward Compatibility</span>

   A TRILL campus might contain TRILL switches supporting E-L1FS
   flooding and legacy TRILL switches that do not support E-L1FS or
   perhaps do not support any [<a href="./rfc7356" title="&quot;IS-IS Flooding Scope Link State PDUs (LSPs)&quot;">RFC7356</a>] scopes.

   A TRILL switch conformant to this document can always tell which
   adjacent TRILL switches support E-L1FS flooding from the adjacency
   table entries on its ports (see <a href="#section-9">Section 9</a>).  In addition, such a
   TRILL switch can tell which remote TRILL switches in a campus support
   E-L1FS by the presence of a TRILL version sub-TLV in that TRILL
   switch's LSP with the E-L1FS support bit set in the Capabilities
   field; this capability bit is ignored for adjacent TRILL switches for
   which only the adjacency table entry is consulted to determine E-L1FS
   support.





<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="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


   TRILL specifications making use of E-L1FS MUST specify how situations
   involving a mixed TRILL campus of TRILL switches will be handled.

<span class="h4"><a class="selflink" id="section-8.1.2" href="#section-8.1.2">8.1.2</a>.  E-L1FS Use for Existing (Sub-)TLVs</span>

   In a campus where all TRILL switches support E-L1FS, all TRILL
   sub-TLVs listed in <a href="./rfc7176#section-2.3">Section&nbsp;2.3 of [RFC7176]</a>, except the TRILL version
   sub-TLV, MAY be advertised by inclusion in Router Capability or
   MT-Capability TLVs in E-L1FS FS-LSPs [<a href="./rfc7356" title="&quot;IS-IS Flooding Scope Link State PDUs (LSPs)&quot;">RFC7356</a>].  (The TRILL version
   sub-TLV still MUST appear in an LSP fragment zero.)

   In a mixed campus where some TRILL switches support E-L1FS and some
   do not, then only the following four sub-TLVs of those listed in
   <a href="./rfc7176#section-2.3">Section&nbsp;2.3 of [RFC7176]</a> can appear in E-L1FS, and then only under
   the conditions discussed below.  In the following list, each sub-TLV
   is preceded by an abbreviated acronym used only in this section of
   this document:

      IV: Interested VLANs and Spanning Tree Roots sub-TLV
      VG: VLAN Group sub-TLV
      IL: Interested Labels and Spanning Tree Roots sub-TLV
      LG: Label Group sub-TLV

   An IV or VG sub-TLV MUST NOT be advertised by TRILL switch RB1 in an
   E-L1FS FS-LSP (and should instead be advertised in an LSP) unless the
   following conditions are met:

   - E-L1FS is supported by all of the TRILL switches that are data
     reachable from RB1 and are interested in the VLANs mentioned in the
     IV or VG sub-TLV, and

   - there is E-L1FS connectivity between all such TRILL switches in the
     campus interested in the VLANs mentioned in the IV or VG sub-TLV
     (connectivity involving only intermediate TRILL switches that also
     support E-L1FS).

   Any IV and VG sub-TLVs MAY still be advertised via core TRILL IS-IS
   LSPs by any TRILL switch that has enough room in its LSPs.

   The conditions for using E-L1FS for the IL and LG sub-TLVs are the
   same as for IV and VG, but with Fine-Grained Labels [<a href="./rfc7172" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Fine-Grained Labeling&quot;">RFC7172</a>]
   substituted for VLANs.

      Note, for example, that the above would permit a contiguous subset
      of the campus that supported Fine-Grained Labels and E-L1FS to use
      E-L1FS to advertise IL and LG sub-TLVs, even if the remainder of
      the campus did not support Fine-Grained Labels or E-L1FS.




<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="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>.  Control Packet Priorities (New)</span>

   When deciding what packet to send out a port, control packets used to
   establish and maintain adjacency between TRILL switches SHOULD be
   treated as being in the highest-priority category.  This includes
   TRILL IS-IS Hello and MTU PDUs, and possibly other adjacency
   [<a href="./rfc7177" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Adjacency&quot;">RFC7177</a>] or link-technology-specific packets.  Other control and
   data packets SHOULD be given lower priority so that a flood of such
   other packets cannot lead to loss of, or inability to establish,
   adjacency.  Loss of adjacency causes a topology transient that can
   result in reduced throughput; reordering; increased probability of
   loss of data; and, in the worst case, network partition if the
   adjacency is a cut point.

   Other important control packets should be given second-highest
   priority.  Lower priorities should be given to data or less important
   control packets.

   Based on the above, control packets can be ordered into priority
   categories as shown below, based on the relative criticality of these
   types of messages, where the most critical control packets relate to
   the core routing between TRILL switches and the less critical control
   packets are closer to "application" information.  (There may be
   additional control packets, not specifically listed in any category
   below, that SHOULD be handled as being in the most nearly analogous
   category.)  Although few implementations will actually treat these
   four categories with different priority, an implementation MAY choose
   to prioritize more critical messages over less critical.  However, an
   implementation SHOULD NOT send control packets in a lower-priority
   category with a priority above those in a higher-priority category
   because, under sufficiently congested conditions, this could block
   control packets in a higher-priority category, resulting in network
   disruption.

      Priority
      Category   Description
      --------  --------------

      4.        Hello, MTU-probe, MTU-ack, and other packets critical
                to establishing and maintaining adjacency.  (Normally
                sent with highest priority, which is priority 7.)

      3.        LSPs, CSNPs/PSNPs, and other important control packets.

      2.        Circuit scoped FS-LSPs, FS-CSNPs, and FS-PSNPs.

      1.        Non-circuit scoped FS-LSPs, FS-CSNPs, and FS-PSNPs.




<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="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


<span class="h3"><a class="selflink" id="section-8.3" href="#section-8.3">8.3</a>.  Unknown PDUs (New)</span>

   TRILL switches MUST silently discard [<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>] PDUs they receive with
   PDU numbers they do not understand, just as they ignore TLVs and
   sub-TLVs they receive that have unknown Types and sub-Types; however,
   they SHOULD maintain a counter of how many such PDUs have been
   received, on a per-PDU-number basis.  (This is not burdensome, as the
   PDU number is only a 5-bit field.)

      Note: The set of valid [<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>] PDUs was stable for so long that
         some IS-IS implementations may treat PDUs with unknown PDU
         numbers as a serious error and, for example, an indication that
         other valid PDUs from the sender are not to be trusted or that
         they should drop adjacency to the sender if it was adjacent.
         However, the MTU-probe and MTU-ack PDUs were added by
         [<a href="./rfc7176" title="&quot;Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS&quot;">RFC7176</a>], and now [<a href="./rfc7356" title="&quot;IS-IS Flooding Scope Link State PDUs (LSPs)&quot;">RFC7356</a>] has added three more new PDUs.
         Although the authors of this document are not aware of any
         Internet-Drafts calling for further PDUs, the eventual addition
         of further new PDUs should not be surprising.

<span class="h3"><a class="selflink" id="section-8.4" href="#section-8.4">8.4</a>.  Nickname Flags APPsub-TLV (New)</span>

   An optional Nickname Flags APPsub-TLV within the TRILL GENINFO TLV
   [<a href="./rfc7357" title="&quot;Transparent Interconnection of Lots of Links (TRILL): End Station Address Distribution Information (ESADI) Protocol&quot;">RFC7357</a>] is specified below.

                           1 1 1 1 1 1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Type = NickFlags (6)          |   (2 bytes)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Length = 4*K                  |   (2 bytes)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   NICKFLAG RECORD 1               (4 bytes)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   NICKFLAG RECORD K               (4 bytes)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      where each NICKFLAG RECORD has the following format:

        0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
      |   Nickname                                    |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
      |IN|      RESV                                  |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+




<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="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


      o  Type: NickFlags TRILL APPsub-TLV, set to 6 (NICKFLAGS).

      o  Length: 4 times the number of NICKFLAG RECORDS present.

      o  Nickname: A 16-bit TRILL nickname held by the advertising TRILL
         switch ([<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] and <a href="#section-4">Section 4</a>).

      o  IN: Ingress.  If this flag is one, it indicates that the
         advertising TRILL switch may use the nickname in the NICKFLAG
         RECORD as the Ingress Nickname of TRILL Headers it creates.  If
         the flag is zero, that nickname will not be used for that
         purpose.

      o  RESV: Reserved for additional flags to be specified in the
         future.  MUST be sent as zero and ignored on receipt.

   The entire NickFlags APPsub-TLV is ignored if the Length is not a
   multiple of 4.  A NICKFLAG RECORD is ignored if the nickname it lists
   is not a nickname owned by the TRILL switch advertising the enclosing
   NickFlags APPsub-TLV.

   If a TRILL switch intends to use a nickname in the Ingress Nickname
   field of TRILL Headers it constructs, it can advertise this through
   E-L1FS FS-LSPs (see <a href="#section-8.1">Section 8.1</a>) using a NickFlags APPsub-TLV entry
   with the IN flag set.  If it owns only one nickname, there is no
   reason to do this because, if a TRILL switch advertises no NickFlags
   APPsub-TLVs with the IN flag set for nicknames it owns, it is assumed
   that the TRILL switch might use any or all nicknames it owns as the
   Ingress Nickname in TRILL Headers it constructs.  If a TRILL switch
   advertises any NickFlags APPsub-TLV entries with the IN flag set,
   then it MUST NOT use any other nickname(s) it owns as the Ingress
   Nickname in TRILL Headers it constructs.

   Every reasonable effort should be made to be sure that Nickname
   sub-TLVs [<a href="./rfc7176" title="&quot;Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS&quot;">RFC7176</a>] and NickFlags APPsub-TLVs remain in sync.  If all
   TRILL switches in a campus support E-L1FS, so that Nickname sub-TLVs
   can be advertised in E-L1FS FS-LSPs, then the Nickname sub-TLV and
   any NickFlags APPsub-TLVs for any particular nickname SHOULD be
   advertised in the same fragment.  If they are not in the same
   fragment, then, to the extent practical, all fragments involving
   those sub-TLVs for the same nickname should be propagated as an
   atomic action.  If a TRILL switch sees multiple NickFlags APPsub-TLV
   entries for the same nickname, it assumes that that nickname might be
   used as the ingress in a TRILL Header if any of the NickFlags
   APPsub-TLV entries have the IN bit set.






<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="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


   It is possible that a NickFlags APPsub-TLV would not be propagated
   throughout the TRILL campus due to legacy TRILL switches not
   supporting E-L1FS.  In that case, Nickname sub-TLVs MUST be
   advertised in LSPs, and TRILL switches not receiving NickFlags
   APPsub-TLVs having entries with the IN flag set will simply assume
   that the source TRILL switch might use any of its nicknames as the
   ingress in constructing TRILL Headers.  Thus, the use of this
   optional APPsub-TLV is backward compatible with legacy lack of E-L1FS
   support.

   (Additional flags are assigned from those labeled RESV above and
   specified in [<a href="#ref-TRILL-L3-GW">TRILL-L3-GW</a>] and [<a href="#ref-Centralized-Replication">Centralized-Replication</a>].)

<span class="h3"><a class="selflink" id="section-8.5" href="#section-8.5">8.5</a>.  Graceful Restart (Unchanged)</span>

   TRILL switches SHOULD support the features specified in [<a href="./rfc5306" title="&quot;Restart Signaling for IS-IS&quot;">RFC5306</a>],
   which describes a mechanism for a restarting IS-IS router to signal
   to its neighbors that it is restarting, allowing them to reestablish
   their adjacencies without cycling through the down state, while still
   correctly initiating link-state database synchronization.  If this
   feature is not supported, it may increase the number of topology
   transients caused by a TRILL switch rebooting due to errors or
   maintenance.

<span class="h3"><a class="selflink" id="section-8.6" href="#section-8.6">8.6</a>.  Purge Originator Identification (New)</span>

   To ease debugging of any purge-related problems, TRILL switches
   SHOULD include the Purge Originator Identification TLV [<a href="./rfc6232" title="&quot;Purge Originator Identification TLV for IS-IS&quot;">RFC6232</a>] in
   all purge PDUs in TRILL IS-IS.  This includes Flooding Scope LSPs
   [<a href="./rfc7356" title="&quot;IS-IS Flooding Scope Link State PDUs (LSPs)&quot;">RFC7356</a>] and ESADI LSPs [<a href="./rfc7357" title="&quot;Transparent Interconnection of Lots of Links (TRILL): End Station Address Distribution Information (ESADI) Protocol&quot;">RFC7357</a>].





















<span class="grey">Eastlake, et al.             Standards Track                   [Page 29]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-30" ></span>
<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Updates to <a href="./rfc7177">RFC 7177</a> (Adjacency) (Changed)</span>

   To support the E-L1FS flooding scope [<a href="./rfc7356" title="&quot;IS-IS Flooding Scope Link State PDUs (LSPs)&quot;">RFC7356</a>] mandated by
   <a href="#section-8.1">Section 8.1</a> and backward compatibility with legacy RBridges not
   supporting E-L1FS flooding, this document updates [<a href="./rfc7177" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Adjacency&quot;">RFC7177</a>] as
   follows:

   1. The list in the second paragraph of <a href="./rfc7177#section-3.1">Section&nbsp;3.1 of [RFC7177]</a> is
      updated by adding the following item:

      o  The Scope Flooding Support TLV.

      In addition, the sentence immediately after that list is updated
      by this document to read as follows:

         Of course, (a) the priority, (b) the Desired Designated VLAN,
         (c) the Scope Flooding Support TLV, and whether or not the
         (d) PORT-TRILL-VER sub-TLV and/or (e) BFD-Enabled TLV are
         included, and their value if included, could change on
         occasion.  However, if these change, the new value(s) must
         similarly be used in all TRILL Hellos on the LAN port,
         regardless of VLAN.

   2. This document adds another bullet item to the end of <a href="./rfc7177#section-3.2">Section&nbsp;3.2
      of [RFC7177]</a>, as follows:

      o  The value from the Scope Flooding Support TLV, or a null string
         if none was included.

   3. Near the bottom of <a href="./rfc7177#section-3.3">Section&nbsp;3.3 of [RFC7177]</a>, this document adds
      the following bullet item:

      o  The variable-length value part of the Scope Flooding Support
         TLV in the Hello, or a null string if that TLV does not occur
         in the Hello.

   4. At the beginning of <a href="./rfc7177#section-4">Section&nbsp;4 of [RFC7177]</a>, this document adds a
      bullet item to the list, as follows:

      o  The variable-length value part of the Scope Flooding Support
         TLV used in TRILL Hellos sent on the port.










<span class="grey">Eastlake, et al.             Standards Track                   [Page 30]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-31" ></span>
<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


   5. This document adds a line to Table 4 ("TRILL Hello Contents") in
      <a href="./rfc7177#section-8.1">Section&nbsp;8.1 of [RFC7177]</a>, as follows:

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

          M    M     1      Scope Flooding Support TLV

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  TRILL Header Update (New)</span>

   The TRILL Header has been updated from its original specification in
   [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] by [<a href="./rfc7455" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Fault Management&quot;">RFC7455</a>] and [<a href="./rfc7179" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Header Extension&quot;">RFC7179</a>] and is further updated by this
   document.  The TRILL Header is now as shown in the figure below
   (which is followed by references for all of the fields).  Those
   fields for which the reference is only to [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] are unchanged
   from that RFC.

                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   | V |A|C|M| RESV  |F| Hop Count |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Egress Nickname             |   Ingress Nickname            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :   Optional Flags Word                                         :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   In calculating a TRILL Data packet hash as part of equal-cost
   multipath selection, a TRILL switch MUST ignore the value of the
   "A" and "C" bits.

   In [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] and [<a href="./rfc7179" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Header Extension&quot;">RFC7179</a>], there is a TRILL Header Extension Length
   field called "Op-Length", which is hereby changed to consist of the
   RESV field and "F" bit shown above.

   o  V (Version): 2-bit unsigned integer.  See <a href="./rfc6325#section-3.2">Section&nbsp;3.2
      of [RFC6325]</a>.

   o  A (Alert): 1 bit.  See [<a href="./rfc7455" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Fault Management&quot;">RFC7455</a>].

   o  C (Color): 1 bit.  See <a href="#section-10.1">Section 10.1</a>.

   o  M (Multi-destination): 1 bit.  See <a href="./rfc6325#section-3.4">Section&nbsp;3.4 of [RFC6325]</a>.

   o  RESV: 4 bits.  These bits are reserved and MUST be sent as zero.
      Due to the previous use of these bits as specified in [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>],
      most TRILL "fast path" hardware implementations trap and do not
      forward TRILL Data packets with these bits non-zero.  A TRILL





<span class="grey">Eastlake, et al.             Standards Track                   [Page 31]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-32" ></span>
<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


      switch receiving a TRILL Data packet with any of these bits
      non-zero MUST discard the packet unless the non-zero bit or bits
      have some future use specified that the TRILL switch understands.

   o  F: 1 bit.  If this field is non-zero, then the optional flags word
      described in <a href="#section-10.2">Section 10.2</a> is present.  If it is zero, the
      flags word is not present.

   o  Hop Count: 6 bits.  See <a href="./rfc6325#section-3.6">Section&nbsp;3.6 of [RFC6325]</a> and
      <a href="#section-10.2.1">Section 10.2.1</a> below.

   o  Egress Nickname: See <a href="./rfc6325#section-3.7.1">Section&nbsp;3.7.1 of [RFC6325]</a>.

   o  Ingress Nickname: See <a href="./rfc6325#section-3.7.2">Section&nbsp;3.7.2 of [RFC6325]</a>.

   o  Optional Flags Word: See [<a href="./rfc7179" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Header Extension&quot;">RFC7179</a>] and <a href="#section-10.2">Section 10.2</a>.

<span class="h3"><a class="selflink" id="section-10.1" href="#section-10.1">10.1</a>.  Color Bit</span>

   The Color bit provides an optional way by which ingress TRILL
   switches MAY mark TRILL Data packets for implementation-specific
   purposes.  Transit TRILL switches MUST NOT change this bit.  Transit
   and egress TRILL switches MAY use the Color bit for implementation-
   dependent traffic labeling, or for statistical analysis or other
   types of traffic study or analysis.

<span class="h3"><a class="selflink" id="section-10.2" href="#section-10.2">10.2</a>.  Flags Word Changes (Update to <a href="./rfc7179">RFC 7179</a>)</span>

   When the "F" bit in the TRILL Header is non-zero, the first 32 bits
   after the Ingress Nickname field provide additional flags.  These
   bits are as specified in [<a href="./rfc7179" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Header Extension&quot;">RFC7179</a>], except as changed by the
   subsections below, in which the Extended Hop Count and Extended Color
   fields are described.  See <a href="#section-10.3">Section 10.3</a> for a diagram and summary of
   these fields.

<span class="h4"><a class="selflink" id="section-10.2.1" href="#section-10.2.1">10.2.1</a>.  Extended Hop Count</span>

   The TRILL base protocol [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] specifies the Hop Count field in
   the header, to avoid packets persisting in the network due to looping
   or the like.  However, the Hop Count field size (6 bits) limits the
   maximum hops a TRILL Data packet can traverse to 64.  Optionally,
   TRILL switches can use a field composed of bits 14 through 16 in the
   flags word, as specified below, to extend this field to 9 bits.  This
   increases the maximum Hop Count to 512.  Except in rare
   circumstances, reliable use of Hop Counts in excess of 64 requires
   support of this optional capability at all TRILL switches along the
   path of a TRILL Data packet.




<span class="grey">Eastlake, et al.             Standards Track                   [Page 32]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-33" ></span>
<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


<span class="h5"><a class="selflink" id="section-10.2.1.1" href="#section-10.2.1.1">10.2.1.1</a>.  Advertising Support</span>

   It may be that not all the TRILL switches support the Extended Hop
   Count mechanism in a TRILL campus and in that campus more than
   64 hops are required either for the distribution tree calculated path
   or for the unicast calculated path plus a reasonable allowance for
   alternate pathing.  As such, it is required that TRILL switches
   advertise their support by setting bit 14 in the TRILL Version
   Sub-TLV Capabilities and Header Flags Supported field [<a href="./rfc7176" title="&quot;Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS&quot;">RFC7176</a>];
   bits 15 and 16 of that field are now specified as Unassigned (see
   <a href="#section-12.2.5">Section 12.2.5</a>).

<span class="h5"><a class="selflink" id="section-10.2.1.2" href="#section-10.2.1.2">10.2.1.2</a>.  Ingress Behavior</span>

   If an ingress TRILL switch determines that it should set the
   Hop Count for a TRILL Data packet to 63 or less, then behavior is as
   specified in the TRILL base protocol [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>].  If the optional
   TRILL Header flags word is present, bits 14, 15, and 16 and the
   critical reserved bit of the critical summary bits are zero.

   If the Hop Count for a TRILL Data packet should be set to some value
   greater than 63 but less than 512 and all TRILL switches that the
   packet is reasonably likely to encounter support Extended Hop Count,
   then the resulting TRILL Header has the flags word extension present,
   the high-order 3 bits of the desired Hop Count are stored in the
   Extended Hop Count field in the flags word, the low-order 5 bits are
   stored in the Hop Count field in the first word of the TRILL Header,
   and bit two (the critical reserved bit of the critical summary bits)
   in the flags word is set to one.

   For known unicast traffic (TRILL Header "M" bit zero), an ingress
   TRILL switch discards the frame if it determines that the least-cost
   path to the egress is (1) more than 64 hops and not all TRILL
   switches on that path support the Extended Hop Count feature or
   (2) more than 512 hops.

   For multi-destination traffic, when a TRILL switch determines that
   one or more tree paths from the ingress are more than 64 hops and not
   all TRILL switches in the campus support the Extended Hop Count
   feature, the encapsulation uses a total Hop Count of 63 to obtain at
   least partial distribution of the traffic.

<span class="h5"><a class="selflink" id="section-10.2.1.3" href="#section-10.2.1.3">10.2.1.3</a>.  Transit Behavior</span>

   A transit TRILL switch supporting Extended Hop Count behaves like a
   base protocol [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] TRILL switch in decrementing the Hop Count,
   except that it considers the Hop Count to be a 9-bit field where the
   Extended Hop Count field constitutes the high-order 3 bits.



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<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


   To be more precise: a TRILL switch supporting Extended Hop Count
   takes the first of the following actions that is applicable:

   1. If both the Hop Count and Extended Hop Count fields are zero, the
      packet is discarded.

   2. If the Hop Count is non-zero, it is decremented.  As long as the
      Extended Hop Count is non-zero, no special action is taken.  If
      the result of this decrement is zero, the packet is processed
      normally.

   3. If the Hop Count is zero, it is set to the maximum value of 63,
      and the Extended Hop Count is decremented.  If this results in the
      Extended Hop Count being zero, the critical reserved bit in the
      critical summary bits is set to zero.

<span class="h5"><a class="selflink" id="section-10.2.1.4" href="#section-10.2.1.4">10.2.1.4</a>.  Egress Behavior</span>

   No special behavior is required when egressing a TRILL Data packet
   that uses the Extended Hop Count.  The flags word, if present, is
   removed along with the rest of the TRILL Header during decapsulation.

<span class="h4"><a class="selflink" id="section-10.2.2" href="#section-10.2.2">10.2.2</a>.  Extended Color Field</span>

   Flags word bits 27 and 28 are specified to be a 2-bit Extended Color
   field (see <a href="#section-10.3">Section 10.3</a>).  These bits are in the non-critical
   ingress-to-egress region of the flags word.

   The Extended Color field provides an optional way by which ingress
   TRILL switches MAY mark TRILL Data packets for implementation-
   specific purposes.  Transit TRILL switches MUST NOT change these
   bits.  Transit and egress TRILL switches MAY use the Extended Color
   bits for implementation-dependent traffic labeling, or for
   statistical analysis or other types of traffic study or analysis.

   Per <a href="./rfc7176#section-2.3.1">Section&nbsp;2.3.1 of [RFC7176]</a>, support for these bits is indicated
   by the same bits (27 and 28) in the Capabilities and Header Flags
   Supported field of the TRILL version sub-TLV.  If these bits are zero
   in those capabilities, Extended Color is not supported.  A TRILL
   switch that does not support Extended Color will ignore the
   corresponding bits in any TRILL Header flags word it receives as part
   of a TRILL Data packet and will set those bits to zero in any TRILL
   Header flags word it creates.  A TRILL switch that sets or senses the
   Extended Color field on transmitting or receiving TRILL Data packets
   MUST set the corresponding 2-bit field in the TRILL version sub-TLV
   to a non-zero value.  Any difference in the meaning of the three
   possible non-zero values of this 2-bit capability field (0b01, 0b10,
   or 0b11) is implementation dependent.



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<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


<span class="h3"><a class="selflink" id="section-10.3" href="#section-10.3">10.3</a>.  Updated Flags Word Summary</span>

   With the changes above, the 32-bit flags word extension to the TRILL
   Header [<a href="./rfc7179" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Header Extension&quot;">RFC7179</a>], which is detailed in the "TRILL Extended Header
   Flags" registry on the "Transparent Interconnection of Lots of Links
   (TRILL) Parameters" IANA web page, is now as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Crit.|  CHbH   |   NCHbH   |CRSV | NCRSV |   CItE    |  NCItE  |
   |.....|.........|...........|.....|.......|...........|.........|
   |C|C|C|       |C|N|         | Ext |       |           |Ext|     |
   |R|R|R|       |R|C|         | Hop |       |           |Clr|     |
   |H|I|R|       |C|C|         | Cnt |       |           |   |     |
   |b|t|s|       |A|A|         |     |       |           |   |     |
   |H|E|v|       |F|F|         |     |       |           |   |     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Bits 0, 1, and 2 are the critical summary bits, as specified in
   [<a href="./rfc7179" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Header Extension&quot;">RFC7179</a>], consisting of the critical hop-by-hop, critical
   ingress-to-egress, and critical reserved bits, respectively.  The
   next two fields are specific critical and non-critical hop-by-hop
   bits -- CHbH and NCHbH, respectively -- containing the Critical and
   Non-critical Channel Alert flags as specified in [<a href="./rfc7179" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Header Extension&quot;">RFC7179</a>].  The next
   field is the critical reserved bits (CRSV), which are specified
   herein to be the Extended Hop Count.  The non-critical reserved bits
   (NCRSV) and the critical ingress-to-egress bits (CItE) as specified
   in [<a href="./rfc7179" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Header Extension&quot;">RFC7179</a>] follow.  Finally, there is the non-critical
   ingress-to-egress field, including bits 27 and 28, which are
   specified herein as the Extended Color field.

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  Appointed Forwarder Status Lost Counter (New)</span>

   Strict conformance to the provisions of <a href="./rfc6325#section-4.8.3">Section&nbsp;4.8.3 of [RFC6325]</a> on
   the value of the Appointed Forwarder Status Lost Counter can result
   in the splitting of Interested VLANs and Spanning Tree Roots sub-TLVs
   [<a href="./rfc7176" title="&quot;Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS&quot;">RFC7176</a>] (or the corresponding Interested Labels and Spanning Tree
   Roots sub-TLVs where a VLAN is mapped to an FGL) due to differences
   in this counter value for adjacent VLAN IDs (or 24-bit FGLs).  This
   counter is a mechanism to optimize data-plane learning by trimming
   the expiration timer for learned addresses on a per-VLAN/FGL basis
   under some circumstances.

   The requirement to increment this counter by one whenever a TRILL
   switch loses Appointed Forwarder status on a port is hereby changed
   from the mandatory provisions of [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] to the enumerated
   provisions below.  To the extent that this might cause the Appointed



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<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


   Forwarder Status Lost Counter to be increased when [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>]
   indicates that it should not, this will cause data-plane address
   learning timeouts at remote TRILL switches to be reduced.  To the
   extent that this might cause the Appointed Forwarder Status Lost
   Counter to remain unchanged when [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] indicates that it should
   be increased, this will defeat a reduction in such timeouts that
   would otherwise occur.

   (1) If any of the following apply, either data-plane address learning
       is not in use or Appointed Forwarder status is irrelevant.  In
       these cases, the Appointed Forwarder Status Lost Counter MAY be
       left at zero or set to any convenient value such as the value of
       the Appointed Forwarder Status Lost Counter for an adjacent
       VLAN ID or FGL.

       (1a) The TRILL switch port has been configured with the
            "end-station service disable" bit (also known as the
            trunk bit) on.

       (1b) The TRILL switch port has been configured in IS-IS as an
            IS-IS point-to-point link.

       (1c) The TRILL switch is relying on ESADI [<a href="./rfc7357" title="&quot;Transparent Interconnection of Lots of Links (TRILL): End Station Address Distribution Information (ESADI) Protocol&quot;">RFC7357</a>] or Directory
            Assist [<a href="./rfc7067" title="&quot;Directory Assistance Problem and High-Level Design Proposal&quot;">RFC7067</a>] and not using data-plane learning.

   (2) In cases other than those enumerated in point 1 above, the
       Appointed Forwarder Status Lost Counter SHOULD be incremented as
       described in [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>].  Such incrementing has the advantage of
       optimizing data-plane learning.  Alternatively, the value of the
       Appointed Forwarder Status Lost Counter can deviate from that
       value -- for example, to make it match the value for an adjacent
       VLAN ID (or FGL), so as to permit greater aggregation of
       Interested VLANs and Spanning Tree Roots sub-TLVs.


















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<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


<span class="h2"><a class="selflink" id="section-12" href="#section-12">12</a>.  IANA Considerations (Changed)</span>

   This section lists IANA actions previously completed and new IANA
   actions.

<span class="h3"><a class="selflink" id="section-12.1" href="#section-12.1">12.1</a>.  Previously Completed IANA Actions (Unchanged)</span>

   The following IANA actions were completed as part of [<a href="./rfc7180" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Clarifications, Corrections, and Updates&quot;">RFC7180</a>] and
   are included here for completeness, since this document obsoletes
   [<a href="./rfc7180" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Clarifications, Corrections, and Updates&quot;">RFC7180</a>].

   1. The nickname 0xFFC1, which was reserved by [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>], is allocated
      for use in the TRILL Header Egress Nickname field to indicate an
      OOMF (Overload Originated Multi-destination Frame).

   2. Bit 1 from the seven previously reserved (RESV) bits in the
      per-neighbor "Neighbor RECORD" in the TRILL Neighbor TLV [<a href="./rfc7176" title="&quot;Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS&quot;">RFC7176</a>]
      is allocated to indicate that the RBridge sending the TRILL Hello
      volunteers to provide the OOMF forwarding service described in
      <a href="#section-2.4.2">Section 2.4.2</a> to such frames originated by the TRILL switch whose
      SNPA (MAC address) appears in that Neighbor RECORD.  The
      description of this bit is "Offering OOMF service".

   3. Bit 0 is allocated from the capability bits in the PORT-TRILL-VER
      sub-TLV [<a href="./rfc7176" title="&quot;Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS&quot;">RFC7176</a>] to indicate support of the VLANs Appointed
      sub-TLV [<a href="./rfc7176" title="&quot;Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS&quot;">RFC7176</a>] and the VLAN inhibition setting mechanisms
      specified in [<a href="#ref-RFC6439bis">RFC6439bis</a>].  The description of this bit is "Hello
      reduction support".

<span class="h3"><a class="selflink" id="section-12.2" href="#section-12.2">12.2</a>.  New IANA Actions (New)</span>

   The following are new IANA actions for this document.

<span class="h4"><a class="selflink" id="section-12.2.1" href="#section-12.2.1">12.2.1</a>.  Reference Updated</span>

   All references to [<a href="./rfc7180" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Clarifications, Corrections, and Updates&quot;">RFC7180</a>] in the "Transparent Interconnection of
   Lots of Links (TRILL) Parameters" registry have been replaced with
   references to this document, except that the Reference for bit 0 in
   the PORT-TRILL-VER Sub-TLV Capability Flags has been changed to
   [<a href="#ref-RFC6439bis">RFC6439bis</a>].

<span class="h4"><a class="selflink" id="section-12.2.2" href="#section-12.2.2">12.2.2</a>.  The "E" Capability Bit</span>

   There is an existing TRILL version sub-TLV, sub-TLV #13, under both
   TLV #242 and TLV #144 [<a href="./rfc7176" title="&quot;Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS&quot;">RFC7176</a>].  This TRILL version sub-TLV contains
   a capability bits field for which assignments are documented in the
   "TRILL-VER Sub-TLV Capability Flags" registry on the TRILL Parameters
   IANA web page.  IANA has allocated 4 from the previously reserved



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<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


   bits in this "TRILL-VER Sub-TLV Capability Flags" registry to
   indicate support of the E-L1FS flooding scope as specified in
   <a href="#section-8.1">Section 8.1</a>.  This capability bit is referred to as the "E" bit.  The
   following is the addition to the "TRILL-VER Sub-TLV Capability Flags"
   registry:

       Bit     Description             References
       ----   ---------------------   ---------------
       4      E-L1FS FS-LSP support   [<a href="./rfc7356" title="&quot;IS-IS Flooding Scope Link State PDUs (LSPs)&quot;">RFC7356</a>], <a href="./rfc7780">RFC 7780</a>

<span class="h4"><a class="selflink" id="section-12.2.3" href="#section-12.2.3">12.2.3</a>.  NickFlags APPsub-TLV Number and Registry</span>

   IANA has assigned an APPsub-TLV number, as follows, under the TRILL
   GENINFO TLV from the range less than 255.

        Type      Name           References
        ----    ---------       -----------
        6       NICKFLAGS       <a href="./rfc7780">RFC 7780</a>

   In addition, IANA has created a registry on its TRILL Parameters web
   page for NickFlags bit assignments, as follows:

        Name: NickFlags Bits
        Registration Procedure: IETF Review [<a href="./rfc5226" title="">RFC5226</a>]
        Reference: <a href="./rfc7780">RFC 7780</a>

         Bit   Mnemonic  Description      Reference
        -----  --------  -----------      ---------
         0       IN      Used as ingress  <a href="./rfc7780">RFC 7780</a>
        1-15      -      Unassigned       <a href="./rfc7780">RFC 7780</a>

<span class="h4"><a class="selflink" id="section-12.2.4" href="#section-12.2.4">12.2.4</a>.  Updated TRILL Extended Header Flags</span>

   The "TRILL Extended Header Flags" registry has been updated as
   follows:

   Bits     Purpose                                  Reference
   -----   ----------------------------------------  ------------

   14-16   Extended Hop Count                        <a href="./rfc7780">RFC 7780</a>

   27-28   Extended Color                            <a href="./rfc7780">RFC 7780</a>

   29-31   Available non-critical ingress-to-egress  [<a href="./rfc7179" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Header Extension&quot;">RFC7179</a>], <a href="./rfc7780">RFC 7780</a>
           flags






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<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


<span class="h4"><a class="selflink" id="section-12.2.5" href="#section-12.2.5">12.2.5</a>.  TRILL-VER Sub-TLV Capability Flags</span>

   The "TRILL-VER Sub-TLV Capability Flags" registry has been updated as
   follows:

   Bit     Description                   Reference
   -----  --------------------------     ----------------

      14  Extended Hop Count support     <a href="./rfc7780">RFC 7780</a>

   15-16  Unassigned                     <a href="./rfc7780">RFC 7780</a>

   27-28  Extended Color support         <a href="./rfc7780">RFC 7780</a>

   29-31  Extended header flag support   [<a href="./rfc7179" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Header Extension&quot;">RFC7179</a>], <a href="./rfc7780">RFC 7780</a>

<span class="h4"><a class="selflink" id="section-12.2.6" href="#section-12.2.6">12.2.6</a>.  Example Nicknames</span>

   As shown in the table below, IANA has assigned a block of eight
   nicknames for use as examples in documentation.  <a href="#appendix-B">Appendix B</a> shows a
   use of some of these nicknames.  The "TRILL Nicknames" registry has
   been updated by changing the previous "0xFFC2-0xFFFE Unassigned" line
   to the following:

       Name        Description                        Reference
   -------------  --------------                     -----------
   0xFFC2-0xFFD7  Unassigned
   0xFFD8-0xFFDF  For use in documentation examples  <a href="./rfc7780">RFC 7780</a>
   0xFFE0-0xFFFE  Unassigned

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

   See [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] for general TRILL security considerations.

   This memo improves the documentation of the TRILL protocol; corrects
   six errata in [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>]; updates [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>], [<a href="./rfc7177" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Adjacency&quot;">RFC7177</a>], and [<a href="./rfc7179" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Header Extension&quot;">RFC7179</a>];
   and obsoletes [<a href="./rfc7180" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Clarifications, Corrections, and Updates&quot;">RFC7180</a>].  It does not change the security
   considerations of those RFCs, except as follows:

   o  E-L1FS FS-LSPs can be authenticated with IS-IS security [<a href="./rfc5310" title="&quot;IS-IS Generic Cryptographic Authentication&quot;">RFC5310</a>],
      that is, through the inclusion of an IS-IS Authentication TLV in
      E-L1FS PDUs.

   o  As discussed in <a href="#section-3.6">Section 3.6</a>, when using an allowed weaker RPF
      check under very rare topologies and transient conditions,
      multi-destination TRILL Data packets can be duplicated; this could
      have security consequences for some protocols.




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

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

   [<a id="ref-802.1Q-2014">802.1Q-2014</a>]
              IEEE, "IEEE Standard for Local and metropolitan area
              networks -- Bridges and Bridged Networks",
              DOI 10.1109/IEEESTD.2014.6991462, IEEE Std 802.1Q-2014.

   [<a id="ref-IS-IS">IS-IS</a>]    International Organization for Standardization,
              "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)", ISO/IEC 10589:2002, Second Edition,
              November 2002.

   [<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>,
              DOI 10.17487/RFC2119, March 1997,
              &lt;<a href="http://www.rfc-editor.org/info/rfc2119">http://www.rfc-editor.org/info/rfc2119</a>&gt;.

   [<a id="ref-RFC5305">RFC5305</a>]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", <a href="./rfc5305">RFC 5305</a>, DOI 10.17487/RFC5305,
              October 2008, &lt;<a href="http://www.rfc-editor.org/info/rfc5305">http://www.rfc-editor.org/info/rfc5305</a>&gt;.

   [<a id="ref-RFC5306">RFC5306</a>]  Shand, M. and L. Ginsberg, "Restart Signaling for IS-IS",
              <a href="./rfc5306">RFC 5306</a>, DOI 10.17487/RFC5306, October 2008,
              &lt;<a href="http://www.rfc-editor.org/info/rfc5306">http://www.rfc-editor.org/info/rfc5306</a>&gt;.

   [<a id="ref-RFC5310">RFC5310</a>]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", <a href="./rfc5310">RFC 5310</a>, DOI 10.17487/RFC5310,
              February 2009, &lt;<a href="http://www.rfc-editor.org/info/rfc5310">http://www.rfc-editor.org/info/rfc5310</a>&gt;.

   [<a id="ref-RFC6232">RFC6232</a>]  Wei, F., Qin, Y., Li, Z., Li, T., and J. Dong, "Purge
              Originator Identification TLV for IS-IS", <a href="./rfc6232">RFC 6232</a>,
              DOI 10.17487/RFC6232, May 2011,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6232">http://www.rfc-editor.org/info/rfc6232</a>&gt;.

   [<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>, DOI 10.17487/RFC6325, July 2011,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6325">http://www.rfc-editor.org/info/rfc6325</a>&gt;.






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<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


   [<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>, DOI 10.17487/RFC6361, August 2011,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6361">http://www.rfc-editor.org/info/rfc6361</a>&gt;.

   [<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>,
              DOI 10.17487/RFC7172, May 2014,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7172">http://www.rfc-editor.org/info/rfc7172</a>&gt;.

   [<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>,
              DOI 10.17487/RFC7176, May 2014,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7176">http://www.rfc-editor.org/info/rfc7176</a>&gt;.

   [<a id="ref-RFC7177">RFC7177</a>]  Eastlake 3rd, D., Perlman, R., Ghanwani, A., Yang, H., and
              V. Manral, "Transparent Interconnection of Lots of Links
              (TRILL): Adjacency", <a href="./rfc7177">RFC 7177</a>, DOI 10.17487/RFC7177,
              May 2014, &lt;<a href="http://www.rfc-editor.org/info/rfc7177">http://www.rfc-editor.org/info/rfc7177</a>&gt;.

   [<a id="ref-RFC7179">RFC7179</a>]  Eastlake 3rd, D., Ghanwani, A., Manral, V., Li, Y., and C.
              Bestler, "Transparent Interconnection of Lots of Links
              (TRILL): Header Extension", <a href="./rfc7179">RFC 7179</a>,
              DOI 10.17487/RFC7179, May 2014,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7179">http://www.rfc-editor.org/info/rfc7179</a>&gt;.

   [<a id="ref-RFC7356">RFC7356</a>]  Ginsberg, L., Previdi, S., and Y. Yang, "IS-IS Flooding
              Scope Link State PDUs (LSPs)", <a href="./rfc7356">RFC 7356</a>,
              DOI 10.17487/RFC7356, September 2014,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7356">http://www.rfc-editor.org/info/rfc7356</a>&gt;.

   [<a id="ref-RFC7455">RFC7455</a>]  Senevirathne, T., Finn, N., Salam, S., Kumar, D., Eastlake
              3rd, D., Aldrin, S., and Y. Li, "Transparent
              Interconnection of Lots of Links (TRILL): Fault
              Management", <a href="./rfc7455">RFC 7455</a>, DOI 10.17487/RFC7455, March 2015,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7455">http://www.rfc-editor.org/info/rfc7455</a>&gt;.













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<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


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

   [<a id="ref-802">802</a>]      IEEE 802, "IEEE Standard for Local and Metropolitan Area
              Networks: Overview and Architecture",
              DOI 10.1109/IEEESTD.2014.6847097, IEEE Std 802-2014.

   [<a id="ref-Centralized-Replication">Centralized-Replication</a>]
              Hao, W., Li, Y., Durrani, M., Gupta, S., Qu, A., and T.
              Han, "Centralized Replication for BUM traffic in
              active-active edge connection", Work in Progress,
              <a href="./draft-ietf-trill-centralized-replication-03">draft-ietf-trill-centralized-replication-03</a>,
              November 2015.

   [<a id="ref-Err3002">Err3002</a>]  RFC Errata, Erratum ID 3002, <a href="./rfc6325">RFC 6325</a>.

   [<a id="ref-Err3003">Err3003</a>]  RFC Errata, Erratum ID 3003, <a href="./rfc6325">RFC 6325</a>.

   [<a id="ref-Err3004">Err3004</a>]  RFC Errata, Erratum ID 3004, <a href="./rfc6325">RFC 6325</a>.

   [<a id="ref-Err3052">Err3052</a>]  RFC Errata, Erratum ID 3052, <a href="./rfc6325">RFC 6325</a>.

   [<a id="ref-Err3053">Err3053</a>]  RFC Errata, Erratum ID 3053, <a href="./rfc6325">RFC 6325</a>.

   [<a id="ref-Err3508">Err3508</a>]  RFC Errata, Erratum ID 3508, <a href="./rfc6325">RFC 6325</a>.

   [<a id="ref-RFC792">RFC792</a>]   Postel, J., "Internet Control Message Protocol", STD 5,
              <a href="./rfc792">RFC 792</a>, DOI 10.17487/RFC0792, September 1981,
              &lt;<a href="http://www.rfc-editor.org/info/rfc792">http://www.rfc-editor.org/info/rfc792</a>&gt;.

   [<a id="ref-RFC826">RFC826</a>]   Plummer, D., "Ethernet Address Resolution Protocol: Or
              Converting Network Protocol Addresses to 48.bit Ethernet
              Address for Transmission on Ethernet Hardware", STD 37,
              <a href="./rfc826">RFC 826</a>, DOI 10.17487/RFC0826, November 1982,
              &lt;<a href="http://www.rfc-editor.org/info/rfc826">http://www.rfc-editor.org/info/rfc826</a>&gt;.

   [<a id="ref-RFC4086">RFC4086</a>]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", <a href="https://www.rfc-editor.org/bcp/bcp106">BCP 106</a>, <a href="./rfc4086">RFC 4086</a>,
              DOI 10.17487/RFC4086, June 2005,
              &lt;<a href="http://www.rfc-editor.org/info/rfc4086">http://www.rfc-editor.org/info/rfc4086</a>&gt;.

   [<a id="ref-RFC5226">RFC5226</a>]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", <a href="https://www.rfc-editor.org/bcp/bcp26">BCP 26</a>, <a href="./rfc5226">RFC 5226</a>,
              DOI 10.17487/RFC5226, May 2008,
              &lt;<a href="http://www.rfc-editor.org/info/rfc5226">http://www.rfc-editor.org/info/rfc5226</a>&gt;.







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   [<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>, DOI 10.17487/RFC6327, July 2011,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6327">http://www.rfc-editor.org/info/rfc6327</a>&gt;.

   [<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>, DOI 10.17487/RFC6439, November 2011,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6439">http://www.rfc-editor.org/info/rfc6439</a>&gt;.

   [<a id="ref-RFC6439bis">RFC6439bis</a>]
              Eastlake 3rd, D., Li, Y., Umair, M., Banerjee, A., and H.
              Fangwei, "TRILL: Appointed Forwarders", Work in Progress,
              <a href="./draft-ietf-trill-rfc6439bis-01">draft-ietf-trill-rfc6439bis-01</a>, January 2016.

   [<a id="ref-RFC7042">RFC7042</a>]  Eastlake 3rd, D. and J. Abley, "IANA Considerations and
              IETF Protocol and Documentation Usage for IEEE 802
              Parameters", <a href="https://www.rfc-editor.org/bcp/bcp141">BCP 141</a>, <a href="./rfc7042">RFC 7042</a>, DOI 10.17487/RFC7042,
              October 2013, &lt;<a href="http://www.rfc-editor.org/info/rfc7042">http://www.rfc-editor.org/info/rfc7042</a>&gt;.

   [<a id="ref-RFC7067">RFC7067</a>]  Dunbar, L., Eastlake 3rd, D., Perlman, R., and I.
              Gashinsky, "Directory Assistance Problem and High-Level
              Design Proposal", <a href="./rfc7067">RFC 7067</a>, DOI 10.17487/RFC7067,
              November 2013, &lt;<a href="http://www.rfc-editor.org/info/rfc7067">http://www.rfc-editor.org/info/rfc7067</a>&gt;.

   [<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>, DOI 10.17487/RFC7175, May 2014,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7175">http://www.rfc-editor.org/info/rfc7175</a>&gt;.

   [<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>,
              DOI 10.17487/RFC7178, May 2014,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7178">http://www.rfc-editor.org/info/rfc7178</a>&gt;.

   [<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>, DOI 10.17487/RFC7180, May 2014,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7180">http://www.rfc-editor.org/info/rfc7180</a>&gt;.









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   [<a id="ref-RFC7357">RFC7357</a>]  Zhai, H., Hu, F., Perlman, R., Eastlake 3rd, D., and O.
              Stokes, "Transparent Interconnection of Lots of Links
              (TRILL): End Station Address Distribution Information
              (ESADI) Protocol", <a href="./rfc7357">RFC 7357</a>, DOI 10.17487/RFC7357,
              September 2014, &lt;<a href="http://www.rfc-editor.org/info/rfc7357">http://www.rfc-editor.org/info/rfc7357</a>&gt;.

   [<a id="ref-TRILL-L3-GW">TRILL-L3-GW</a>]
              Hao, W., Li, Y., Qu, A., Durrani, M., Sivamurugan, P., and
              L. Xia, "TRILL Distributed Layer 3 Gateway", Work in
              Progress, <a href="./draft-ietf-trill-irb-10">draft-ietf-trill-irb-10</a>, January 2016.









































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<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>.  Life Cycle of a TRILL Switch Port (New)</span>

   Text from &lt;<a href="http://www.ietf.org/mail-archive/web/trill/current/msg06355.html">http://www.ietf.org/mail-archive/web/trill/</a>
   <a href="http://www.ietf.org/mail-archive/web/trill/current/msg06355.html">current/msg06355.html</a>&gt; is paraphrased in this informational appendix.

   Question:
      Suppose we are developing a TRILL implementation to run on
      different machines.  Then what happens first?  Is LSP flooding or
      ESADI started first?  -&gt; Link-state database creation -&gt;
      Designated RBridge election (How to set priority?  Any fixed
      process that depends on user settings?) -&gt; etc.

   Answer:
      The first thing that happens on a port/link is any link setup that
      is needed.  For example, on a PPP link [<a href="./rfc6361" title="&quot;PPP Transparent Interconnection of Lots of Links (TRILL) Protocol Control Protocol&quot;">RFC6361</a>], you need to
      negotiate that you will be using TRILL.  However, if you have
      Ethernet links [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>], which are probably the most common type,
      there isn't any link setup needed.

      As soon as the port is set up, it can ingress or egress native
      frames if end-station service is being offered on that port.
      Offering end-station service is the default.  However, if the port
      trunk bit (end-station service disable) is set or the port is
      configured as an IS-IS point-to-point link port, then end-station
      service is not offered; therefore, native frames received are
      ignored, and native frames are not egressed.

      TRILL IS-IS Hellos then get sent out the port to be exchanged with
      any other TRILL switches on the link [<a href="./rfc7177" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Adjacency&quot;">RFC7177</a>].  Only the Hellos
      are required; optionally, you might also exchange MTU-probe/ack
      PDUs [<a href="./rfc7177" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Adjacency&quot;">RFC7177</a>], BFD PDUs [<a href="./rfc7175" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Bidirectional Forwarding Detection (BFD) Support&quot;">RFC7175</a>], or other link test packets.

      TRILL doesn't send any TRILL Data or TRILL IS-IS packets out the
      port to the link, except for Hellos, until the link gets to the
      2-Way or Report state [<a href="./rfc7177" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Adjacency&quot;">RFC7177</a>].

      If a link is configured as a point-to-point link, there is no
      Designated RBridge (DRB) election.  By default, an Ethernet link
      is considered a LAN link, and the DRB election occurs when the
      link is in any state other than Down.  You don't have to configure
      priorities for each TRILL switch (RBridge) to be the DRB.  Things
      will work fine with all the RBridges on a link using default
      priority.  But if the network manager wants to control this, there
      should be a way for them to configure the priority to be the DRB
      of the TRILL switch ports on the link.






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      (To avoid complexity, this appendix generally describes the
      life cycle for a link that only has two TRILL switches on it.  But
      TRILL works fine as currently specified on a broadcast link with
      multiple TRILL switches on it -- actually, multiple TRILL switch
      ports -- since a TRILL switch can have multiple ports connected to
      the same link.  The most likely way to get such a multi-access
      link with current technology and the existing TRILL standards is
      to have more than two TRILL switch Ethernet ports connected to a
      bridged LAN.  The TRILL protocol operates above all bridging; in
      general, the bridged LAN looks like a transparent broadcast link
      to TRILL.)

      When a link gets to the 2-Way or Report state, LSPs, CSNPs, and
      PSNPs will start to flow on the link (as well as FS-LSPs,
      FS-CSNPs, and FS-PSNPs for E-L1FS (see <a href="#section-8.1">Section 8.1</a>)).

      When a link gets to the Report state, there is adjacency.  The
      existence of that adjacency is flooded (reported) to the campus in
      LSPs.  TRILL Data packets can then start to flow on the link as
      TRILL switches recalculate the least-cost paths and distribution
      trees to take the new adjacency into account.  Until it gets to
      the Report state, there is no adjacency, and no TRILL Data packets
      can flow over that link (with the minor corner case exception that
      an RBridge Channel message can, for its first hop only, be sent on
      a port where there is no adjacency (<a href="./rfc7178#section-2.4">Section&nbsp;2.4 of [RFC7178]</a>).
      (Although this paragraph seems to be talking about link state, it
      is actually port state.  It is possible for different TRILL switch
      ports on the same link to temporarily be in different states.  The
      adjacency state machinery runs independently on each port.)

      ESADI [<a href="./rfc7357" title="&quot;Transparent Interconnection of Lots of Links (TRILL): End Station Address Distribution Information (ESADI) Protocol&quot;">RFC7357</a>] is built on top of the regular TRILL Data routing.
      Since ESADI PDUs look, to transit TRILL switches, like regular
      TRILL Data packets, no ESADI PDUs can flow until adjacencies are
      established and TRILL Data is flowing.  Of course, ESADI is
      optional and is not used unless configured.
















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   Question:
      Does it require TRILL Full Headers at the time TRILL LSPs start
      being broadcast on a link?  Because at that time it's not defined
      egress and ingress nicknames.

   Answer:
      TRILL Headers are only for TRILL Data packets.  TRILL IS-IS
      packets, such as TRILL LSPs, are sent in a different way that does
      not use a TRILL Header and does not depend on nicknames.

      Probably, in most implementations, a TRILL switch will start up
      using the same nickname it had when it shut down or last got
      disconnected from a campus.  If you want, you can implement TRILL
      to come up initially not reporting any nickname (by not including
      a Nickname sub-TLV in its LSPs) until you get the link-state
      database or most of the link-state database, and then choose a
      nickname no other TRILL switch in the campus is using.  Of course,
      if a TRILL switch does not have a nickname, then it cannot ingress
      data, cannot egress known unicast data, and cannot be a tree root.

      TRILL IS-IS PDUs such as LSPs, and the link-state database, all
      work based on the 7-byte IS-IS System ID (sometimes called the
      LAN ID [<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>]).  Since topology determination uses System IDs,
      which are always unique across the campus, it is not affected by
      the nickname assignment state.  The nickname system is built on
      top of that.

























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<span class="h2"><a class="selflink" id="appendix-B" href="#appendix-B">Appendix B</a>.  Example TRILL PDUs (New)</span>

   This appendix shows example TRILL IS-IS PDUs.  The primary purpose of
   these examples is to clarify issues related to bit ordering.

   The examples in this appendix concentrate on the format of the packet
   header and trailer.  There are frequently unspecified optional items
   or data in the packet that would affect header or trailer fields like
   the packet length or checksum.  Thus, an "Xed out" placeholder is
   used for such fields, where each X represents one hex nibble.

<span class="h3"><a class="selflink" id="appendix-B.1" href="#appendix-B.1">B.1</a>.  LAN Hello over Ethernet</span>

   A TRILL Hello sent from a TRILL switch (RBridge) with 7-byte
   System ID 0x30033003300300 holding nickname 0xFFDE over Ethernet from
   a port with MAC address 0x00005E0053DE on VLAN 1 at priority 7.
   There is one neighbor that is the DRB.  The neighbor's port MAC is
   0x00005E0053E3, and the neighbor's System ID is 0x44444444444400.

      Ethernet Header
        Outer.MacDA, Outer.MacSA
          0x0180C2000041   All-IS-IS-RBridges Destination MAC Address
          0x00005E0053DE   Source MAC Address
        Outer VLAN Tag (optional)
          0x8100           C-VLAN Ethertype [<a href="#ref-802.1Q-2014">802.1Q-2014</a>]
          0xE001           Priority 7, Outer.VLAN
        IS-IS
          0x22F4           L2-IS-IS Ethertype
      IS-IS Payload
        Common Header
          0x83             Intradomain Routeing Protocol Discriminator
          0x08             Header Length
          0x01             IS-IS Version Number
          0x06             ID Length of 6 Bytes
          0x0F             PDU Type (Level 1 LAN Hello)
          0x01             Version
          0x00             Reserved
          0x01             Maximum Area Addresses
        Hello PDU Specific Fields
          0x01             Circuit Type (Level 1)
          0x30033003300300 Source System ID
          0x0009           Holding Time
          0xXXXX           PDU Length
          0x40             Priority to be DRB
          0x44444444444400 LAN ID
        TLVs (the following order of TLVs or of sub-TLVs in a TLV
          is not significant)




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        Area Addresses TLV
          0x01             Area Addresses Type
          0x02             Length of Value
          0x01             Length of Address
          0x00             The fixed TRILL Area Address
        MT Port Capabilities TLV
          0x8F             MT Port Capabilities Type
          0x0011           Length of Value
          0x0000           Topology
            Special VLANs and Flags Sub-TLV
              0x01            Sub-TLV Type
              0x08            Length
              0x0123          Port ID
              0xFFDE          Sender Nickname
              0x0001          Outer.VLAN
              0x0001          Designated VLAN
            Enabled VLANs Sub-TLV (optional)
              0x02            Sub-TLV Type
              0x03            Length
              0x0001          Start VLAN 1
              0x80            VLAN 1
        TRILL Neighbor TLV
          0x91            Neighbor Type
          0x0A            Length of Value
          0xC0            S Flag = 1, L Flag = 1, SIZE field 0
            NEIGHBOR RECORD
              0x00            Flags
              0x2328          MTU = 9 KB
              0x00005E0053E3  Neighbor MAC Address
        Scope Flooding Support TLV
        0xF3              Scope Flooding Support Type
        0x01              Length of Value
        0x40              E-L1FS Flooding Scope
        More TLVs (optional)
          ...
      Ethernet Trailer
        0xXXXXXXXX      Ethernet Frame Check Sequence (FCS)














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<span class="h3"><a class="selflink" id="appendix-B.2" href="#appendix-B.2">B.2</a>.  LSP over PPP</span>

   Here is an example of a TRILL LSP sent over a PPP link by the same
   source TRILL switch as the example in <a href="#appendix-B.1">Appendix B.1</a>.

      PPP Header
        0x405D               PPP TRILL Link State Protocol
      IS-IS Payload
        Common Header
          0x83               Intradomain Routeing Protocol Discriminator
          0x08               Header Length
          0x01               IS-IS Version Number
          0x06               ID Length of 6 Bytes
          0x12               PDU Type (Level 1 LSP)
          0x01               Version
          0x00               Reserved
          0x01               Maximum Area Addresses
        LSP Specific Fields
          0xXXXX             PDU Length
          0x0123             Remaining Lifetime
          0x3003300330030009 LSP ID (fragment 9)
          0x00001234         Sequence Number
          0xXXXX             Checksum
          0x01               Flags = Level 1
        TLVs (the following order of TLVs or of sub-TLVs in a TLV
          is not significant)
        Router Capability TLV
          0xF2               Router Capability Type
          0x0F               Length of Value
          0x00               Flags
            Nickname Sub-TLV
              0x06           Sub-TLV Type
              0x05           Length of Value
              NICKNAME RECORD
                0x33         Nickname Priority
                0x1234       Tree Root Priority
                0xFFDE       Nickname
            TRILL Version Sub-TLV
              0x0D           Sub-TLV Type
              0x05
              0x00           Max Version
              0x40000000     Flags = FGL Support
        More TLVs (optional
          ...
      PPP Trailer
        0xXXXXXX        PPP Frame Check Sequence (FCS)





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<span class="h3"><a class="selflink" id="appendix-B.3" href="#appendix-B.3">B.3</a>.  TRILL Data over Ethernet</span>

   Below is an IPv4 ICMP Echo [<a href="./rfc792" title="&quot;Internet Control Message Protocol&quot;">RFC792</a>] sent in a TRILL Data packet from
   the TRILL switch that sent the Hello in <a href="#appendix-B.1">Appendix B.1</a> to the neighbor
   TRILL switch on the link used in <a href="#appendix-B.1">Appendix B.1</a>.

      Ethernet Header
        Outer.MacDA, Outer.MacSA
          0x00005E0053E3  Destination MAC Address
          0x00005E0053DE  Source MAC Address
        Outer VLAN Tag (optional)
          0x8100          C-VLAN Ethertype [<a href="#ref-802.1Q-2014">802.1Q-2014</a>]
          0x0001          Priority 0, Outer.VLAN 1
        TRILL
          0x22F3          TRILL Ethertype
      TRILL Header
          0X000E          Flags, Hop Count 14
          0xFFDF          Egress Nickname
          0xFFDC          Ingress Nickname
      Inner Ethernet Header
        Inner.MacDA, Inner.MacSA
          0x00005E005322  Destination MAC Address
          0x00005E005344  Source MAC Address
        Inner VLAN Tag
          0x8100          C-VLAN Ethertype
          0x0022          Priority 0, Inner.VLAN 34
        Ethertype
          0x0800          IPv4 Ethertype
      IP Header
          0x4500          Version 4, Header Length 5, ToS 0
          0xXXXX          Total Length
          0x3579          Identification
          0x0000          Flags, Fragment Offset
          0x1101          TTL 17, ICMP = Protocol 1
          0xXXXX          Header Checksum
          0xC0000207      Source IP 192.0.2.7
          0xC000020D      Destination IP 192.0.2.13
          0x00000000      Options, Padding
      ICMP
          0x0800          ICMP Echo
          0xXXXX          Checksum
          0x87654321      Identifier, Sequence Number
          ...             Echo Data
      Ethernet Trailer
        0xXXXXXXXX      Ethernet Frame Check Sequence (FCS)






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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-52" ></span>
<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


<span class="h3"><a class="selflink" id="appendix-B.4" href="#appendix-B.4">B.4</a>.  TRILL Data over PPP</span>

   Below is an ARP Request [<a href="./rfc826" title="&quot;Ethernet Address Resolution Protocol: Or Converting Network Protocol Addresses to 48.bit Ethernet Address for Transmission on Ethernet Hardware&quot;">RFC826</a>] sent in a TRILL Data packet from the
   TRILL switch that sent the Hello in <a href="#appendix-B.1">Appendix B.1</a> over a PPP link.

      PPP Header
        0x005D          PPP TRILL Network Protocol
      TRILL Header
          0X080D          Flags (M = 1), Hop Count 13
          0xFFDD          Distribution Tree Root Nickname
          0xFFDC          Ingress Nickname
      Inner Ethernet Header
        Inner.MacDA, Inner.MacSA
          0xFFFFFFFFFFFF  Destination MAC Address
          0x00005E005344  Source MAC Address
        Inner VLAN Tag
          0x8100          C-VLAN Ethertype
          0x0022          Priority 0, Inner.VLAN 34
        Ethertype
          0x0806          ARP Ethertype
      ARP
          0x0001          Hardware Address Space = Ethernet
          0x0001          Protocol Address Space = IPv4
          0x06            Size of Hardware Address
          0x04            Size of Protocol Address
          0x0001          OpCode = Request
          0x00005E005344  Sender Hardware Address
          0xC0000207      Sender Protocol Address 192.0.2.7
          0x000000000000  Target Hardware Address
          0xC000020D      Target Protocol Address 192.0.2.13
      PPP Trailer
        0xXXXXXX        PPP Frame Check Sequence (FCS)



















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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-53" ></span>
<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


<span class="h2"><a class="selflink" id="appendix-C" href="#appendix-C">Appendix C</a>.  Changes to Previous RFCs (New)</span>

<span class="h3"><a class="selflink" id="appendix-C.1" href="#appendix-C.1">C.1</a>.  Changes to Obsoleted <a href="./rfc7180">RFC 7180</a></span>

   This section summarizes the changes, augmentations, and excisions
   this document specifies for [<a href="./rfc7180" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Clarifications, Corrections, and Updates&quot;">RFC7180</a>], which it obsoletes and
   replaces.

<span class="h4"><a class="selflink" id="appendix-C.1.1" href="#appendix-C.1.1">C.1.1</a>.  Changes</span>

   For each section header in this document ending with "(Changed)",
   this section summarizes the changes that are made by this document:

   <a href="#section-1">Section 1</a> ("Introduction"): Numerous changes to reflect the overall
   changes in contents.

   <a href="#section-1.1">Section 1.1</a> ("Precedence"): Changed to add mention of [<a href="./rfc7179" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Header Extension&quot;">RFC7179</a>].

   <a href="#section-1.3">Section 1.3</a> ("Terminology and Acronyms"): Numerous terms added.

   <a href="#section-3">Section 3</a> ("Distribution Trees and RPF Check"): Changed by the
   addition of the new material in <a href="#section-3.6">Section 3.6</a>.  See <a href="#appendix-C.1.2">Appendix C.1.2</a>,
   Item 1.

   <a href="#section-8">Section 8</a> ("Other IS-IS Considerations"): Changed by the addition of
   Sections <a href="#section-8.1">8.1</a>, <a href="#section-8.2">8.2</a>, <a href="#section-8.3">8.3</a>, and <a href="#section-8.4">8.4</a>.  See <a href="#appendix-C.1.2">Appendix C.1.2</a> -- Items 2, 3,
   4, and 5, respectively.

   <a href="#section-9">Section 9</a> ("Updates to <a href="./rfc7177">RFC 7177</a> (Adjacency)": Changes and additions
   to [<a href="./rfc7177" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Adjacency&quot;">RFC7177</a>] to support E-L1FS.  See <a href="#appendix-C.1.2">Appendix C.1.2</a>, Item 2.

   <a href="#section-12">Section 12</a> ("IANA Considerations"): Changed by the addition of
   material in <a href="#section-12.2">Section 12.2</a>.  See <a href="#appendix-C.1.2">Appendix C.1.2</a>, Item 7.

   <a href="#section-13">Section 13</a> ("Security Considerations"): Minor changes in the RFCs
   listed.

<span class="h4"><a class="selflink" id="appendix-C.1.2" href="#appendix-C.1.2">C.1.2</a>.  Additions</span>

   This document contains the following material not present in
   [<a href="./rfc7180" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Clarifications, Corrections, and Updates&quot;">RFC7180</a>]:

   1.  Support for an alternative Reverse Path Forwarding Check (RPFC),
       along with considerations for deciding between the original
       [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] RPFC and this alternative RPFC.  This alternative RPFC
       was originally discussed on the TRILL WG mailing list in
       &lt;<a href="http://www.ietf.org/mail-archive/web/trill/current/msg01852.html">http://www.ietf.org/mail-archive/web/trill/current/</a>
       <a href="http://www.ietf.org/mail-archive/web/trill/current/msg01852.html">msg01852.html</a>&gt; and subsequent messages (<a href="#section-3.6">Section 3.6</a>).



<span class="grey">Eastlake, et al.             Standards Track                   [Page 53]</span></pre>
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<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


   2.  Mandatory E-L1FS [<a href="./rfc7356" title="&quot;IS-IS Flooding Scope Link State PDUs (LSPs)&quot;">RFC7356</a>] support (Sections <a href="#section-8.1">8.1</a> and <a href="#section-9">9</a>).

   3.  Recommendations concerning control packet priorities
       (<a href="#section-8.2">Section 8.2</a>).

   4.  Implementation requirements concerning unknown IS-IS PDU types
       (<a href="#section-8.3">Section 8.3</a>).

   5.  Specification of an optional Nickname Flags APPsub-TLV and an
       ingress flag within that APPsub-TLV (<a href="#section-8.4">Section 8.4</a>).

   6.  Update to the TRILL Header to allocate a Color bit
       (<a href="#section-10.1">Section 10.1</a>), and update to the optional TRILL Header Extension
       flags word to allocate a 2-bit Extended Color field
       (<a href="#section-10.2">Section 10.2</a>).

   7.  Some new IANA Considerations in <a href="#section-12.2">Section 12.2</a>, including
       reservation of nicknames for use as examples in documentation.

   8.  A new "Appointed Forwarder Status Lost Counter" section
       (<a href="#section-11">Section 11</a> of this document) that loosens the mandatory update
       requirements specified in [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>].

   9.  Informative <a href="#appendix-A">Appendix A</a> on the life cycle of a TRILL port.

   10. A new <a href="#appendix-B">Appendix B</a> containing example TRILL PDUs.

   11. Recommendation to use the Purge Originator Identification TLV
       (<a href="#section-8.6">Section 8.6</a>).

<span class="h4"><a class="selflink" id="appendix-C.1.3" href="#appendix-C.1.3">C.1.3</a>.  Deletions</span>

   This document omits the following material that was present in
   [<a href="./rfc7180" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Clarifications, Corrections, and Updates&quot;">RFC7180</a>]:

   1.  All updates to [<a href="./rfc6327" title="&quot;Routing Bridges (RBridges): Adjacency&quot;">RFC6327</a>] that occurred in [<a href="./rfc7180" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Clarifications, Corrections, and Updates&quot;">RFC7180</a>].  These have
       been rolled into [<a href="./rfc7177" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Adjacency&quot;">RFC7177</a>], which obsoletes [<a href="./rfc6327" title="&quot;Routing Bridges (RBridges): Adjacency&quot;">RFC6327</a>].  However,
       new updates to [<a href="./rfc7177" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Adjacency&quot;">RFC7177</a>] are included (see <a href="#appendix-C.3">Appendix C.3</a>).

   2.  All updates to [<a href="./rfc6439" title="&quot;Routing Bridges (RBridges): Appointed Forwarders&quot;">RFC6439</a>].  These have been rolled into
       [<a href="#ref-RFC6439bis">RFC6439bis</a>], which is intended to obsolete [<a href="./rfc6439" title="&quot;Routing Bridges (RBridges): Appointed Forwarders&quot;">RFC6439</a>].










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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-55" ></span>
<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


<span class="h3"><a class="selflink" id="appendix-C.2" href="#appendix-C.2">C.2</a>.  Changes to <a href="./rfc6325">RFC 6325</a></span>

   This document contains many normative updates to [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>], some of
   which were also in [<a href="./rfc7180" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Clarifications, Corrections, and Updates&quot;">RFC7180</a>], which this document replaces.  These
   changes include the following:

   1.  Changing nickname allocation to ignore conflicts with RBridges
       that are IS-IS unreachable.

   2.  Fixing errors: [<a href="#ref-Err3002" title="Erratum ID 3002">Err3002</a>], [<a href="#ref-Err3003" title="Erratum ID 3003">Err3003</a>], [<a href="#ref-Err3004" title="Erratum ID 3004">Err3004</a>], [<a href="#ref-Err3052" title="Erratum ID 3052">Err3052</a>],
       [<a href="#ref-Err3053" title="Erratum ID 3053">Err3053</a>], and [<a href="#ref-Err3508" title="Erratum ID 3508">Err3508</a>].

   3.  Changing the requirement to use the RPF check described in
       [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>] for multi-destination TRILL Data packets by providing
       an alternative stronger RPF check.

   4.  Adoption of the change of the CFI bit, which was required to be
       zero in the inner frame, to the DEI bit, which is obtained from
       inner frame ingress or creation.

   5.  Requiring that all RBridges support E-L1FS FS-LSP flooding.

   6.  Reducing the variable-length TRILL Header extensions area to one
       optional flags word.  The Extension Length field (called
       "Op-Length" in [<a href="./rfc6325" title="&quot;Routing Bridges (RBridges): Base Protocol Specification&quot;">RFC6325</a>]) is reduced to 1 bit that indicates
       whether the flags word is present.  The rest of that Length field
       is now reserved.

   7.  Changing the mandatory Appointed Forwarder Status Lost Counter
       increment provisions, as specified in <a href="#section-11">Section 11</a>.

<span class="h3"><a class="selflink" id="appendix-C.3" href="#appendix-C.3">C.3</a>.  Changes to <a href="./rfc7177">RFC 7177</a></span>

   All of the updates to [<a href="./rfc7177" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Adjacency&quot;">RFC7177</a>] herein are in <a href="#section-9">Section 9</a>.  Basically,
   this document requires that a Scope Flooding Support TLV [<a href="./rfc7356" title="&quot;IS-IS Flooding Scope Link State PDUs (LSPs)&quot;">RFC7356</a>]
   appear in all Hellos and that TRILL switches retain in their
   adjacency state the information received in that TLV.

<span class="h3"><a class="selflink" id="appendix-C.4" href="#appendix-C.4">C.4</a>.  Changes to <a href="./rfc7179">RFC 7179</a></span>

   The updates to [<a href="./rfc7179" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Header Extension&quot;">RFC7179</a>] herein are in Sections <a href="#section-10.2">10.2</a> and <a href="#section-10.3">10.3</a>.










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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-56" ></span>
<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


Acknowledgments

   The contributions of the following individuals to this document are
   gratefully acknowledged:

      Santosh Rajagopalan and Gayle Noble

   The contributions of the following (listed in alphabetical order) to
   the preceding version of this document, [<a href="./rfc7180" title="&quot;Transparent Interconnection of Lots of Links (TRILL): Clarifications, Corrections, and Updates&quot;">RFC7180</a>], are gratefully
   acknowledged:

      Somnath Chatterjee, Weiguo Hao, Rakesh Kumar, Yizhou Li, Radia
      Perlman, Varun Shah, Mike Shand, and Meral Shirazipour.

Authors' Addresses

   Donald Eastlake 3rd
   Huawei Technology
   155 Beaver Street
   Milford, MA  01757
   United States

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


   Mingui Zhang
   Huawei Technologies
   No. 156 Beiqing Rd., Haidian District
   Beijing  100095
   China

   Email: zhangmingui@huawei.com


   Radia Perlman
   EMC
   2010 256th Avenue NE, #200
   Bellevue, WA  98007
   United States

   Email: radia@alum.mit.edu


   Ayan Banerjee
   Cisco

   Email: ayabaner@cisco.com



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<span class="grey"><a href="./rfc7780">RFC 7780</a>       TRILL Clarifications, Corrections, Updates  February 2016</span>


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

   Email: anoop@alumni.duke.edu


   Sujay Gupta
   IP Infusion
   RMZ Centennial
   Mahadevapura Post
   Bangalore  560048
   India

   Email: sujay.gupta@ipinfusion.com


































Eastlake, et al.             Standards Track                   [Page 57]
</pre>