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rfc:rfc6325

Internet Engineering Task Force (IETF) R. Perlman Request for Comments: 6325 Intel Labs Category: Standards Track D. Eastlake 3rd ISSN: 2070-1721 Huawei

                                                               D. Dutt
                                                                S. Gai
                                                         Cisco Systems
                                                           A. Ghanwani
                                                               Brocade
                                                             July 2011
      Routing Bridges (RBridges): Base Protocol Specification

Abstract

 Routing Bridges (RBridges) provide optimal pair-wise forwarding
 without configuration, safe forwarding even during periods of
 temporary loops, and support for multipathing of both unicast and
 multicast traffic.  They achieve these goals using IS-IS routing and
 encapsulation of traffic with a header that includes a hop count.
 RBridges are compatible with previous IEEE 802.1 customer bridges as
 well as IPv4 and IPv6 routers and end nodes.  They are as invisible
 to current IP routers as bridges are and, like routers, they
 terminate the bridge spanning tree protocol.
 The design supports VLANs and the optimization of the distribution of
 multi-destination frames based on VLAN ID and based on IP-derived
 multicast groups.  It also allows unicast forwarding tables at
 transit RBridges to be sized according to the number of RBridges
 (rather than the number of end nodes), which allows their forwarding
 tables to be substantially smaller than in conventional customer
 bridges.

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 Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6325.

Perlman, et al. Standards Track [Page 1] RFC 6325 RBridge Protocol July 2011

Copyright Notice

 Copyright (c) 2011 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) 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.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Perlman, et al. Standards Track [Page 2] RFC 6325 RBridge Protocol July 2011

Table of Contents

 1. Introduction ....................................................6
    1.1. Algorhyme V2, by Ray Perlner ...............................7
    1.2. Normative Content and Precedence ...........................7
    1.3. Terminology and Notation in This Document ..................7
    1.4. Categories of Layer 2 Frames ...............................8
    1.5. Acronyms ...................................................9
 2. RBridges .......................................................11
    2.1. General Overview ..........................................11
    2.2. End-Station Addresses .....................................12
    2.3. RBridge Encapsulation Architecture ........................13
    2.4. Forwarding Overview .......................................15
         2.4.1. Known-Unicast ......................................16
         2.4.2. Multi-Destination ..................................16
    2.5. RBridges and VLANs ........................................17
         2.5.1. Link VLAN Assumptions ..............................17
    2.6. RBridges and IEEE 802.1 Bridges ...........................18
         2.6.1. RBridge Ports and 802.1 Layering ...................18
         2.6.2. Incremental Deployment .............................20
 3. Details of the TRILL Header ....................................20
    3.1. TRILL Header Format .......................................20
    3.2. Version (V) ...............................................21
    3.3. Reserved (R) ..............................................21
    3.4. Multi-destination (M) .....................................22
    3.5. Op-Length .................................................22
    3.6. Hop Count .................................................22
    3.7. RBridge Nicknames .........................................23
         3.7.1. Egress RBridge Nickname ............................23
         3.7.2. Ingress RBridge Nickname ...........................24
         3.7.3. RBridge Nickname Selection .........................24
    3.8. TRILL Header Options ......................................26
 4. Other RBridge Design Details ...................................27
    4.1. Ethernet Data Encapsulation ...............................27
         4.1.1. VLAN Tag Information ...............................30
         4.1.2. Inner VLAN Tag .....................................31
         4.1.3. Outer VLAN Tag .....................................31
         4.1.4. Frame Check Sequence (FCS) .........................32
    4.2. Link State Protocol (IS-IS) ...............................32
         4.2.1. IS-IS RBridge Identity .............................32
         4.2.2. IS-IS Instances ....................................33
         4.2.3. TRILL IS-IS Frames .................................33
         4.2.4. TRILL Link Hellos, DRBs, and Appointed Forwarders ..34
                4.2.4.1. P2P Hello Links ...........................35
                4.2.4.2. Designated RBridge ........................35
                4.2.4.3. Appointed VLAN-x Forwarder ................36
                4.2.4.4. TRILL LSP Information .....................37
         4.2.5. The TRILL ESADI Protocol ...........................40

Perlman, et al. Standards Track [Page 3] RFC 6325 RBridge Protocol July 2011

                4.2.5.1. TRILL ESADI Participation .................42
                4.2.5.2. TRILL ESADI Information ...................42
         4.2.6. SPF, Forwarding, and Ambiguous Destinations ........43
    4.3. Inter-RBridge Link MTU Size ...............................43
         4.3.1. Determining Campus-Wide TRILL IS-IS MTU Size .......44
         4.3.2. Testing Link MTU Size ..............................44
    4.4. TRILL-Hello Protocol ......................................45
         4.4.1. TRILL-Hello Rationale ..............................45
         4.4.2. TRILL-Hello Contents and Timing ....................46
                4.4.2.1. TRILL Neighbor List .......................48
         4.4.3. TRILL MTU-Probe and TRILL Hello VLAN Tagging .......49
         4.4.4. Multiple Ports on the Same Link ....................50
         4.4.5. VLAN Mapping within a Link .........................51
    4.5. Distribution Trees ........................................52
         4.5.1. Distribution Tree Calculation ......................54
         4.5.2. Multi-Destination Frame Checks .....................55
         4.5.3. Pruning the Distribution Tree ......................57
         4.5.4. Tree Distribution Optimization .....................58
         4.5.5. Forwarding Using a Distribution Tree ...............59
    4.6. Frame Processing Behavior .................................60
         4.6.1. Receipt of a Native Frame ..........................60
                4.6.1.1. Native Unicast Case .......................60
                4.6.1.2. Native Multicast and Broadcast Frames .....61
         4.6.2. Receipt of a TRILL Frame ...........................62
                4.6.2.1. TRILL Control Frames ......................63
                4.6.2.2. TRILL ESADI Frames ........................63
                4.6.2.3. TRILL Data Frames .........................63
                4.6.2.4. Known Unicast TRILL Data Frames ...........63
                4.6.2.5. Multi-Destination TRILL Data Frames .......64
         4.6.3. Receipt of a Layer 2 Control Frame .................65
    4.7. IGMP, MLD, and MRD Learning ...............................66
    4.8. End-Station Address Details ...............................66
         4.8.1. Learning End-Station Addresses .....................67
         4.8.2. Learning Confidence Level Rationale ................68
         4.8.3. Forgetting End-Station Addresses ...................69
         4.8.4. Shared VLAN Learning ...............................70
    4.9. RBridge Ports .............................................71
         4.9.1. RBridge Port Configuration .........................71
         4.9.2. RBridge Port Structure .............................73
         4.9.3. BPDU Handling ......................................76
                4.9.3.1. Receipt of BPDUs ..........................76
                4.9.3.2. Root Bridge Changes .......................76
                4.9.3.3. Transmission of BPDUs .....................77
         4.9.4. Dynamic VLAN Registration ..........................77
 5. RBridge Parameters .............................................77
    5.1. Per RBridge ...............................................78
    5.2. Per Nickname Per RBridge ..................................79
    5.3. Per Port Per RBridge ......................................79

Perlman, et al. Standards Track [Page 4] RFC 6325 RBridge Protocol July 2011

    5.4. Per VLAN Per RBridge ......................................80
 6. Security Considerations ........................................80
    6.1. VLAN Security Considerations ..............................81
    6.2. BPDU/Hello Denial-of-Service Considerations ...............82
 7. Assignment Considerations ......................................82
    7.1. IANA Considerations .......................................83
    7.2. IEEE Registration Authority Considerations ................83
 8. Normative References ...........................................83
 9. Informative References .........................................85
 Appendix A. Incremental Deployment Considerations .................87
    A.1. Link Cost Determination ...................................87
    A.2. Appointed Forwarders and Bridged LANs .....................87
    A.3. Wiring Closet Topology ....................................89
         A.3.1. The RBridge Solution ...............................90
         A.3.2. The VLAN Solution ..................................90
         A.3.3. The Spanning Tree Solution .........................90
         A.3.4. Comparison of Solutions ............................91
 Appendix B. Trunk and Access Port Configuration ...................92
 Appendix C. Multipathing ..........................................92
 Appendix D. Determination of VLAN and Priority ....................95
 Appendix E. Support of IEEE 802.1Q-2005 Amendments ................95
    E.1. Completed Amendments ......................................96
    E.2. In-Process Amendments .....................................97
 Appendix F. Acknowledgements ......................................98

Table of Figures

 Figure 1: Interconnected RBridges .................................14
 Figure 2: An Ethernet Encapsulated TRILL Frame ....................14
 Figure 3: A PPP Encapsulated TRILL Frame ..........................14
 Figure 4: RBridge Port Model ......................................19
 Figure 5: TRILL Header ............................................21
 Figure 6: Options Area Initial Flags Octet ........................26
 Figure 7: TRILL Data Encapsulation over Ethernet ..................29
 Figure 8: VLAN Tag Information ....................................30
 Figure 9: TRILL IS-IS Frame Format ................................34
 Figure 10: TRILL ESADI Frame Format ...............................41
 Figure 11: Detailed RBridge Port Model ............................74
 Figure 12: Link Cost of a Bridged Link ............................87
 Figure 13: Wiring Closet Topology .................................89
 Figure 14: Multi-Destination Multipath ............................93
 Figure 15: Known Unicast Multipath ................................94

Perlman, et al. Standards Track [Page 5] RFC 6325 RBridge Protocol July 2011

1. Introduction

 In traditional IPv4 and IPv6 networks, each subnet has a unique
 prefix.  Therefore, a node in multiple subnets has multiple IP
 addresses, typically one per interface.  This also means that when an
 interface moves from one subnet to another, it changes its IP
 address.  Administration of IP networks is complicated because IP
 routers require per-port subnet address configuration.  Careful IP
 address management is required to avoid creating subnets that are
 sparsely populated, wasting addresses.
 IEEE 802.1 bridges avoid these problems by transparently gluing many
 physical links into what appears to IP to be a single LAN [802.1D].
 However, 802.1 bridge forwarding using the spanning tree protocol has
 some disadvantages:
 o  The spanning tree protocol works by blocking ports, limiting the
    number of forwarding links, and therefore creates bottlenecks by
    concentrating traffic onto selected links.
 o  Forwarding is not pair-wise shortest path, but is instead whatever
    path remains after the spanning tree eliminates redundant paths.
 o  The Ethernet header does not contain a hop count (or Time to Live
    (TTL)) field.  This is dangerous when there are temporary loops
    such as when spanning tree messages are lost or components such as
    repeaters are added.
 o  VLANs can partition when the spanning tree reconfigures.
 This document presents the design for RBridges (Routing Bridges
 [RBridges]) that implement the TRILL protocol and are poetically
 summarized below.  Rbridges combine the advantages of bridges and
 routers and, as specified in this document, are the application of
 link state routing to the VLAN-aware customer bridging problem.  With
 the exceptions discussed in this document, RBridges can incrementally
 replace IEEE [802.1Q-2005] or [802.1D] customer bridges.
 While RBridges can be applied to a variety of link protocols, this
 specification focuses on IEEE [802.3] links.  Use with other link
 types is expected to be covered in other documents.
 The TRILL protocol, as specified herein, is designed to be a Local
 Area Network protocol and not designed with the goal of scaling
 beyond the size of existing bridged LANs.  For further discussion of
 the problem domain addressed by RBridges, see [RFC5556].

Perlman, et al. Standards Track [Page 6] RFC 6325 RBridge Protocol July 2011

1.1. Algorhyme V2, by Ray Perlner

 I hope that we shall one day see
 A graph more lovely than a tree.
 A graph to boost efficiency
 While still configuration-free.
 A network where RBridges can
 Route packets to their target LAN.
 The paths they find, to our elation,
 Are least cost paths to destination!
 With packet hop counts we now see,
 The network need not be loop-free!
 RBridges work transparently,
 Without a common spanning tree.

1.2. Normative Content and Precedence

 The bulk of the normative material in this specification appears in
 Sections 1 through 4.  In case of conflict between provisions in
 these four sections, the provision in the higher numbered section
 prevails.

1.3. Terminology and Notation in This Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].
 "TRILL" is the protocol specified herein while an "RBridge" is a
 device that implements that protocol.  The second letter in Rbridge
 is case insensitive.  Both Rbridge and RBridge are correct.
 In this document, the term "link", unless otherwise qualified, means
 "bridged LAN", that is to say, the combination of one or more [802.3]
 links with zero or more bridges, hubs, repeaters, or the like.  The
 term "simple link" or the like is used indicate a point-to-point or
 multi-access link with no included bridges or RBridges.
 In this document, the term "port", unless otherwise qualified,
 includes physical, virtual [802.1AE], and pseudo [802.1X] ports.  The
 term "physical port" or the like is used to indicate the physical
 point of connection between an RBridge and a link.

Perlman, et al. Standards Track [Page 7] RFC 6325 RBridge Protocol July 2011

 A "campus" is to RBridges as a "bridged LAN" is to bridges.  An
 RBridge campus consists of a network of RBridges, bridges, hubs,
 repeaters, simple links, and the like and it is bounded by end
 stations and routers.
 The term "spanning tree" in this document includes both classic
 spanning tree and rapid spanning tree (as in the Rapid Spanning Tree
 Protocol).
 This document uses hexadecimal notation for MAC addresses.  Two
 hexadecimal digits represent each octet (that is, 8-bit byte), giving
 the value of the octet as an unsigned integer.  A hyphen separates
 successive octets.  This document consistently uses IETF bit
 ordering, although the physical order of bit transmission within an
 octet on an IEEE [802.3] link is from the lowest order bit to the
 highest order bit, the reverse of IETF ordering.

1.4. Categories of Layer 2 Frames

 In this document, Layer 2 frames are divided into five categories:
 o  Layer 2 control frames (such as Bridge PDUs (BPDUs))
 o  native frames (non-TRILL-encapsulated data frames)
 o  TRILL Data frames (TRILL-encapsulated data frames)
 o  TRILL control frames
 o  TRILL other frames
 The way these five types of frames are distinguished is as follows:
 o  Layer 2 control frames are those with a multicast destination
    address in the range 01-80-C2-00-00-00 to 01-80-C2-00-00-0F or
    equal to 01-80-C2-00-00-21.  RBridges MUST NOT encapsulate and
    forward such frames, though they MAY, unless otherwise specified
    in this document, perform the Layer 2 function (such as MAC-level
    security) of the control frame.  Frames with a destination address
    of 01-80-C2-00-00-00 (BPDU) or 01-80-C2-00-00-21 (VLAN
    Registration Protocol) are called "high-level control frames" in
    this document.  All other Layer 2 control frames are called "low-
    level control frames".
 o  Native frames are those that are not control frames and have an
    Ethertype other than "TRILL" or "L2-IS-IS" and have a destination
    MAC address that is not one of the 16 multicast addresses reserved
    for TRILL.
 o  TRILL Data frames have the Ethertype "TRILL".  In addition, TRILL
    data frames, if multicast, have the multicast destination MAC
    address "All-RBridges".

Perlman, et al. Standards Track [Page 8] RFC 6325 RBridge Protocol July 2011

 o  TRILL control frames have the Ethertype "L2-IS-IS".  In addition,
    TRILL control frames, if multicast, have the multicast destination
    MAC addresses of "All-IS-IS-RBridges".  (Note that ESADI frames
    look on the outside like TRILL data and are so handled but, when
    decapsulated, have the L2-IS-IS Ethertype.)
 o  TRILL other frames are those with any of the 16 multicast
    destination addresses reserved for TRILL other than All-RBridges
    and All-IS-IS-RBridges.  RBridges conformant to this specification
    MUST discard TRILL other frames.

1.5. Acronyms

 AllL1ISs - All Level 1 Intermediate Systems
 AllL2ISs - All Level 2 Intermediate Systems
 BPDU - Bridge PDU
 CHbH - Critical Hop-by-Hop
 CItE - Critical Ingress-to-Egress
 CSNP - Complete Sequence Number PDU
 DA - Destination Address
 DR - Designated Router
 DRB - Designated RBridge
 EAP - Extensible Authentication Protocol
 ECMP - Equal Cost Multipath
 EISS - Extended Internal Sublayer Service
 ESADI - End-Station Address Distribution Information
 FCS - Frame Check Sequence
 GARP - Generic Attribute Registration Protocol
 GVRP - GARP VLAN Registration Protocol
 IEEE - Institute of Electrical and Electronics Engineers
 IGMP - Internet Group Management Protocol

Perlman, et al. Standards Track [Page 9] RFC 6325 RBridge Protocol July 2011

 IP - Internet Protocol
 IS-IS - Intermediate System to Intermediate System
 ISS - Internal Sublayer Service
 LAN - Local Area Network
 LSP - Link State PDU
 MAC - Media Access Control
 MLD - Multicast Listener Discovery
 MRD - Multicast Router Discovery
 MTU - Maximum Transmission Unit
 MVRP - Multiple VLAN Registration Protocol
 NSAP - Network Service Access Point
 P2P - Point-to-point
 PDU - Protocol Data Unit
 PPP - Point-to-Point Protocol
 RBridge - Routing Bridge
 RPF - Reverse Path Forwarding
 SA - Source Address
 SNMP - Simple Network Management Protocol
 SPF - Shortest Path First
 TLV - Type, Length, Value
 TRILL - TRansparent Interconnection of Lots of Links
 VLAN - Virtual Local Area Network
 VRP - VLAN Registration Protocol

Perlman, et al. Standards Track [Page 10] RFC 6325 RBridge Protocol July 2011

2. RBridges

 This section provides a high-level overview of RBridges, which
 implement the TRILL protocol, omitting some details.  Sections 3 and
 4 below provide more detailed specifications.
 TRILL, as described in this document and with the exceptions
 discussed herein, provides [802.1Q-2005] VLAN-aware customer bridging
 service.  As described below, TRILL is layered above the ports of an
 RBridge.
 The RBridges specified by this document do not supply provider
 [802.1ad] or provider backbone [802.1ah] bridging or the like.  The
 extension of TRILL to provide such provider services is left for
 future work that will be separately documented.  However, provider or
 provider backbone bridges may be used to interconnect parts of an
 RBridge campus.

2.1. General Overview

 RBridges run a link state protocol amongst themselves.  This gives
 them enough information to compute pair-wise optimal paths for
 unicast, and calculate distribution trees for delivery of frames
 either to destinations whose location is unknown or to
 multicast/broadcast groups [RBridges] [RP1999].
 To mitigate temporary loop issues, RBridges forward based on a header
 with a hop count.  RBridges also specify the next hop RBridge as the
 frame destination when forwarding unicast frames across a shared-
 media link, which avoids spawning additional copies of frames during
 a temporary loop.  A Reverse Path Forwarding Check and other checks
 are performed on multi-destination frames to further control
 potentially looping traffic (see Section 4.5.2).
 The first RBridge that a unicast frame encounters in a campus, RB1,
 encapsulates the received frame with a TRILL header that specifies
 the last RBridge, RB2, where the frame is decapsulated.  RB1 is known
 as the "ingress RBridge" and RB2 is known as the "egress RBridge".
 To save room in the TRILL header and simplify forwarding lookups, a
 dynamic nickname acquisition protocol is run among the RBridges to
 select 2-octet nicknames for RBridges, unique within the campus,
 which are an abbreviation for the IS-IS ID of the RBridge.  The
 2-octet nicknames are used to specify the ingress and egress RBridges
 in the TRILL header.
 Multipathing of multi-destination frames through alternative
 distribution trees and ECMP (Equal Cost Multipath) of unicast frames
 are supported (see Appendix C).

Perlman, et al. Standards Track [Page 11] RFC 6325 RBridge Protocol July 2011

 Networks with a more mesh-like structure will benefit to a greater
 extent from the multipathing and optimal paths provided by TRILL than
 will more tree-like networks.
 RBridges run a protocol on a link to elect a "Designated RBridge"
 (DRB).  The TRILL-IS-IS election protocol on a link is a little
 different from the Layer 3 IS-IS [ISO10589] election protocol,
 because in TRILL it is essential that only one RBridge be elected
 DRB, whereas in Layer 3 IS-IS it is possible for multiple routers to
 be elected Designated Router (also known as Designated Intermediate
 System).  As with an IS-IS router, the DRB may give a pseudonode name
 to the link, issue an LSP (Link State PDU) on behalf of the
 pseudonode, and issues CSNPs (Complete Sequence Number PDUs) on the
 link.  Additionally, the DRB has some TRILL-specific duties,
 including specifying which VLAN will be the Designated VLAN used for
 communication between RBridges on that link (see Section 4.2.4.2).
 The DRB either encapsulates/decapsulates all data traffic to/from the
 link, or, for load splitting, delegates this responsibility, for one
 or more VLANs, to other RBridges on the link.  There must at all
 times be at most one RBridge on the link that
 encapsulates/decapsulates traffic for a particular VLAN.  We will
 refer to the RBridge appointed to forward VLAN-x traffic on behalf of
 the link as the "appointed VLAN-x forwarder" (see Section 4.2.4.3).
 (Section 2.5 discusses VLANs further.)
 Rbridges SHOULD support SNMPv3 [RFC3411].  The Rbridge MIB will be
 specified in a separate document.  If IP service is available to an
 RBridge, it SHOULD support SNMPv3 over UDP over IPv4 [RFC3417] and
 IPv6 [RFC3419]; however, management can be used, within a campus,
 even for an RBridge that lacks an IP or other Layer 3 transport stack
 or which does not have a Layer 3 address, by transporting SNMP with
 Ethernet [RFC4789].

2.2. End-Station Addresses

 An RBridge, RB1, that is the VLAN-x forwarder on any of its links
 MUST learn the location of VLAN-x end nodes, both on the links for
 which it is VLAN-x forwarder and on other links in the campus.  RB1
 learns the port, VLAN, and Layer 2 (MAC) addresses of end nodes on
 links for which it is VLAN-x forwarder from the source address of
 frames received, as bridges do (for example, see Section 8.7 of
 [802.1Q-2005]), or through configuration or a Layer 2 explicit
 registration protocol such as IEEE 802.11 association and
 authentication.  RB1 learns the VLAN and Layer 2 address of distant
 VLAN-x end nodes, and the corresponding RBridge to which they are

Perlman, et al. Standards Track [Page 12] RFC 6325 RBridge Protocol July 2011

 attached, by looking at the ingress RBridge nickname in the TRILL
 header and the VLAN and source MAC address of the inner frame of
 TRILL Data frames that it decapsulates.
 Additionally, an RBridge that is the appointed VLAN-x forwarder on
 one or more links MAY use the End-Station Address Distribution
 Information (ESADI) protocol to announce some or all of the attached
 VLAN-x end nodes on those links.
 The ESADI protocol could be used to announce end nodes that have been
 explicitly enrolled.  Such information might be more authoritative
 than that learned from data frames being decapsulated onto the link.
 Also, the addresses enrolled and distributed in this way can be more
 secure for two reasons: (1) the enrollment might be authenticated
 (for example, by cryptographically based EAP methods via [802.1X]),
 and (2) the ESADI protocol also supports cryptographic authentication
 of its messages [RFC5304] [RFC5310] for more secure transmission.
 If an end station is unplugged from one RBridge and plugged into
 another, then, depending on circumstances, frames addressed to that
 end station can be black-holed.  That is, they can be sent just to
 the older RBridge that the end station used to be connected to until
 cached address information at some remote RBridge(s) times out,
 possibly for a number of minutes or longer.  With the ESADI protocol,
 the link interruption from the unplugging can cause an immediate
 update to be sent.
 Even if the ESADI protocol is used to announce or learn attached end
 nodes, RBridges MUST still learn from received native frames and
 decapsulated TRILL Data frames unless configured not to do so.
 Advertising end nodes using ESADI is optional, as is learning from
 these announcements.
 (See Section 4.8 for further end-station address details.)

2.3. RBridge Encapsulation Architecture

 The Layer 2 technology used to connect Rbridges may be either IEEE
 [802.3] or some other link technology such as PPP [RFC1661].  This is
 possible since the RBridge relay function is layered on top of the
 Layer 2 technologies.  However, this document specifies only an IEEE
 802.3 encapsulation.
 Figure 1 shows two RBridges, RB1 and RB2, interconnected through an
 Ethernet cloud.  The Ethernet cloud may include hubs, point-to-point
 or shared media, IEEE 802.1D bridges, or 802.1Q bridges.

Perlman, et al. Standards Track [Page 13] RFC 6325 RBridge Protocol July 2011

  1. ———–

/ \

               +-----+     /   Ethernet   \    +-----+
               | RB1 |----<                >---| RB2 |
               +-----+     \    Cloud     /    +-----+
                            \            /
                             ------------
                   Figure 1: Interconnected RBridges
 Figure 2 shows the format of a TRILL data or ESADI frame traveling
 through the Ethernet cloud between RB1 and RB2.
                  +--------------------------------+
                  |     Outer Ethernet Header      |
                  +--------------------------------+
                  |          TRILL Header          |
                  +--------------------------------+
                  |     Inner Ethernet Header      |
                  +--------------------------------+
                  |        Ethernet Payload        |
                  +--------------------------------+
                  |         Ethernet FCS           |
                  +--------------------------------+
            Figure 2: An Ethernet Encapsulated TRILL Frame
 In the case of media different from Ethernet, the header specific to
 that media replaces the outer Ethernet header.  For example, Figure 3
 shows a TRILL encapsulation over PPP.
                  +--------------------------------+
                  |           PPP Header           |
                  +--------------------------------+
                  |          TRILL Header          |
                  +--------------------------------+
                  |     Inner Ethernet Header      |
                  +--------------------------------+
                  |        Ethernet Payload        |
                  +--------------------------------+
                  |             PPP FCS            |
                  +--------------------------------+
               Figure 3: A PPP Encapsulated TRILL Frame
 The outer header is link-specific and, although this document
 specifies only [802.3] links, other links are allowed.

Perlman, et al. Standards Track [Page 14] RFC 6325 RBridge Protocol July 2011

 In both cases, the inner Ethernet header and the Ethernet Payload
 come from the original frame and are encapsulated with a TRILL header
 as they travel between RBridges.  Use of a TRILL header offers the
 following benefits:
 1. loop mitigation through use of a hop count field;
 2. elimination of the need for end-station VLAN and MAC address
    learning in transit RBridges;
 3. direction of unicast frames towards the egress RBridge (this
    enables unicast forwarding tables of transit RBridges to be sized
    with the number of RBridges rather than the total number of end
    nodes); and
 4. provision of a separate VLAN tag for forwarding traffic between
    RBridges, independent of the VLAN of the native frame.
 When forwarding unicast frames between RBridges, the outer header has
 the MAC destination address of the next hop Rbridge, to avoid frame
 duplication if the inter-RBridge link is multi-access.  This also
 enables multipathing of unicast, since the transmitting RBridge can
 specify the next hop.  Having the outer header specify the
 transmitting RBridge as the source address ensures that any bridges
 inside the Ethernet cloud will not get confused, as they might be if
 multipathing is in use and they were to see the original source or
 ingress RBridge in the outer header.

2.4. Forwarding Overview

 RBridges are true routers in the sense that, in the forwarding of a
 frame by a transit RBridge, the outer Layer 2 header is replaced at
 each hop with an appropriate Layer 2 header for the next hop, and a
 hop count is decreased.  Despite these modifications of the outer
 Layer 2 header and the hop count in the TRILL header, the original
 encapsulated frame is preserved, including the original frame's VLAN
 tag.  See Section 4.6 for more details.
 From a forwarding standpoint, transit frames may be classified into
 two categories: known-unicast and multi-destination.  Layer 2 control
 frames and TRILL control and TRILL other frames are not transit
 frames, are not forwarded by RBridges, and are not included in these
 categories.

Perlman, et al. Standards Track [Page 15] RFC 6325 RBridge Protocol July 2011

2.4.1. Known-Unicast

 These frames have a unicast inner MAC destination address
 (Inner.MacDA) and are those for which the ingress RBridge knows the
 egress RBridge for the destination MAC address in the frame's VLAN.
 Such frames are forwarded Rbridge hop by Rbridge hop to their egress
 Rbridge.

2.4.2. Multi-Destination

 These are frames that must be delivered to multiple destinations.
 Multi-destination frames include the following:
 1. unicast frames for which the location of the destination is
    unknown: the Inner.MacDA is unicast, but the ingress RBridge does
    not know its location in the frame's VLAN.
 2. multicast frames for which the Layer 2 destination address is
    derived from an IP multicast address: the Inner.MacDA is
    multicast, from the set of Layer 2 multicast addresses derived
    from IPv4 [RFC1112] or IPv6 [RFC2464] multicast addresses.  These
    frames are handled somewhat differently in different subcases:
    2.1. IGMP [RFC3376] and MLD [RFC2710] multicast group membership
         reports
    2.2. IGMP [RFC3376] and MLD [RFC2710] queries and MRD [RFC4286]
         announcement messages
    2.3. other IP-derived Layer 2 multicast frames
 3. multicast frames for which the Layer 2 destination address is not
    derived from an IP multicast address: the Inner.MacDA is
    multicast, and not from the set of Layer 2 multicast addresses
    derived from IPv4 or IPv6 multicast addresses.
 4. broadcast frames: the Inner.MacDA is broadcast
    (FF-FF-FF-FF-FF-FF).
 RBridges build distribution trees (see Section 4.5) and use these
 trees for forwarding multi-destination frames.  Each distribution
 tree reaches all RBridges in the campus, is shared across all VLANs,
 and may be used for the distribution of a native frame that is in any
 VLAN.  However, the distribution of any particular frame on a
 distribution tree is pruned in different ways for different cases to
 avoid unnecessary propagation of the frame.

Perlman, et al. Standards Track [Page 16] RFC 6325 RBridge Protocol July 2011

2.5. RBridges and VLANs

 A VLAN is a way to partition end nodes in a campus into different
 Layer 2 communities [802.1Q-2005].  Use of VLANs requires
 configuration.  By default, the port of receipt determines the VLAN
 of a frame sent by an end station.  End stations can also explicitly
 insert this information in a frame.
 IEEE [802.1Q-2005] bridges can be configured to support multiple
 customer VLANs over a single simple link by inserting/removing a VLAN
 tag in the frame.  VLAN tags used by TRILL have the same format as
 VLAN tags defined in IEEE [802.1Q-2005].  As shown in Figure 2, there
 are two places where such tags may be present in a TRILL-encapsulated
 frame sent over an IEEE [802.3] link: one in the outer header
 (Outer.VLAN) and one in the inner header (Inner.VLAN).  Inner and
 outer VLANs are further discussed in Section 4.1.
 RBridges enforce delivery of a native frame originating in a
 particular VLAN only to other links in the same VLAN; however, there
 are a few differences in the handling of VLANs between an RBridge
 campus and an 802.1 bridged LAN as described below.
 (See Section 4.2.4 for further discussion of TRILL IS-IS operation on
 a link.)

2.5.1. Link VLAN Assumptions

 Certain configurations of bridges may cause partitions of a VLAN on a
 link.  For such configurations, a frame sent by one RBridge to a
 neighbor on that link might not arrive, if tagged with a VLAN that is
 partitioned due to bridge configuration.
 TRILL requires at least one VLAN per link that gives full
 connectivity to all the RBridges on that link.  The default VLAN is
 1, though RBridges may be configured to use a different VLAN.  The
 DRB dictates to the other RBridges which VLAN to use.
 Since there will be only one appointed forwarder for any VLAN, say,
 VLAN-x, on a link, if bridges are configured to cause VLAN-x to be
 partitioned on a link, some VLAN-x end nodes on that link may be
 orphaned (unable to communicate with the rest of the campus).
 It is possible for bridge and port configuration to cause VLAN
 mapping on a link (where a VLAN-x frame turns into a VLAN-y frame).
 TRILL detects this by inserting a copy of the outer VLAN into TRILL-
 Hello messages and checking it on receipt.  If detected, it takes

Perlman, et al. Standards Track [Page 17] RFC 6325 RBridge Protocol July 2011

 steps to ensure that there is at most a single appointed forwarder on
 the link, to avoid possible frame duplication or loops (see Section
 4.4.5).
 TRILL behaves as conservatively as possible, avoiding loops rather
 than avoiding partial connectivity.  As a result, lack of
 connectivity may result from bridge or port misconfiguration.

2.6. RBridges and IEEE 802.1 Bridges

 RBridge ports are, except as described below, layered on top of IEEE
 [802.1Q-2005] port facilities.

2.6.1. RBridge Ports and 802.1 Layering

 RBridge ports make use of [802.1Q-2005] port VLAN and priority
 processing.  In addition, they MAY implement other lower-level 802.1
 protocols as well as protocols for the link in use, such as PAUSE
 (Annex 31B of [802.3]), port-based access control [802.1X], MAC
 security [802.1AE], or link aggregation [802.1AX].
 However, RBridges do not use spanning tree and do not block ports as
 spanning tree does.  Figure 4 shows a high-level diagram of an
 RBridge with one port connected to an IEEE 802.3 link.  Single lines
 represent the flow of control information, double lines the flow of
 both frames and control information.

Perlman, et al. Standards Track [Page 18] RFC 6325 RBridge Protocol July 2011

                        +-----------------------------------------
                        |                RBridge
                        |
                        |     Forwarding Engine, IS-IS, etc.
                        | Processing of native and TRILL frames
                        |
                        +----+---+--------++----------------------
                             |   |        ||         other ports...
               +-------------+   |        ||
               |                 |        ||
  +------------+-------------+   |        ||
  |         RBridge          |   |   +----++-------+ <- EISS
  |                          |   |   |             |
  | High-Level Control Frame |   |   | 802.1Q-2005 |
  |  Processing (BPDU, VRP)  |   |   |  Port VLAN  |
  |                          |   |   |  & Priority |
  +-----------++-------------+   |   |  Processing |
              ||                 |   |             |
    +---------++-----------------+---+-------------+ <-- ISS
    |                                              |
    |    802.1/802.3 Low-Level Control Frame       |
    |    Processing, Port/Link Control Logic       |
    |                                              |
    +-----------++---------------------------------+
                ||
                ||        +------------+
                ||        | 802.3 PHY  |
                |+--------+ (Physical  +--------- 802.3
                +---------+ Interface) +--------- Link
                          |            |
                          +------------+
                     Figure 4: RBridge Port Model
 The upper interface to the low-level port/link control logic
 corresponds to the Internal Sublayer Service (ISS) in [802.1Q-2005].
 In RBridges, high-level control frames are processed above the ISS
 interface.
 The upper interface to the port VLAN and priority processing
 corresponds to the Extended Internal Sublayer Service (EISS) in
 [802.1Q-2005].  In RBridges, native and TRILL frames are processed
 above the EISS interface and are subject to port VLAN and priority
 processing.

Perlman, et al. Standards Track [Page 19] RFC 6325 RBridge Protocol July 2011

2.6.2. Incremental Deployment

 Because RBridges are compatible with IEEE [802.1Q-2005] customer
 bridges, except as discussed in this document, a bridged LAN can be
 upgraded by incrementally replacing such bridges with RBridges.
 Bridges that have not yet been replaced are transparent to RBridge
 traffic.  The physical links directly interconnected by such bridges,
 together with the bridges themselves, constitute bridged LANs.  These
 bridged LANs appear to RBridges to be multi-access links.
 If the bridges replaced by RBridges were default configuration
 bridges, then their RBridge replacements will not require
 configuration.
 Because RBridges, as described in this document, only provide
 customer services, they cannot replace provider bridges or provider
 backbone bridges, just as a customer bridge can't replace a provider
 bridge.  However, such provider devices can be part of the bridged
 LAN between RBridges.  Extension of TRILL to support provider
 services is left for future work and will be separately documented.
 Of course, if the bridges replaced had any port level protocols
 enabled, such as port-based access control [802.1X] or MAC security
 [802.1AE], replacement RBridges would need the same port level
 protocols enabled and similarly configured.  In addition, the
 replacement RBridges would have to support the same link type and
 link level protocols as the replaced bridges.
 An RBridge campus will work best if all IEEE [802.1D] and
 [802.1Q-2005] bridges are replaced with RBridges, assuming the
 RBridges have the same speed and capacity as the bridges.  However,
 there may be intermediate states, where only some bridges have been
 replaced by RBridges, with inferior performance.
 See Appendix A for further discussion of incremental deployment.

3. Details of the TRILL Header

 This section specifies the TRILL header.  Section 4 below provides
 other RBridge design details.

3.1. TRILL Header Format

 The TRILL header is shown in Figure 5 and is independent of the data
 link layer used.  When that layer is IEEE [802.3], it is prefixed
 with the 16-bit TRILL Ethertype [RFC5342], making it 64-bit aligned.
 If Op-Length is a multiple of 64 bits, then 64-bit alignment is
 normally maintained for the content of an encapsulated frame.

Perlman, et al. Standards Track [Page 20] RFC 6325 RBridge Protocol July 2011

                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                 | V | R |M|Op-Length| Hop Count |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Egress RBridge Nickname     |  Ingress RBridge Nickname     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Options...
 +-+-+-+-+-+-+-+-+-+-+-+-
                        Figure 5: TRILL Header
 The header contains the following fields that are described in the
 sections referenced:
 o  V (Version): 2-bit unsigned integer.  See Section 3.2.
 o  R (Reserved): 2 bits.  See Section 3.3.
 o  M (Multi-destination): 1 bit.  See Section 3.4.
 o  Op-Length (Options Length): 5-bit unsigned integer.  See Section
    3.5.
 o  Hop Count: 6-bit unsigned integer.  See Section 3.6.
 o  Egress RBridge Nickname: 16-bit identifier.  See Section 3.7.1.
 o  Ingress RBridge Nickname: 16-bit identifier.  See Section 3.7.2.
 o  Options: present if Op-Length is non-zero.  See Section 3.8.

3.2. Version (V)

 Version (V) is a 2-bit field.  Version zero of TRILL is specified in
 this document.  An RBridge RB1 MUST check the V field in a received
 TRILL-encapsulated frame.  If the V field has a value not recognized
 by RB1, then RB1 MUST silently discard the frame.  The allocation of
 new TRILL Version numbers requires an IETF Standards Action.

3.3. Reserved (R)

 The two R bits are reserved for future use in extensions to this
 version zero of the TRILL protocol.  They MUST be set to zero when
 the TRILL header is added by an ingress RBridge, transparently copied
 but otherwise ignored by transit RBridges, and ignored by egress
 RBridges.  The allocation of reserved TRILL header bits requires an
 IETF Standards Action.

Perlman, et al. Standards Track [Page 21] RFC 6325 RBridge Protocol July 2011

3.4. Multi-destination (M)

 The Multi-destination bit (see Section 2.4.2) indicates that the
 frame is to be delivered to a class of destination end stations via a
 distribution tree and that the egress RBridge nickname field
 specifies this tree.  In particular:
 o  M = 0 (FALSE) - The egress RBridge nickname contains a nickname of
    the egress Rbridge for a known unicast MAC address.
 o  M = 1 (TRUE) - The egress RBridge nickname field contains a
    nickname that specifies a distribution tree.  This nickname is
    selected by the ingress RBridge for a TRILL Data frame or by the
    source RBridge for a TRILL ESADI frame.

3.5. Op-Length

 There are provisions to express in the TRILL header that a frame is
 using an optional capability and to encode information into the
 header in connection with that capability.
 The Op-Length header field gives the length of the TRILL header
 options in units of 4 octets, which allows up to 124 octets of
 options area.  If Op-Length is zero, there are no options present.
 If options are present, they follow immediately after the Ingress
 Rbridge Nickname field.
 See Section 3.8 for more information on TRILL header options.

3.6. Hop Count

 The Hop Count field is a 6-bit unsigned integer.  An Rbridge drops
 frames received with a hop count of zero, otherwise it decrements the
 hop count.  (This behavior is different from IPv4 and IPv6 in order
 to support the later addition of a traceroute-like facility that
 would be able to get a hop count exceeded from an egress RBridge.)
 For known unicast frames, the ingress RBridge SHOULD set the Hop
 Count in excess of the number of RBridge hops it expects to the
 egress RBridge to allow for alternate routing later in the path.
 For multi-destination frames, the Hop Count SHOULD be set by the
 ingress RBridge (or source RBridge for a TRILL ESADI frame) to at
 least the expected number of hops to the most distant RBridge.  To
 accomplish this, RBridge RBn calculates, for each branch from RBn of
 the specified distribution tree rooted at RBi, the maximum number of
 hops in that branch.

Perlman, et al. Standards Track [Page 22] RFC 6325 RBridge Protocol July 2011

 Multi-destination frames are of particular danger because a loop
 involving one or more distribution tree forks could result in the
 rapid generation of multiple copies of the frame, even with the
 normal hop count mechanism.  It is for this reason that multi-
 destination frames are subject to a stringent Reverse Path Forwarding
 Check and other checks as described in Section 4.5.2.  As an optional
 additional traffic control measure, when forwarding a multi-
 destination frame onto a distribution tree branch, transit RBridge
 RBm MAY decrease the hop count by more than 1, unless decreasing the
 hop count by more than 1 would result in a hop count insufficient to
 reach all destinations in that branch of the tree rooted at RBi.
 Using a hop count close or equal to the minimum needed on multi-
 destination frames provides additional protection against problems
 with temporary loops when forwarding.
 Although the RBridge MAY decrease the hop count of multi-destination
 frames by more than 1, under the circumstances described above, the
 RBridge forwarding a frame MUST decrease the hop count by at least 1,
 and discards the frame if it cannot do so because the hop count is 0.
 The option to decrease the hop count by more than 1 under the
 circumstances described above applies only to multi-destination
 frames, not to known unicast frames.

3.7. RBridge Nicknames

 Nicknames are 16-bit dynamically assigned quantities that act as
 abbreviations for RBridges' IS-IS IDs to achieve a more compact
 encoding and can be used to specify potentially different trees with
 the same root.  This assignment allows specifying up to 2**16
 RBridges; however, the value 0x0000 is reserved to indicate that a
 nickname is not specified, the values 0xFFC0 through 0xFFFE are
 reserved for future specification, and the value 0xFFFF is
 permanently reserved.  RBridges piggyback a nickname acquisition
 protocol on the link state protocol (see Section 3.7.3) to acquire
 one or more nicknames unique within the campus.

3.7.1. Egress RBridge Nickname

 There are two cases for the contents of the egress RBridge nickname
 field, depending on the M bit (see Section 3.4).  The nickname is
 filled in by the ingress RBridge for TRILL Data frames and by the
 source RBridge for TRILL ESADI frames.
 o  For known unicast TRILL Data frames, M == 0 and the egress RBridge
    nickname field specifies the egress RBridge; that is, it specifies
    the RBridge that needs to remove the TRILL encapsulation and
    forward the native frame.  Once the egress nickname field is set,
    it MUST NOT be changed by any subsequent transit RBridge.

Perlman, et al. Standards Track [Page 23] RFC 6325 RBridge Protocol July 2011

 o  For multi-destination TRILL Data frames and for TRILL ESADI
    frames, M == 1.  The egress RBridge nickname field contains a
    nickname specifying the distribution tree selected to be used to
    forward the frame.  This root nickname MUST NOT be changed by
    transit RBridges.

3.7.2. Ingress RBridge Nickname

 The ingress RBridge nickname is set to a nickname of the ingress
 RBridge for TRILL Data frames and to a nickname of the source RBridge
 for TRILL ESADI frames.  If the RBridge setting the ingress nickname
 has multiple nicknames, it SHOULD use the same nickname in the
 ingress field whenever it encapsulates a frame with any particular
 Inner.MacSA and Inner.VLAN value.  This simplifies end node learning.
 Once the ingress nickname field is set, it MUST NOT be changed by any
 subsequent transit RBridge.

3.7.3. RBridge Nickname Selection

 The nickname selection protocol is piggybacked on TRILL IS-IS as
 follows:
 o  The nickname or nicknames being used by an RBridge are carried in
    an IS-IS TLV (type-length-value data element) along with a
    priority of use value [RFC6326].  Each RBridge chooses its own
    nickname or nicknames.
 o  Nickname values MAY be configured.  An RBridge that has been
    configured with one or more nickname values will have priority for
    those nickname values over all Rbridges with non-configured
    nicknames.
 o  The nickname value 0x0000 and the values from 0xFFC0 through
    0xFFFF are reserved and MUST NOT be selected by or configured for
    an RBridge.  The value 0x0000 is used to indicate that a nickname
    is not known.
 o  The priority of use field reported with a nickname is an unsigned
    8-bit value, where the most significant bit (0x80) indicates that
    the nickname value was configured.  The bottom 7 bits have the
    default value 0x40, but MAY be configured to be some other value.
    Additionally, an RBridge MAY increase its priority after holding a
    nickname for some amount of time.  However, the most significant
    bit of the priority MUST NOT be set unless the nickname value was
    configured.

Perlman, et al. Standards Track [Page 24] RFC 6325 RBridge Protocol July 2011

 o  Once an RBridge has successfully acquired a nickname, it SHOULD
    attempt to reuse it in the case of a reboot.
 o  Each RBridge is responsible for ensuring that its nickname or each
    of its nicknames is unique.  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 IS-IS ID (LAN ID) keeps the nickname, or if
    there is a tie in priority, the RBridge with the numerically
    higher IS-IS System ID keeps the nickname, and the other RBridge
    MUST select a new nickname.  This can require an RBridge with a
    configured nickname to select a replacement nickname.
 o  To minimize the probability of nickname collisions, an RBridge
    selects a nickname randomly from the apparently available
    nicknames, based on its copy of the link state.  This random
    selection can be by the RBridge hashing some of its parameters,
    e.g., SystemID, time and date, and other entropy sources, such as
    those given in [RFC4086], each time or by the RBridge using such
    hashing to create a seed and making any selections based on
    pseudo-random numbers generated from that seed [RFC4086].  The
    random numbers or seed and the algorithm used SHOULD make
    uniformly distributed selections over the available nicknames.
    Convergence to a nickname-collision-free campus is accelerated by
    selecting new nicknames only from those that appear to be
    available and by having the highest priority nickname involved in
    a nickname conflict retain its value.  There is no reason for all
    Rbridges to use the same algorithm for selecting nicknames.
 o  If two RBridge campuses merge, then transient nickname collisions
    are possible.  As soon as each RBridge receives the LSPs from the
    other RBridges, the RBridges that need to change nicknames select
    new nicknames that do not, to the best of their knowledge, collide
    with any existing nicknames.  Some RBridges may need to change
    nicknames more than once before the situation is resolved.
 o  To minimize the probability of a new RBridge usurping a nickname
    already in use, an RBridge SHOULD wait to acquire the link state
    database from a neighbor before it announces any nicknames that
    were not configured.
 o  An RBridge by default has only a single nickname but MAY be
    configured to request multiple nicknames.  Each such nickname
    would specify a shortest path tree with the RBridge as root but,
    since the tree number is used in tiebreaking when there are
    multiple equal cost paths (see Section 4.5.1), the trees for the
    different nicknames will likely utilize different links.  Because
    of the potential tree computation load it imposes, this capability

Perlman, et al. Standards Track [Page 25] RFC 6325 RBridge Protocol July 2011

    to request multiple nicknames for an RBridge should be used
    sparingly.  For example, it should be used at a few RBridges that,
    because of campus topology, are particularly good places from
    which to calculate multiple different shortest path distribution
    trees.  Such trees need separate nicknames so traffic can be
    multipathed across them.
 o  If it is desired for a pseudonode to be a tree root, the DRB MAY
    request one or more nicknames in the pseudonode LSP.
 Every nickname in use in a campus identifies an RBridge (or
 pseudonode) and every nickname designates a distribution tree rooted
 at the RBridge (or pseudonode) it identifies.  However, only a
 limited number of these potential distribution trees are actually
 computed by all the RBridges in a campus as discussed in Section 4.5.

3.8. TRILL Header Options

 All Rbridges MUST be able to skip the number of 4-octet chunks
 indicated by the Op-Length field (see Section 3.5) in order to find
 the inner frame, since RBridges must be able to find the destination
 MAC address and VLAN tag in the inner frame.  (Transit RBridges need
 such information to filter VLANs, IP multicast, and the like.  Egress
 Rbridges need to find the inner header to correctly decapsulate and
 handle the inner frame.)
 To ensure backward-compatible safe operation, when Op-Length is non-
 zero indicating that options are present, the top two bits of the
 first octet of the options area are specified as follows:
             +------+------+----+----+----+----+----+----+
             | CHbH | CItE |          Reserved           |
             +------+------+----+----+----+----+----+----+
              Figure 6: Options Area Initial Flags Octet
 If the CHbH (Critical Hop-by-Hop) bit is one, one or more critical
 hop-by-hop options are present.  Transit RBridges that do not support
 all of the critical hop-by-hop options present, for example, an
 RBridge that supported no options, MUST drop the frame.  If the CHbH
 bit is zero, the frame is safe, from the point of view of options
 processing, for a transit RBridge to forward, regardless of what
 options that RBridge does or does not support.  A transit RBridge
 that supports none of the options present MUST transparently forward
 the options area when it forwards a frame.
 If the CItE (Critical Ingress-to-Egress) bit is one, one or more
 critical ingress-to-egress options are present.  If it is zero, no

Perlman, et al. Standards Track [Page 26] RFC 6325 RBridge Protocol July 2011

 such options are present.  If either CHbH or CItE is non-zero, egress
 RBridges that don't support all critical options present, for
 example, an RBridge that supports no options, MUST drop the frame.
 If both CHbH and CItE are zero, the frame is safe, from the point of
 view of options, for any egress RBridge to process, regardless of
 what options that RBridge does or does not support.
 Options, including the meaning of the bits labeled as Reserved in
 Figure 6, will be further specified in other documents and are
 expected to include provisions for hop-by-hop and ingress-to-egress
 options as well as critical and non-critical options.
 Note: Most RBridge implementations are expected to be optimized for
    the simplest and most common cases of frame forwarding and
    processing.  The inclusion of options may, and the inclusion of
    complex or lengthy options likely will, cause frame processing
    using a "slow path" with inferior performance to "fast path"
    processing.  Limited slow path throughput may cause such frames to
    be discarded.

4. Other RBridge Design Details

 Section 3 above specifies the TRILL header, while this section
 specifies other RBridge design details.

4.1. Ethernet Data Encapsulation

 TRILL data and ESADI frames in transit on Ethernet links are
 encapsulated with an outer Ethernet header (see Figure 2).  This
 outer header looks, to a bridge on the path between two RBridges,
 like the header of a regular Ethernet frame; therefore, bridges
 forward the frame as they normally would.  To enable RBridges to
 distinguish such TRILL Data frames, a new TRILL Ethertype (see
 Section 7.2) is used in the outer header.
 Figure 7 details a TRILL Data frame with an outer VLAN tag traveling
 on an Ethernet link as shown at the top of the figure, that is,
 between transit RBridges RB3 and RB4.  The native frame originated at
 end station ESa, was encapsulated by ingress RBridge RB1, and will
 ultimately be decapsulated by egress RBridge RB2 and delivered to
 destination end station ESb.  The encapsulation shown has the
 advantage, if TRILL options are absent or the length of such options
 is a multiple of 64 bits, of aligning the original Ethernet frame at
 a 64-bit boundary.
 When a TRILL Data frame is carried over an Ethernet cloud, it has
 three pairs of addresses:

Perlman, et al. Standards Track [Page 27] RFC 6325 RBridge Protocol July 2011

 o  Outer Ethernet Header: Outer Destination MAC Address (Outer.MacDA)
    and Outer Source MAC Address (Outer.MacSA): These addresses are
    used to specify the next hop RBridge and the transmitting RBridge,
    respectively.
 o  TRILL Header: Egress Nickname and Ingress Nickname.  These specify
    nicknames of the egress and ingress RBridges, respectively, unless
    the frame is multi-destination, in which case the Egress Nickname
    specifies the distribution tree on which the frame is being sent.
 o  Inner Ethernet Header: Inner Destination MAC Address (Inner.MacDA)
    and Inner Source MAC Address (Inner.MacSA): These addresses are as
    transmitted by the original end station, specifying, respectively,
    the destination and source of the inner frame.
 A TRILL Data frame also potentially has two VLAN tags, as discussed
 in Sections 4.1.2 and 4.1.3 below, that can carry two different VLAN
 Identifiers and specify priority.

Perlman, et al. Standards Track [Page 28] RFC 6325 RBridge Protocol July 2011

 Flow:
   +-----+  +-------+   +-------+       +-------+   +-------+  +----+
   | ESa +--+  RB1  +---+  RB3  +-------+  RB4  +---+  RB2  +--+ESb |
   +-----+  |ingress|   |transit|   ^   |transit|   |egress |  +----+
            +-------+   +-------+   |   +-------+   +-------+
                                    |
 Outer Ethernet Header:             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Outer Destination MAC Address  (RB4)              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Outer Destination MAC Address | Outer Source MAC Address      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Outer Source MAC Address  (RB3)                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Ethertype = C-Tag [802.1Q-2005]| Outer.VLAN Tag Information    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 TRILL Header:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Ethertype = TRILL             | V | R |M|Op-Length| Hop Count |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Egress (RB2) Nickname         | Ingress (RB1) Nickname        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Inner Ethernet Header:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Inner Destination MAC Address  (ESb)              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Inner Destination MAC Address | Inner Source MAC Address      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  Inner Source MAC Address  (ESa)              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Ethertype = C-Tag [802.1Q-2005]| Inner.VLAN Tag Information    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Payload:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Ethertype of Original Payload |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
    |                                  Original Ethernet Payload    |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Frame Check Sequence:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               New FCS (Frame Check Sequence)                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           Figure 7: TRILL Data Encapsulation over Ethernet

Perlman, et al. Standards Track [Page 29] RFC 6325 RBridge Protocol July 2011

4.1.1. VLAN Tag Information

 A "VLAN Tag" (formerly known as a Q-tag), also known as a "C-tag" for
 customer tag, includes a VLAN ID and a priority field as shown in
 Figure 8.  The "VLAN ID" may be zero, indicating that no VLAN is
 specified, just a priority, although such frames are called "priority
 tagged" rather than "VLAN tagged" [802.1Q-2005].
 Use of [802.1ad] S-tags, also known as service tags, and use of
 stacked tags, are beyond the scope of this document.
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   | Priority  | C |                  VLAN ID                      |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
                    Figure 8: VLAN Tag Information
 As recommended in [802.1Q-2005], Rbridges SHOULD be implemented so as
 to allow use of the full range of VLAN IDs from 0x001 through 0xFFE.
 Rbridges MAY support a smaller number of simultaneously active VLAN
 IDs.  VLAN ID zero is the null VLAN identifier and indicates that no
 VLAN is specified while VLAN ID 0xFFF is reserved.
 The VLAN ID 0xFFF MUST NOT be used.  Rbridges MUST discard any frame
 they receive with an Outer.VLAN ID of 0xFFF.  Rbridges MUST discard
 any frame for which they examine the Inner.VLAN ID and find it to be
 0xFFF; such examination is required at all egress Rbridges that
 decapsulate a frame.
 The "C" bit shown in Figure 8 is not used in the Inner.VLAN in TRILL.
 It MUST be set to zero there by ingress RBridges, transparently
 forwarded by transit RBridges, and is ignored by egress RBridges.
 As specified in [802.1Q-2005], the priority field contains an
 unsigned value from 0 through 7 where 1 indicates the lowest
 priority, 7 the highest priority, and the default priority zero is
 considered to be higher than priority 1 but lower than priority 2.
 The [802.1ad] amendment to [802.1Q-2005] permits mapping some
 adjacent pairs of priority levels into a single priority level with
 and without drop eligibility.  Ongoing work in IEEE 802.1 (802.1az,
 Appendix E) suggests the ability to configure "priority groups" that
 have a certain guaranteed bandwidth.  RBridges ports MAY also
 implement such options.  RBridges are not required to implement any
 particular number of distinct priority levels but may treat one or
 more adjacent priority levels in the same fashion.

Perlman, et al. Standards Track [Page 30] RFC 6325 RBridge Protocol July 2011

 Frames with the same source address, destination address, VLAN, and
 priority that are received on the same port as each other and are
 transmitted on the same port MUST be transmitted in the order
 received unless the RBridge classifies the frames into more fine-
 grained flows, in which case this ordering requirement applies to
 each such flow.  Frames in the same VLAN with the same priority and
 received on the same port may be sent out different ports if
 multipathing is in effect.  (See Appendix C.)
 The C-Tag Ethertype [RFC5342] is 0x8100.

4.1.2. Inner VLAN Tag

 The "Inner VLAN Tag Information" (Inner.VLAN) field contains the VLAN
 tag information associated with the native frame when it was
 ingressed or the VLAN tag information associated with a TRILL ESADI
 frame when that frame was created.  When a TRILL frame passes through
 a transit RBridge, the Inner.VLAN MUST NOT be changed except when
 VLAN mapping is being intentionally performed within that RBridge.
 When a native frame arrives at an RBridge, the associated VLAN ID and
 priority are determined as specified in [802.1Q-2005] (see Appendix D
 and [802.1Q-2005], Section 6.7).  If the RBridge is an appointed
 forwarder for that VLAN and the delivery of the frame requires
 transmission to one or more other links, this ingress RBridge forms a
 TRILL Data frame with the associated VLAN ID and priority placed in
 the Inner.VLAN information.
 The VLAN ID is required at the ingress Rbridge as one element in
 determining the appropriate egress Rbridge for a known unicast frame
 and is needed at the ingress and every transit Rbridge for multi-
 destination frames to correctly prune the distribution tree.

4.1.3. Outer VLAN Tag

 TRILL frames sent by an RBridge, except for some TRILL-Hello frames,
 use an Outer.VLAN ID specified by the Designated RBridge (DRB) for
 the link onto which they are being sent, referred to as the
 Designated VLAN.  For TRILL data and ESADI frames, the priority in
 the Outer.VLAN tag SHOULD be set to the priority in the Inner.VLAN
 tag.
 TRILL frames forwarded by a transit RBridge use the priority present
 in the Inner.VLAN of the frame as received.  TRILL Data frames are
 sent with the priority associated with the corresponding native frame
 when received (see Appendix D).  TRILL IS-IS frames SHOULD be sent
 with priority 7.

Perlman, et al. Standards Track [Page 31] RFC 6325 RBridge Protocol July 2011

 Whether an Outer.VLAN tag actually appears on the wire when a TRILL
 frame is sent depends on the configuration of the RBridge port
 through which it is sent in the same way as the appearance of a VLAN
 tag on a frame sent by an [802.1Q-2005] bridge depends on the
 configuration of the bridge port (see Section 4.9.2).

4.1.4. Frame Check Sequence (FCS)

 Each Ethernet frame has a single Frame Check Sequence (FCS) that is
 computed to cover the entire frame, for detecting frame corruption
 due to bit errors on a link.  Thus, when a frame is encapsulated, the
 original FCS is not included but is discarded.  Any received frame
 for which the FCS check fails SHOULD be discarded (this may not be
 possible in the case of cut through forwarding).  The FCS normally
 changes on encapsulation, decapsulation, and every TRILL hop due to
 changes in the outer destination and source addresses, the
 decrementing of the hop count, etc.
 Although the FCS is normally calculated just before transmission, it
 is desirable, when practical, for an FCS to accompany a frame within
 an RBridge after receipt.  That FCS could then be dynamically updated
 to account for changes to the frame during Rbridge processing and
 used for transmission or checked against the FCS calculated for frame
 transmission.  This optional, more continuous use of an FCS would be
 helpful in detecting some internal RBridge failures such as memory
 errors.

4.2. Link State Protocol (IS-IS)

 TRILL uses an extension of IS-IS [ISO10589] [RFC1195] as its routing
 protocol.  IS-IS has the following advantages:
 o  It runs directly over Layer 2, so therefore it may be run without
    configuration (no IP addresses need to be assigned).
 o  It is easy to extend by defining new TLV (type-length-value) data
    elements and sub-elements for carrying TRILL information.
 This section describes TRILL use of IS-IS, except for the TRILL-Hello
 protocol, which is described in Section 4.4, and the MTU-probe and
 MTU-ack messages that are described in Section 4.3.

4.2.1. IS-IS RBridge Identity

 Each RBridge has a unique 48-bit (6-octet) IS-IS System ID.  This ID
 may be derived from any of the RBridge's unique MAC addresses.

Perlman, et al. Standards Track [Page 32] RFC 6325 RBridge Protocol July 2011

 A pseudonode is assigned a 7-octet ID by the DRB that created it, by
 taking a 6-octet ID owned by the DRB, and appending another octet.
 The 6-octet ID used to form a pseudonode ID SHOULD be the DRB's ID
 unless the DRB has to create IDs for pseudonodes for more than 255
 links.  The only constraint for correct operation is that the 7-octet
 ID be unique within the campus, and that the 7th octet be nonzero.
 An RBridge has a 7-octet ID consisting of its 6-octet system ID
 concatenated with a zero octet.
 In this document, we use the term "IS-IS ID" to refer to the 7-octet
 quantity that can be either the ID of an RBridge or a pseudonode.

4.2.2. IS-IS Instances

 TRILL implements a separate IS-IS instance from any used by Layer 3,
 that is, different from the one used by routers.  Layer 3 IS-IS
 frames must be distinguished from TRILL IS-IS frames even when those
 Layer 3 IS-IS frames are transiting an RBridge campus.
 Layer 3 IS-IS native frames have special multicast destination
 addresses specified for that purpose, such as AllL1ISs or AllL2ISs.
 When they are TRILL encapsulated, these multicast addresses appear as
 the Inner.MacDA and the Outer.MacDA will be the All-RBridges
 multicast address.
 Within TRILL, there is an IS-IS instance across all Rbridges in the
 campus as described in Section 4.2.3.  This instance uses TRILL IS-IS
 frames that are distinguished by having a different Ethertype
 "L2-IS-IS".  Additionally, for TRILL IS-IS frames that are multicast,
 there is a distinct multicast destination address of
 All-IS-IS-RBridges.  TRILL IS-IS frames do not have a TRILL header.
 ESADI is a separate protocol from the IS-IS instance implemented by
 all the RBridges.  There is a separate ESADI instance for each VLAN,
 and ESADI frames are encapsulated just like TRILL Data frames.  After
 the TRILL header, the ESADI frame has an inner Ethernet header with
 the Inner.MacDA of "All-ESADI-RBridges" and the "L2-IS-IS" Ethertype
 followed by the ESADI frame.

4.2.3. TRILL IS-IS Frames

 All Rbridges MUST participate in the TRILL IS-IS instance, which
 constitutes a single Level 1 IS-IS area using the fixed area address
 zero.  TRILL IS-IS frames are never forwarded by an RBridge but are
 locally processed on receipt.  (Such processing may cause the RBridge
 to send additional TRILL IS-IS frames.)

Perlman, et al. Standards Track [Page 33] RFC 6325 RBridge Protocol July 2011

 A TRILL IS-IS frame on an 802.3 link is structured as shown below.
 All such frames are Ethertype encoded.  The RBridge port out of which
 such a frame is sent will strip the outer VLAN tag if configured to
 do so.
 Outer Ethernet Header:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             All-IS-IS-RBridges Multicast Address              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | All-IS-IS-RBridges continued  | Source RBridge MAC Address    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Source RBridge MAC Address continued              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Ethertype = C-Tag [802.1Q-2005]| Outer.VLAN Tag Information    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   L2-IS-IS Ethertype          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 IS-IS Payload:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | IS-IS Common Header, IS-IS PDU Specific Fields, IS-IS TLVs    |
 Frame Check Sequence:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 FCS (Frame Check Sequence)                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure 9: TRILL IS-IS Frame Format
 The VLAN specified in the Outer.VLAN information will be the
 Designated VLAN for the link on which the frame is sent, except in
 the case of some TRILL Hellos.

4.2.4. TRILL Link Hellos, DRBs, and Appointed Forwarders

 RBridges default to using TRILL Hellos unless, on a per-port basis,
 they are configured to use P2P Hellos.  TRILL-Hello frames are
 specified in Section 4.4.
 RBridges are normally configured to use P2P Hellos only when there
 are exactly two of them on a link.  However, it can occur that
 RBridges are misconfigured as to which type of hello to use.  This is
 safe but may cause lack of RBridge-to-RBridge connectivity.  An
 RBridge port configured to use P2P Hellos ignores TRILL Hellos, and
 an RBridge port configured to use TRILL Hellos ignores P2P Hellos.
 If any of the RBridge ports on a link is configured to use TRILL
 Hellos, one of such RBridge ports using TRILL Hellos is elected DRB
 (Designated RBridge) for the link.  This election is based on

Perlman, et al. Standards Track [Page 34] RFC 6325 RBridge Protocol July 2011

 configured priority (most significant field), and source MAC address,
 as communicated by TRILL-Hello frames.  The DRB, as described in
 Section 4.2.4.2, designates the VLAN to be used on the link for
 inter-RBridge communication by the non-P2P RBridge ports and appoints
 itself or other RBridges on the link as appointed forwarder (see
 Section 4.2.4.3) for VLANs on the link.

4.2.4.1. P2P Hello Links

 RBridge ports can be configured to use IS-IS P2P Hellos.  This
 implies that the port is a point-to-point link to another RBridge.
 An RBridge MUST NOT provide any end-station (native frame) service on
 a port configured to use P2P Hellos.
 As with Layer 3 IS-IS, such P2P ports do not participate in a DRB
 election.  They send all frames VLAN tagged as being in the Desired
 Designated VLAN configured for the port, although this tag may be
 stripped if the port is so configured.  Since all traffic through the
 port should be TRILL frames or Layer 2 control frames, such a port
 cannot be an appointed forwarder.  RBridge P2P ports MUST use the
 IS-IS three-way handshake [RFC5303] so that extended circuit IDs are
 associated with the link for tie breaking purposes (see Section
 4.5.2).
 Even if all simple links in a network are physically point-to-point,
 if some of the nodes are bridges, the bridged LANs that include those
 bridges appear to be multi-access links to attached RBridges.  This
 would necessitate using TRILL Hellos for proper operation in many
 cases.
 While it is safe to erroneously configure ports as P2P, this may
 result in lack of connectivity.

4.2.4.2. Designated RBridge

 TRILL IS-IS elects one RBridge for each LAN link to be the Designated
 RBridge (DRB), that is, to have special duties.  The Designated
 RBridge:
 o  Chooses, for the link, and announces in its TRILL Hellos, the
    Designated VLAN ID to be used for inter-RBridge communication.
    This VLAN is used for all TRILL-encapsulated data and ESADI frames
    and TRILL IS-IS frames except some TRILL-Hello frames.
 o  If the link is represented in the IS-IS topology as a pseudonode,
    chooses a pseudonode ID and announces that in its TRILL Hellos and
    issues an LSP on behalf of the pseudonode.

Perlman, et al. Standards Track [Page 35] RFC 6325 RBridge Protocol July 2011

 o  Issues CSNPs.
 o  For each VLAN-x appearing on the link, chooses an RBridge on the
    link to be the appointed VLAN-x forwarder (the DRB MAY choose
    itself to be the appointed VLAN-x forwarder for all or some of the
    VLANs).
 o  Before appointing a VLAN-x forwarder (including appointing
    itself), wait at least its Holding Time (to ensure it is the DRB).
 o  If configured to send TRILL-Hello frames, continues to send them
    on all its enabled VLANs that have been configured in the
    Announcing VLANs set of the DRB, which defaults to all enabled
    VLANs.

4.2.4.3. Appointed VLAN-x Forwarder

 The appointed VLAN-x forwarder for a link is responsible for the
 following points.  In connection with the loop avoidance points, when
 an appointed forwarder for a port is "inhibited", it drops any native
 frames it receives and does not transmit but instead drops any native
 frames it decapsulates, in the VLAN for which it is appointed.
 o  Loop avoidance:
  1. Inhibiting itself for a time, configurable per port from zero

to 30 seconds, which defaults to 30 seconds, after it sees a

       root bridge change on the link (see Section 4.9.3.2).
  1. Inhibiting itself for VLAN-x, if it has received a Hello in

which the sender asserts that it is appointed forwarder and

       that is either
       +  received on VLAN-x (has VLAN-x as its Outer.VLAN) or
       +  was originally sent on VLAN-x as indicated inside the body
          of the Hello.
  1. Optionally, not decapsulating a frame from ingress RBridge RBm

unless it has RBm's LSP, and the root bridge on the link it is

       about to forward onto is not listed in RBm's list of root
       bridges for VLAN-x.  This is known as the "decapsulation check"
       or "root bridge collision check".
 o  Unless inhibited (see above), receiving VLAN-x native traffic from
    the link and forwarding it as appropriate.
 o  Receiving VLAN-x traffic for the link and, unless inhibited,
    transmitting it in native form after decapsulating it as
    appropriate.

Perlman, et al. Standards Track [Page 36] RFC 6325 RBridge Protocol July 2011

 o  Learning the MAC address of local VLAN-x nodes by looking at the
    source address of VLAN-x frames from the link.
 o  Optionally learning the port of local VLAN-x nodes based on any
    sort of Layer 2 registration protocols, such as IEEE 802.11
    association and authentication.
 o  Keeping track of the { egress RBridge, VLAN, MAC address } of
    distant VLAN-x end nodes, learned by looking at the fields
    { ingress RBridge, Inner.VLAN ID, Inner.MacSA } from VLAN-x frames
    being received for decapsulation onto the link.
 o  Optionally observe native IGMP [RFC3376], MLD [RFC2710], and MRD
    [RFC4286] frames to learn the presence of local multicast
    listeners and multicast routers.
 o  Optionally listening to TRILL ESADI messages for VLAN-x to learn
    { egress RBridge, VLAN-x, MAC address } triplets and the
    confidence level of such explicitly advertised end nodes.
 o  Optionally advertising VLAN-x end nodes, on links for which it is
    appointed VLAN-x forwarder, in ESADI messages.
 o  Sending TRILL-Hello frames on VLAN-x unless the Announcing VLANs
    set for the port has been configured to disable them.
 o  Listening to BPDUs on the common spanning tree to learn the root
    bridge, if any, for that link and to report in its LSP the
    complete set of root bridges seen on any of its links for which it
    is appointed forwarder for VLAN-x.
 When an appointed forwarder observes that the DRB on a link has
 changed, it no longer considers itself appointed for that link until
 appointed by the new DRB.

4.2.4.4. TRILL LSP Information

 The information items in the TRILL IS-IS LSP that are mentioned
 elsewhere in this document are listed below.  Unless an item is
 stated in the list below to be optional, it MUST be included.  Other
 items MAY be included unless their inclusion is prohibited elsewhere
 in this document.  The actual encoding of this information and the
 IS-IS Type or sub-Type values for any new IS-IS TLV or sub-TLV data
 elements are specified in separate documents [RFC6165] [RFC6326].
 1. The IS-IS IDs of neighbors (pseudonodes as well as RBridges) of
    RBridge RBn, and the cost of the link to each of those neighbors.
    RBridges MUST use the Extended IS Reachability TLV (#22, also

Perlman, et al. Standards Track [Page 37] RFC 6325 RBridge Protocol July 2011

    known as "wide metric" [RFC5305]) and MUST NOT use the IS
    Reachability TLV (#2, also known as "narrow metric").  To
    facilitate efficient operation without configuration and
    consistent with [802.1D], RBridges SHOULD, by default, set the
    cost of a link to the integer part of twenty trillion
    (20,000,000,000,000) divided by the RBridge port's bit rate but
    not more than 2**24-2 (16,777,214); for example, the cost for a
    link accessed by a 1Gbps port would default to 20,000.  (Note that
    2**24-1 has a special meaning in IS-IS and would exclude the link
    from SPF routes.)  However, the link cost MAY, by default, be
    decreased for aggregated links and/or increased to not more than
    2**24-2 if the link appears to be a bridged LAN.  The tested MTU
    for the link (see Section 4.3) MAY be included via a sub-TLV.
 2. The following information in connection with the nickname or each
    of the nicknames of RBridge RBn:
    2.1. The nickname value (2 octets).
    2.2. The unsigned 8-bit priority for RBn to have that nickname
         (see Section 3.7.3).
    2.3. The 16-bit unsigned priority of that nickname to becoming a
         distribution tree root.
 3. The maximum TRILL Header Version supported by RBridge RBn.
 4. The following information, in addition to the per-nickname tree
    root priority, in connection with distribution tree determination
    and announcement.  (See Section 4.5 for further details on how
    this information is used.)
    4.1. An unsigned 16-bit number that is the number of trees all
         RBridges in the campus calculate if RBn has the highest
         priority tree root.
    4.2. A second unsigned 16-bit number that is the number of trees
         RBn would like to use.
    4.3. A third unsigned 16-bit number that is the maximum number of
         distribution trees that RBn is able to calculate.
    4.4. A first list of nicknames that are intended distribution
         trees for all RBridges in the campus to calculate.
    4.5. A second list of nicknames that are distribution trees RBn
         would like to use when ingressing multi-destination frames.

Perlman, et al. Standards Track [Page 38] RFC 6325 RBridge Protocol July 2011

 5. The list of VLAN IDs of VLANs directly connected to RBn for links
    on which RBn is the appointed forwarder for that VLAN.  (Note: An
    RBridge may advertise that it is connected to additional VLANs in
    order to receive additional frames to support certain VLAN-based
    features beyond the scope of this specification as mentioned in
    Section 4.8.4 and in a separate document concerning VLAN mapping
    inside RBridges.) RBridges may associate advertised connectivity
    to different groups of VLANs with specific nicknames they hold.
    In addition, the LSP contains the following information on a per-
    VLAN basis:
    5.1. Per-VLAN Multicast Router attached flags: This is two bits of
         information that indicate whether there is an IPv4 and/or
         IPv6 multicast router attached to the Rbridge on that VLAN.
         An RBridge that does not do IP multicast control snooping
         MUST set both of these bits (see Section 4.5.4).  This
         information is used because IGMP [RFC3376] and MLD [RFC2710]
         Membership Reports MUST be transmitted to all links with IP
         multicast routers, and SHOULD NOT be transmitted to links
         without such routers.  Also, all frames for IP-derived
         multicast addresses MUST be transmitted to all links with IP
         multicast routers (within a VLAN), in addition to links from
         which an IP node has explicitly asked to join the group the
         frame is for, except for some IP multicast addresses that
         MUST be treated as broadcast.
    5.2. Per-VLAN mandatory announcement of the set of IDs of Root
         bridges for any of RBn's links on which RBn is appointed
         forwarder for that VLAN.  Where MSTP (Multiple Spanning Tree
         Protocol) is running on a link, this is the root bridge of
         the CIST (Common and Internal Spanning Tree).  This is to
         quickly detect cases where two Layer 2 clouds accidentally
         get merged, and where there might otherwise temporarily be
         two DRBs for the same VLAN on the same link.  (See Section
         4.2.4.3.)
    5.3. Optionally, per-VLAN Layer 2 multicast addresses derived from
         IPv4 IGMP and IPv6 MLD notification messages received from
         attached end nodes on that VLAN, indicating the location of
         listeners for these multicast addresses (see Section 4.5.5).
    5.4. Per-VLAN ESADI protocol participation flag, priority, and
         holding time.  If this flag is one, it indicates that the
         RBridge wishes to receive such TRILL ESADI frames (see
         Section 4.2.5.1).
    5.5. Per-VLAN appointed forwarder status lost counter (see Section
         4.8.3).

Perlman, et al. Standards Track [Page 39] RFC 6325 RBridge Protocol July 2011

 6. Optionally, the largest TRILL IS-IS frame that the RBridge can
    handle using the originatingLSPBufferSize TLV #14 (see Section
    4.3).
 7. Optionally, a list of VLAN groups where address learning is shared
    across that VLAN group (see Section 4.8.4).  Each VLAN group is a
    list of VLAN IDs, where the first VLAN ID listed in a group, if
    present, is the "primary" and the others are "secondary".  This is
    to detect misconfiguration of features outside the scope of this
    document.  RBridges that do not support features such as "shared
    VLAN learning" ignore this field.
 8. Optionally, the Authentication TLV #10 (see Section 6).

4.2.5. The TRILL ESADI Protocol

 RBridges that are the appointed VLAN-x forwarder for a link MAY
 participate in the TRILL ESADI protocol for that VLAN.  But all
 transit RBridges MUST properly forward TRILL ESADI frames as if they
 were multicast TRILL Data frames.  TRILL ESADI frames are structured
 like IS-IS frames but are always TRILL encapsulated on the wire as if
 they were TRILL Data frames.
 Because of this forwarding, it appears to the ESADI protocol at an
 RBridge that it is directly connected by a shared virtual link to all
 other RBridges in the campus running ESADI for that VLAN.  RBridges
 that do not implement the ESADI protocol or are not appointed
 forwarder for that VLAN do not decapsulate or locally process any
 TRILL ESADI frames they receive for that VLAN.  In other words, these
 frames are transparently tunneled through transit RBridges.  Such
 transit RBridges treat them exactly as multicast TRILL Data frames
 and no special processing is invoked due to such forwarding.
 TRILL ESADI frames sent on an IEEE 802.3 link are structured as shown
 below.  The outer VLAN tag will not be present if it was stripped by
 the port out of which the frame was sent.

Perlman, et al. Standards Track [Page 40] RFC 6325 RBridge Protocol July 2011

 Outer Ethernet Header:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Next Hop Destination Address                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Next Hop Destination Address  | Sending RBridge MAC Address   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               Sending RBridge Port MAC Address                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Ethertype = C-Tag [802.1Q-2005]| Outer.VLAN Tag Information    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 TRILL Header:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Ethertype = TRILL             | V | R |M|Op-Length| Hop Count |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Egress (Dist. Tree) Nickname  | Ingress (Origin) Nickname     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Inner Ethernet Header:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             All-ESADI-RBridges Multicast Address              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | All-ESADI-RBridges continued  | Origin RBridge MAC Address    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Origin RBridge MAC Address continued           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Ethertype = C-Tag [802.1Q-2005]| Inner.VLAN Tag Information    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Ethertype = L2-IS-IS          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 ESADI Payload (formatted as IS-IS):
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | IS-IS Common Header, IS-IS PDU Specific Fields, IS-IS TLVs    |
 Frame Check Sequence:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  FCS (Frame Check Sequence)                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure 10: TRILL ESADI Frame Format
 The Next Hop Destination Address or Outer.MacDA is the All-RBridges
 multicast address.  The VLAN specified in the Outer.VLAN information
 will always be the Designated VLAN for the link on which the frame is
 sent.  The V and R fields will be zero while the M field will be one.
 The VLAN specified in the Inner.VLAN information will be the VLAN to
 which the ESADI frame applies.  The Origin RBridge MAC Address or
 Inner.MacSA MUST be a globally unique MAC address owned by the

Perlman, et al. Standards Track [Page 41] RFC 6325 RBridge Protocol July 2011

 RBridge originating the ESADI frame, for example, any of its port MAC
 addresses, and each RBridge MUST use the same Inner.MacSA for all of
 the ESADI frames that RBridge originates.

4.2.5.1. TRILL ESADI Participation

 An RBridge does not send any Hellos because of participation in the
 ESADI protocol.  The information available in the TRILL IS-IS link
 state database is sufficient to determine the ESADI DRB on the
 virtual link for the ESADI protocol for each VLAN.  In particular,
 the link state database information for each RBridge includes the
 VLANs, if any, for which that RBridge is participating in the ESADI
 protocol, its priority for being selected as DRB for the ESADI
 protocol for each of those VLANs, its holding time, and its IS-IS
 system ID for breaking ties in priority.
 An RBridge need not perform any routing calculation because of
 participation in the ESADI protocol.  Since all RBridges
 participating in ESADI for a particular VLAN appear to be connected
 to the same single virtual link, there are no routing decisions to be
 made.  A participating RBridge merely transmits the ESADI frames it
 originates on this virtual link.
 The ESADI DRB sends TRILL-ESADI-CSNP frames on the ESADI virtual
 link.  For robustness, a participating RBridge that determines that
 some other RBridge should be ESADI DRB on such a virtual link but has
 not received or sent a TRILL-ESADI-CSNP in at least the ESADI DRB
 holding time MAY also send a TRILL-ESADI-CSNP on the virtual link.  A
 participating RBridge that determines that no other RBridges are
 participating in the ESADI protocol for a particular VLAN SHOULD NOT
 send ESADI information or TRILL-ESADI-CSNPs on the virtual link for
 that VLAN.

4.2.5.2. TRILL ESADI Information

 The information distributed with the ESADI protocol is the list of
 local end-station MAC addresses known to the originating RBridge and,
 for each such address, a one-octet unsigned "confidence" rating in
 the range 0-254 (see Section 4.8).
 It is intended to optionally provide for VLAN ID translation within
 RBridges, as specified in [VLAN-MAPPING].  This includes translating
 TRILL ESADI frames.  If TRILL ESADI frames could contain VLAN IDs in
 arbitrary internal locations, such translation would be impractical.
 Thus, TRILL ESADI frames MUST NOT contain the VLAN ID of the VLAN to
 which they apply in the body of the frame after the Inner.VLAN tag.

Perlman, et al. Standards Track [Page 42] RFC 6325 RBridge Protocol July 2011

4.2.6. SPF, Forwarding, and Ambiguous Destinations

 This section describes the logical result desired.  Alternative
 implementation methods may be used as long as they produce the same
 forwarding behavior.
 When building a forwarding table, an RBridge RB1 calculates shortest
 paths from itself as described in Appendix C.1 of [RFC1195].
 Nicknames are added into the shortest path calculation as a final
 step, just as with an end node.  If multiple RBridges, say, RBa and
 RBb, claim the same nickname, this is a transitory condition and one
 of RBa or RBb will defer and choose a new nickname.  However, RB1
 simply adds that nickname as if it were attached to both RBa and RBb,
 and uses its standard shortest path calculation to choose the next
 hop.
 An ingress RBridge RB2 maps a native frame's known unicast
 destination MAC address and VLAN into an egress RBridge nickname.  If
 RB2 learns addresses only from the observation of received and
 decapsulated frames, then such MAC addresses cannot be duplicated
 within a VLAN in RB2 tables because more recent learned information,
 if of a higher or equal confidence, overwrites previous information
 and, if of a lower confidence, is ignored.  However, duplicates of
 the same MAC within a VLAN can appear in ESADI data and between ESADI
 data and addresses learned from the observation of received and
 decapsulated frames, entered by manual configuration, or learned
 through Layer 2 registration protocols.  If duplicate MAC addresses
 occur within a VLAN, RB2 sends frames to the MAC with the highest
 confidence.  If confidences are also tied between the duplicates, for
 consistency it is suggested that RB2 direct all such frames (or all
 such frames in the same ECMP flow) toward the same egress RBridge;
 however, the use of other policies will not cause a network problem
 since transit RBridges do not examine the Inner.MacDA for known
 unicast frames.

4.3. Inter-RBridge Link MTU Size

 There are two reasons why it is important to know what size of frame
 each inter-RBridge link in the campus can support:
 1. RBridge RB1 must know the size of link state information messages
    it can generate that will be guaranteed to be forwardable across
    all inter-RBridge links in the campus.
 2. If traffic engineering tools know which links support larger than
    minimally acceptable data packet sizes, paths can be computed that
    can support large data packets.

Perlman, et al. Standards Track [Page 43] RFC 6325 RBridge Protocol July 2011

4.3.1. Determining Campus-Wide TRILL IS-IS MTU Size

 In a stable campus, there must ultimately be agreement among all
 RBridges on the value of "Sz", the minimum acceptable inter-RBridge
 link size for the campus, for the proper operation of TRILL IS-IS.
 All RBridges MUST format their link state information messages to be
 in chunks of size no larger than what they believe Sz to be.  Also,
 every RBridge RB1 SHOULD test each of its RBridge adjacencies, say,
 to RB2, to ensure that the RB1-RB2 link can forward packets of at
 least size Sz.
 Sz has no direct effect on end stations and is not directly related
 to any end-station-to-end-station "path MTU".  Methods of using Sz or
 any link MTU information gathered by TRILL IS-IS in the traffic
 engineering of routes or the determination of any path MTU is beyond
 the scope of this document.  Native frames that, after TRILL
 encapsulation, exceed the MTU of a link on which they are sent will
 generally be discarded.
 Sz is determined by having each RBridge (optionally) advertise, in
 its LSP, its assumption of the value of the campus-wide Sz.  This LSP
 element is known in IS-IS as the originatingLSPBufferSize, TLV #14.
 The default and minimum value for Sz, and the implicitly advertised
 value of Sz if the TLV is absent, is 1470 octets.  This length (which
 is also the maximum size of a TRILL-Hello) was chosen to make it
 extremely unlikely that a TRILL control frame, even with reasonable
 additional headers, tags, and/or encapsulation, would encounter MTU
 problems on an inter-RBridge link.
 The campus-wide value of Sz is the smallest value of Sz advertised by
 any RBridge.

4.3.2. Testing Link MTU Size

 There are two new TRILL IS-IS message types for use between pairs of
 RBridge neighbors to test the bidirectional packet size capacity of
 their connection.  These messages are:
  1. - MTU-probe
  2. - MTU-ack
 Both the MTU-probe and the MTU-ack are padded to the size being
 tested.
 Sending of MTU-probes is optional; however, an RBridge RB2 that
 receives an MTU-probe from RB1 MUST respond with an MTU-ack padded to
 the same size as the MTU-probe.  The MTU-probe MAY be multicast to

Perlman, et al. Standards Track [Page 44] RFC 6325 RBridge Protocol July 2011

 All-RBridges, or unicast to a specific RBridge.  The MTU-ack is
 normally unicast to the source of the MTU-probe to which it responds
 but MAY be multicast to All-RBridges.
 If RB1 fails to receive an MTU-ack to a probe of size X from RB2
 after k tries (where k is a configurable parameter whose default is
 3), then RB1 assumes the RB1-RB2 link cannot support size X.  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 > 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.

4.4. TRILL-Hello Protocol

 The TRILL-Hello protocol is a little different from the Layer 3 IS-IS
 LAN Hello protocol and uses a new type of IS-IS message known as a
 TRILL-Hello.

4.4.1. TRILL-Hello Rationale

 The reason for defining this new type of link in TRILL is that in
 Layer 3 IS-IS, the LAN Hello protocol may elect multiple Designated
 Routers (DRs) since, when choosing a DR, routers ignore other routers
 with whom they do not have 2-way connectivity.  Also, Layer 3 IS-IS
 LAN Hellos are padded, to avoid forming adjacencies between neighbors
 that can't speak the maximum-sized packet to each other.  This means,
 in Layer 3 IS-IS, that neighbors that have connectivity to each
 other, but with an MTU on that connection less than what they
 perceive as maximum sized packets, will not see each other's Hellos.
 The result is that routers might form cliques, resulting in the link
 turning into multiple pseudonodes.
 This behavior is fine for Layer 3, but not for Layer 2, where loops
 may form if there are multiple DRBs.  Therefore, the TRILL-Hello
 protocol is a little different from Layer 3 IS-IS's LAN Hello
 protocol.
 One other issue with TRILL-Hellos is to ensure that subsets of the
 information can appear in any single message, and be processable, in
 the spirit of IS-IS LSPs and CSNPs.  TRILL-Hello frames, even though
 they are not padded, can become very large.  An example where this
 might be the case is when some sort of backbone technology
 interconnects hundreds of TRILL sites over what would appear to TRILL
 to be a giant Ethernet, where the RBridges connected to that cloud
 will perceive that backbone to be a single link with hundreds of
 neighbors.

Perlman, et al. Standards Track [Page 45] RFC 6325 RBridge Protocol July 2011

 In TRILL (unlike in Layer 3 IS-IS), the DRB is selected based solely
 on priority and MAC address.  In other words, if RB2 receives a
 TRILL-Hello from RB1 with higher (priority, MAC), RB2 defers to RB1
 as DRB, regardless of whether RB1 lists RB2 in RB1's TRILL-Hello.
 Although the neighbor list in a TRILL-Hello does not influence the
 DRB election, it does determine what is announced in LSPs.  RB1 only
 reports links to RBridges with which it has two-way connectivity.  If
 RB1 is the DRB on a link, and for whatever reason (MTU mismatch, or
 one-way connectivity) RB1 and RB2 do not have two-way connectivity,
 then RB2 does not report a link to RB1 (or the pseudonode), and RB1
 (or RB1 on behalf of the pseudonode) does not report a link to RB2.

4.4.2. TRILL-Hello Contents and Timing

 The TRILL-Hello has a new IS-IS message type.  It starts with the
 same fixed header as an IS-IS LAN Hello, which includes the 7-bit
 priority for the issuing RBridge to be DRB on that link.  TRILL-
 Hellos are sent with the same timing as IS-IS LAN Hellos.
 TRILL-Hello messages, including their Outer.MacDA and Outer.MacSA,
 but excluding any Outer.VLAN or other tags, MUST NOT exceed 1470
 octets in length and SHOULD NOT be padded.  The following information
 MUST appear in every TRILL-Hello.  References to "TLV" may actually
 be a "sub-TLV" as specified in separate documents [RFC6165]
 [RFC6326].
 1. The VLAN ID of the Designated VLAN for the link.
 2. A copy of the Outer.VLAN ID with which the Hello was tagged on
    sending.
 3. A 16-bit port ID assigned by the sending RBridge to the port the
    TRILL-Hello is sent on such that no two ports of that RBridge have
    the same port ID.
 4. A nickname of the sending RBridge.
 5. Two flags as follows:
    5.a. A flag that, if set, indicates that the sender has detected
         VLAN mapping on the link, within the past 2 of its Holding
         Times.
    5.b. A flag that, if set, indicates that the sender believes it is
         appointed forwarder for the VLAN and port on which the TRILL-
         Hello was sent.

Perlman, et al. Standards Track [Page 46] RFC 6325 RBridge Protocol July 2011

 The following information MAY appear:
 1. The set of VLANs for which end-station service is enabled on the
    port.
 2. Several flags as follows:
    2.a. A flag that, if set, indicates that the sender's port was
         configured as an access port.
    2.b. A flag that, if set, indicates that the sender's port was
         configured as a trunk port.
    2.c. A bypass pseudonode flag, as described below in this section.
 3. If the sender is the DRB, the Rbridges (excluding itself) that it
    appoints as forwarders for that link and the VLANs for which it
    appoints them.  As described below, this TLV is designed so that
    not all the appointment information need be included in each
    Hello.  Its absence means that appointed forwarders should
    continue as previously assigned.
 4. The TRILL neighbor list.  This is a new TLV, not the same as the
    IS-IS Neighbor TLV, in order to accommodate fragmentation and
    reporting MTU on the link (see Section 4.4.2.1).
 The Appointed Forwarders TLV specifies a range of VLANs and, within
 that range, specifies which Rbridge, if any, other than the DRB, is
 appointed forwarder for the VLANs in that range [RFC6326].
 Appointing an RBridge as forwarder on a port for a VLAN that is not
 enabled on that port has no effect.
 It is anticipated that many links between RBridges will be point-to-
 point, in which case using a pseudonode merely adds to the
 complexity.  If the DRB specifies the bypass pseudonode bit in its
 TRILL-Hellos, the RBridges on the link just report their adjacencies
 as point-to-point.  This has no effect on how LSPs are flooded on a
 link.  It only affects what LSPs are generated.
 For example, if RB1 and RB2 are the only RBridges on the link and RB1
 is the DRB, then if RB1 creates a pseudonode that is used, there are
 3 LSPs: for, say, RB1.25 (the pseudonode), RB1, and RB2, where RB1.25
 reports connectivity to RB1 and RB2, and RB1 and RB2 each just say
 they are connected to RB1.25.  Whereas if DRB RB1 sets the bypass
 pseudonode bit in its Hellos, then there will be only 2 LSPs: RB1 and
 RB2 each reporting connectivity to each other.

Perlman, et al. Standards Track [Page 47] RFC 6325 RBridge Protocol July 2011

 A DRB SHOULD set the bypass pseudonode bit for its links unless, for
 a particular link, it has seen at least two simultaneous adjacencies
 on the link at some point since it last rebooted.

4.4.2.1. TRILL Neighbor List

 The new TRILL Neighbor TLV includes the following information for
 each neighbor it lists:
 1.  The neighbor's MAC address.
 2.  MTU size to this neighbor as a 2-octet unsigned integer in units
    of 4-octet chunks.  The value zero indicates that the MTU is
    untested.
 3.  A flag for "failed minimum MTU test".
 To allow partial reporting of neighbors, the neighbor IDs MUST be
 sorted by ID.  If a set of neighbors { X1, X2, X3, ...  Xn } is
 reported in RB1's Hello, then X1 < X2 < X3, ...  < Xn.  If RBridge
 RB2's ID is between X1 and Xn, and does not appear in RB1's Hello,
 then RB2 knows that RB1 has not heard RB2's Hello.
 Additionally there are two overall TRILL Neighbor List TLV flags:
 "the smallest ID I reported in this Hello is the smallest ID of any
 neighbor", and "the largest ID I reported in this Hello is the
 largest ID of any neighbor".  If all the neighbors fit in RB1's
 TRILL-Hello, both flags will be set.
 If RB1 reports { X1, ...  Xn } in its Hello, with the "smallest" flag
 set, and RB2's ID is smaller than X1, then RB2 knows that RB1 has not
 heard RB2's Hello.  Similarly, if RB2's ID is larger than Xn and the
 "largest" flag is set, then RB2 knows that RB1 has not heard RB2's
 Hello.
 To ensure that any RBridge RB2 can definitively determine whether RB1
 can hear RB2, RB1's neighbor list MUST eventually cover every
 possible range of IDs, that is, within a period that depends on RB1's
 policy and not necessarily within any specific period such as the
 holding time.  In other words, if X1 is the smallest ID reported in
 one of RB1's neighbor lists, and the "smallest" flag is not set, then
 X1 MUST also appear as the largest ID reported in a different TRILL-
 Hello neighbor list.  Or, fragments may overlap, as long as there is
 no gap, such that some range, say, between Xi and Xj, never appears
 in any fragment.

Perlman, et al. Standards Track [Page 48] RFC 6325 RBridge Protocol July 2011

4.4.3. TRILL MTU-Probe and TRILL Hello VLAN Tagging

 The MTU-probe mechanism is designed to determine the MTU for
 transmissions between RBridges.  MTU-probes and probe
 acknowledgements are only sent on the Designated VLAN.
 An RBridge RBn maintains for each port the same VLAN information as a
 customer IEEE [802.1Q-2005] bridge, including the set of VLANs
 enabled for output through that port (see Section 4.9.2).  In
 addition, RBn maintains the following TRILL-specific VLAN parameters
 per port:
    a) Desired Designated VLAN: the VLAN that RBn, if it is the DRB,
       will specify in its TRILL-Hellos as the VLAN to be used by all
       RBridges on the link to communicate all TRILL frames, except
       some TRILL-Hellos.  This MUST be a VLAN enabled on RBn's port.
       It defaults to the numerically lowest enabled VLAN ID, which is
       VLAN 1 for a default configuration RBridge.
    b) Designated VLAN: the VLAN being used on the link for all TRILL
       frames except some TRILL Hellos.  This is RBn's Desired
       Designated VLAN if RBn believes it is the DRB or the Designated
       VLAN in the DRB's Hellos if RBn is not the DRB.
    c) Announcing VLANs set.  This defaults to the enabled VLANs set
       on the port but may be configured to be a subset of the enabled
       VLANs.
    d) Forwarding VLANs set: the set of VLANs for which an RBridge
       port is appointed VLAN forwarder on the port.  This MUST
       contain only enabled VLANs for the port, possibly all enabled
       VLANs.
 On each of its ports that is not configured to use P2P Hellos, an
 RBridge sends TRILL-Hellos Outer.VLAN tagged with each VLAN in a set
 of VLANs.  This set depends on the RBridge's DRB status and the above
 VLAN parameters.  RBridges send TRILL Hellos Outer.VLAN tagged with
 the Designated VLAN, unless that VLAN is not enabled on the port.  In
 addition, the DRB sends TRILL Hellos Outer.VLAN tagged with each
 enabled VLAN in its Announcing VLANs set.  All non-DRB RBridges send
 TRILL-Hellos Outer.VLAN tagged with all enabled VLANs that are in the
 intersection of their Forwarding VLANs set and their Announcing VLANs
 set.  More symbolically, TRILL-Hello frames, when sent, are sent as
 follows:
 If sender is DRB
    intersection ( Enabled VLANs,
    union ( Designated VLAN, Announcing VLANs ) )

Perlman, et al. Standards Track [Page 49] RFC 6325 RBridge Protocol July 2011

 If sender is not DRB
    intersection ( Enabled VLANs,
    union ( Designated VLAN,
    intersection ( Forwarding VLANs, Announcing VLANs ) ) )
 Configuring the Announcing VLANs set to be null minimizes the number
 of TRILL-Hellos.  In that case, TRILL-Hellos are only tagged with the
 Designated VLAN.  Great care should be taken in configuring an
 RBridge to not send TRILL Hellos on any VLAN where that RBridge is
 appointed forwarder as, under some circumstances, failure to send
 such Hellos can lead to loops.
 The number of TRILL-Hellos is maximized, within this specification,
 by configuring the Announcing VLANs set to be the set of all enabled
 VLAN IDs, which is the default.  In that case, the DRB will send
 TRILL-Hello frames tagged with all its Enabled VLAN tags; in
 addition, any non-DRB RBridge RBn will send TRILL-Hello frames tagged
 with the Designated VLAN, if enabled, and tagged with all VLANs for
 which RBn is an appointed forwarder.  (It is possible to send even
 more TRILL-Hellos.  In particular, non-DRB RBridges could send TRILL-
 Hellos on enabled VLANs for which they are not an appointed forwarder
 and which are not the Designated VLAN.  This would cause no harm
 other than a further communications and processing burden.)
 When an RBridge port comes up, until it has heard a TRILL-Hello from
 a higher priority RBridge, it considers itself to be DRB on that port
 and sends TRILL-Hellos on that basis.  Similarly, even if it has at
 some time recognized some other RBridge on the link as DRB, if it
 receives no TRILL-Hellos on that port from an RBridge with higher
 priority as DRB for a long enough time, as specified by IS-IS, it
 will revert to believing itself DRB.

4.4.4. Multiple Ports on the Same Link

 It is possible for an RBridge RB1 to have multiple ports to the same
 link.  It is important for RB1 to recognize which of its ports are on
 the same link, so, for instance, if RB1 is appointed forwarder for
 VLAN A, RB1 knows that only one of its ports acts as appointed
 forwarder for VLAN A on that link.
 RB1 detects this condition based on receiving TRILL-Hello messages
 with the same IS-IS pseudonode ID on multiple ports.  RB1 might have
 one set of ports, say, { p1, p2, p3 } on one link, and another set of
 ports { p4, p5 } on a second link, and yet other ports, say, p6, p7,
 p8, that are each on distinct links.  Let us call a set of ports on
 the same link a "port group".

Perlman, et al. Standards Track [Page 50] RFC 6325 RBridge Protocol July 2011

 If RB1 detects that a set of ports, say, { p1, p2, p3 }, is a port
 group on a link, then RB1 MUST ensure that it does not cause loops
 when it encapsulates and decapsulates traffic from/to that link.  If
 RB1 is appointed forwarder for VLAN A on that Ethernet link, RB1 MUST
 encapsulate/decapsulate VLAN A on only one of the ports.  However, if
 RB1 is appointed forwarder for more than one VLAN, RB1 MAY choose to
 load split among its ports, using one port for some set of VLANs, and
 another port for a disjoint set of VLANs.
 If RB1 detects VLAN mapping occurring (see Section 4.4.5), then RB1
 MUST NOT load split as appointed forwarder, and instead MUST act as
 appointed VLAN forwarder on that link on only one of its ports in the
 port group.
 When forwarding TRILL-encapsulated multi-destination frames to/from a
 link on which RB1 has a port group, RB1 MAY choose to load split
 among its ports, provided that it does not duplicate frames, and
 provided that it keeps frames for the same flow on the same port.  If
 RB1's neighbor on that link, RB2, accepts multi-destination frames on
 that tree on that link from RB1, RB2 MUST accept the frame from any
 of RB2's adjacencies to RB1 on that link.
 If an RBridge has more than one port connected to a link and those
 ports have the same MAC address, they can be distinguished by the
 port ID contained in TRILL-Hellos.

4.4.5. VLAN Mapping within a Link

 IEEE [802.1Q-2005] does not provide for bridges changing the C-tag
 VLAN ID for a tagged frame they receive, that is, mapping VLANs.
 Nevertheless, some bridge products provide this capability and, in
 any case, bridged LANs can be configured to display this behavior.
 For example, a bridge port can be configured to strip VLAN tags on
 output and send the resulting untagged frames onto a link leading to
 another bridge's port configured to tag these frames with a different
 VLAN.  Although each port's configuration is legal under
 [802.1Q-2005], in the aggregate they perform manipulations not
 permitted on a single customer [802.1Q-2005] bridge.  Since RBridge
 ports have the same VLAN capabilities as customer [802.1Q-2005]
 bridges, this can occur even in the absence of bridges.  (VLAN
 mapping is referred to in IEEE 802.1 as "VLAN ID translation".)
 RBridges include the Outer.VLAN ID inside every TRILL-Hello message.
 When a TRILL-Hello is received, RBridges compare this saved copy with
 the Outer.VLAN ID information associated with the received frame.  If
 these differ and the VLAN ID inside the Hello is X and the Outer.VLAN
 is Y, it can be assumed that VLAN ID X is being mapped into VLAN ID
 Y.

Perlman, et al. Standards Track [Page 51] RFC 6325 RBridge Protocol July 2011

 When non-DRB RB2 detects VLAN mapping, based on receiving a TRILL-
 Hello where the VLAN tag in the body of the Hello differs from the
 one in the outer header, it sets a flag in all of its TRILL-Hellos
 for a period of two of its Holding Times since the last time RB2
 detected VLAN mapping.  When DRB RB1 is informed of VLAN mapping,
 either because of receiving a TRILL-Hello that has been VLAN-mapped,
 or because of seeing the "VLAN mapping detected" flag in a neighbor's
 TRILL-Hello on the link, RB1 re-assigns VLAN forwarders to ensure
 there is only a single forwarder on the link for all VLANs.

4.5. Distribution Trees

 RBridges use distribution trees to forward multi-destination frames
 (see Section 2.4.2).  Distribution trees are bidirectional.  Although
 a single tree is logically sufficient for the entire campus, the
 computation of additional distribution trees is warranted for the
 following reasons: it enables multipathing of multi-destination
 frames and enables the choice of a tree root closer to or, in the
 limit, identical with the ingress RBridge.  Such a closer tree root
 improves the efficiency of the delivery of multi-destination frames
 that are being delivered to a subset of the links in the campus and
 reduces out-of-order delivery when a unicast address transitions
 between unknown and known.  If applications are in use where
 occasional out-of-order unicast frames due to such transitions are a
 problem, the RBridge campus should be engineered to make sure they
 are of extremely low probability, such as by using the ESADI
 protocol, configuring addresses to eliminate unknown destination
 unicast frames, or using keep alive frames.
 An additional level of flexibility is the ability of an RBridge to
 acquire multiple nicknames, and therefore have multiple trees rooted
 at the same RBridge.  Since the tree number is used as a tiebreaker
 for equal cost paths, the different trees, even if rooted at the same
 RBridge, will likely utilize different equal cost paths.
 How an ingress RBridge chooses the distribution tree or trees that it
 uses for multi-destination frames is beyond the scope of this
 document.  However, for the reasons stated above, in the absence of
 other factors, a good choice is the tree whose root is least cost
 from the ingress RBridge and that is the default for an ingress
 RBridge that uses a single tree to distribute multi-destination
 frames.
 RBridges will precompute all the trees that might be used, and keep
 state for Reverse Path Forwarding Check filters (see Section 4.5.2).
 Also, since the tree number is used as a tiebreaker, it is important
 for all RBridges to know:

Perlman, et al. Standards Track [Page 52] RFC 6325 RBridge Protocol July 2011

 o  how many trees to compute
 o  which trees to compute
 o  what the tree number for each tree is
 o  which trees each ingress RBridge might choose (for building
    Reverse Path Forwarding Check filters)
 Each RBridge advertises in its LSP a "tree root" priority for its
 nickname or for each of its nicknames if it has been configured to
 have more than one.  This is a 16-bit unsigned integer that defaults,
 for an unconfigured RBridge, to 0x8000.  Tree roots are ordered with
 highest numerical priority being highest priority, then with system
 ID of the RBridge (numerically higher = higher priority) as
 tiebreaker, and if that is equal, by the numerically higher nickname
 value, as an unsigned integer, having priority.
 Each RBridge advertises in its LSP the maximum number of distribution
 trees that it can compute and the number of trees that it wants all
 RBridges in the campus to compute.  The number of trees, k, that are
 computed for the campus is the number wanted by the RBridge RB1,
 which has the nickname with the highest "tree root" priority, but no
 more than the number of trees supported by the RBridge in the campus
 that supports the fewest trees.  If RB1 does not specify the specific
 distribution tree roots as described below, then the k highest
 priority trees are the trees that will be computed by all RBridges.
 Note that some of these k highest priority trees might be rooted at
 the same RBridge, if that RBridge has multiple nicknames.
 If an RBridge specifies the number of trees it can compute, or the
 number of trees it wants computed for the campus, as 0, it is treated
 as specifying them as 1.  Thus, k defaults to 1.
 In addition, the RBridge RB1 having the highest root priority
 nickname might explicitly advertise a set of s trees by providing a
 list of s nicknames.  In that case, the first k of those s trees will
 be computed.  If s is less than k, or if any of the s nicknames
 associated with the trees RB1 is advertising does not exist within
 the LSP database, then the RBridges still compute k trees, but the
 remaining trees they select are the highest priority trees, such that
 k trees are computed.
 There are two exceptions to the above, which can cause fewer
 distribution trees to be computed, as follows:
 o  A nickname whose tree root priority is zero is not selected as a
    tree root based on priority, although it may be selected by being
    listed by the RBridge holding the highest priority tree root
    nickname.  The one exception to this is that if all nicknames have
    priority zero, then the highest priority among them as determined

Perlman, et al. Standards Track [Page 53] RFC 6325 RBridge Protocol July 2011

    by the tiebreakers is used as a tree root so that there is always
    guaranteed to be at least one distribution tree.
 o  As a transient condition, two or more identical nicknames can
    appear in the list of roots for trees to be computed.  In such a
    case, it is useless to compute a tree for the nickname(s) that are
    about to be lost by the RBridges holding them.  So a distribution
    tree is only computed for the instance of the nickname where the
    priority to hold that nickname value is highest, reducing the
    total number of trees computed.  (It would also be of little use
    to go further down the priority ordered list of possible tree root
    nicknames to maintain the number of trees as the additional tree
    roots found this way would only be valid for a very brief nickname
    transition period.)
 The k trees calculated for a campus are ordered and numbered from 1
 to k.  In addition to advertising the number k, RB1 might explicitly
 advertise a set of s trees by providing a list of s nicknames as
 described above.
  1. If s == k, then the trees are numbered in the order that RB1

advertises them.

  1. If s == 0, then the trees are numbered in order of decreasing

priority. For example, if RB1 advertises only that k=2, then the

   highest priority tree is number 1 and the 2nd highest priority tree
   is number 2.
  1. If s < k, then those advertised by RB1 are numbered from 1 in the

order advertised. Then the remainder are chosen by priority order

   from among the remaining possible trees with the numbering
   continuing.  For example, if RB1 advertises k=4, advertises
   { Tx, Ty } as the nicknames of the root of the trees, and the
   campus-wide priority ordering of trees in decreasing order is Ty >
   Ta > Tc > Tb > Tx, the numbering will be as follows: Tx is 1 and Ty
   is 2 since that is the order they are advertised in by RB1.  Then
   Ta is 3 and Tc is 4 because they are the highest priority trees
   that have not already been numbered.

4.5.1. Distribution Tree Calculation

 RBridges do not use spanning tree to calculate distribution trees.
 Instead, distribution trees are calculated based on the link state
 information, selecting a particular RBridge nickname as the root.
 Each RBridge RBn independently calculates a tree rooted at RBi by
 performing the SPF (Shortest Path First) calculation with RBi as the
 root without requiring any additional exchange of information.

Perlman, et al. Standards Track [Page 54] RFC 6325 RBridge Protocol July 2011

 It is important, when building a distribution tree, that all RBridges
 choose the same links for that tree.  Therefore, when there are equal
 cost paths for a particular tree, all RBridges need to use the same
 tiebreakers.  It is also desirable to allow splitting of traffic on
 as many links as possible.  For this reason, a simple tiebreaker such
 as "always choose the parent with lower ID" would not be desirable.
 Instead, TRILL uses the tree number as a parameter in the tiebreaking
 algorithm.
 When building the tree number j, remember all possible equal cost
 parents for node N.  After calculating the entire "tree" (actually,
 directed graph), for each node N, if N has "p" parents, then order
 the parents in ascending order according to the 7-octet IS-IS ID
 considered as an unsigned integer, and number them starting at zero.
 For tree j, choose N's parent as choice j mod p.
 Note that there might be multiple equal cost links between N and
 potential parent P that have no pseudonodes, because they are either
 point-to-point links or pseudonode-suppressed links.  Such links will
 be treated as a single link for the purpose of tree building, because
 they all have the same parent P, whose IS-IS ID is "P.0".
 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 that have distinct IS-IS IDs, based on what is
 reported in LSPs.

4.5.2. Multi-Destination Frame Checks

 When a multi-destination TRILL-encapsulated frame is received by an
 RBridge, there are four checks performed, each of which may cause the
 frame to be discarded:
 1. Tree Adjacency Check: Each RBridge RBn keeps a set of adjacencies
    ( { port, neighbor } pairs ) for each distribution tree it is
    calculating.  One of these adjacencies is toward the tree root
    RBi, and the others are toward the leaves.  Once the adjacencies
    are chosen, it is irrelevant which ones are towards the root RBi
    and which are away from RBi.  RBridges MUST drop a multi-
    destination frame that arrives at a port from an RBridge that is
    not an adjacency for the tree on which the frame is being
    distributed.  Let's suppose that RBn has calculated that
    adjacencies a, c, and f are in the RBi tree.  A multi-destination
    frame for the distribution tree RBi is received only from one of
    the adjacencies a, c, or f (otherwise it is discarded) and
    forwarded to the other two adjacencies.  Should RBn have multiple

Perlman, et al. Standards Track [Page 55] RFC 6325 RBridge Protocol July 2011

    ports on a link, a multi-destination frame it sends on one of
    these ports will be received by the others but will be discarded
    as an RBridge is not adjacent to itself.
 2. RPF Check: Another technique used by RBridges for avoiding
    temporary multicast loops during topology changes is the Reverse
    Path Forwarding Check.  It involves checking that a multi-
    destination frame, based on the tree and the ingress RBridge,
    arrives from the expected link.  RBridges MUST drop multi-
    destination frames that fail the RPF check.
    To limit the amount of state necessary to perform the RPF check,
    each RBridge RB2 MUST announce which trees RB2 may choose when RB2
    ingresses a multi-destination packet.  When any RBridge, say, RB3,
    is computing the tree from nickname X, RB3 computes, for each
    RBridge RB2 that might act as ingress for tree X, the link on
    which RB3 should receive a packet from ingress RB2 on tree X, and
    note for that link that RB2 is a legal ingress RBridge for tree X.
    The information to determine which trees RB2 might choose is
    included in RB2's LSP.  Similarly to how the highest priority
    RBridge RB1 specifies the k trees that will be computed by all
    RBridges, RB2 specifies a number j, which is the total number of
    different trees RB2 might specify, and the specific trees RB2
    might specify are a combination of specified trees and trees
    selected from highest priority trees.  If RB2 specifies any trees
    that are not in the k trees as specified by RB1, they are ignored.
    The j potential ingress trees for RB2 are the ones with nicknames
    that RB2 has explicitly specified in "specified ingress tree
    nicknames" (and that are included in the k campus-wide trees
    selected by highest priority RBridge RB1), with the remainder (up
    to the maximum of {j,k}) being the highest priority of the k
    campus-wide trees.
    The default value for j is 1.  The value 0 for j is special and
    means that RB2 can pick any of the k trees being computed for the
    campus.
 3. Parallel Links Check: If the tree-building and tiebreaking for a
    particular multi-destination frame distribution tree selects a
    non-pseudonode link between RB1 and RB2, that "RB1-RB2 link" might
    actually consist of multiple links.  These parallel links would be
    visible to RB1 and RB2, but not to the rest of the campus (because
    the links are not represented by pseudonodes).  If this bundle of
    parallel links is included in a tree, it is important for RB1 and
    RB2 to decide which link to use, but is irrelevant to other
    RBridges, and therefore, the tiebreaking algorithm need not be

Perlman, et al. Standards Track [Page 56] RFC 6325 RBridge Protocol July 2011

    visible to any RBridges other than RB1 and RB2.  In this case,
    RB1-RB2 adjacencies are ordered as follows, with the one "most
    preferred" adjacency being the one on which RB1 and RB2 transmit
    to and receive multi-destination frames from each other.
    a) Most preferred are those established by P2P Hellos.
       Tiebreaking among those is based on preferring the one with the
       numerically highest Extended Circuit ID as associated with the
       adjacency by the RBridge with the highest System ID.
    b) Next considered are those established through TRILL-Hello
       frames, with suppressed pseudonodes.  Note that the pseudonode
       is suppressed in LSPs, but still appears in the TRILL-Hello,
       and therefore is available for this tiebreaking.  Among these
       links, the one with the numerically largest pseudonode ID is
       preferred.
 4. Port Group Check: If an RBridge has multiple ports attached to the
    same link, a multi-destination frame it is receiving will arrive
    on all of them.  All but one received copy of such a frame MUST be
    discarded to avoid duplication.  All such frames that are part of
    the same flow must be accepted on the same port to avoid re-
    ordering.
 When a topology change occurs (including apparent changes during
 start up), an RBridge MUST adjust its input distribution tree filters
 no later than it adjusts its output forwarding.

4.5.3. Pruning the Distribution Tree

 Each distribution tree SHOULD be pruned per VLAN, eliminating
 branches that have no potential receivers downstream.  Multi-
 destination TRILL Data frames SHOULD only be forwarded on branches
 that are not pruned.
 Further pruning SHOULD be done in two cases: (1) IGMP [RFC3376], MLD
 [RFC2710], and MRD [RFC4286] messages, where these are to be
 delivered only to links with IP multicast routers; and (2) other
 multicast frames derived from an IP multicast address that should be
 delivered only to links that have registered listeners, plus links
 that have IP multicast routers, except for IP multicast addresses
 that must be broadcast.  Each of these cases is scoped per VLAN.
 Let's assume that RBridge RBn knows that adjacencies (a, c, and f)
 are in the nickname1 distribution tree.  RBn marks pruning
 information for each of the adjacencies in the nickname1-tree.  For
 each adjacency and for each tree, RBn marks:

Perlman, et al. Standards Track [Page 57] RFC 6325 RBridge Protocol July 2011

 o  the set of VLANs reachable downstream,
 o  for each one of those VLANs, flags indicating whether there are
    IPv4 or IPv6 multicast routers downstream, and
 o  the set of Layer 2 multicast addresses derived from IP multicast
    groups for which there are receivers downstream.

4.5.4. Tree Distribution Optimization

 RBridges MUST determine the VLAN associated with all native frames
 they ingress and properly enforce VLAN rules on the emission of
 native frames at egress RBridge ports according to how those ports
 are configured and designated as appointed forwarders.  RBridges
 SHOULD also prune the distribution tree of multi-destination frames
 according to VLAN.  But, since they are not required to do such
 pruning, they may receive TRILL data or ESADI frames that should have
 been VLAN pruned earlier in the tree distribution.  They silently
 discard such frames.  A campus may contain some Rbridges that prune
 distribution trees on VLAN and some that do not.
 The situation is more complex for multicast.  RBridges SHOULD analyze
 IP-derived native multicast frames, and learn and announce listeners
 and IP multicast routers for such frames as discussed in Section 4.7
 below.  And they SHOULD prune the distribution of IP-derived
 multicast frames based on such learning and announcements.  But, they
 are not required to prune based on IP multicast listener and router
 attachment state.  And, unlike VLANs, where VLAN attachment state of
 ports MUST be maintained and honored, RBridges are not required to
 maintain IP multicast listener and router attachment state.
 An RBridge that does not examine native IGMP [RFC3376], MLD
 [RFC2710], or MRD [RFC4286] frames that it ingresses MUST advertise
 that it has IPv4 and IPv6 IP multicast routers attached for all the
 VLANs for which it is an appointed forwarder.  It need not advertise
 any IP-derived multicast listeners.  This will cause all IP-derived
 multicast traffic to be sent to this RBridge for those VLANs.  It
 then egresses that traffic onto the links for which it is appointed
 forwarder where the VLAN of the traffic matches the VLAN for which it
 is appointed forwarder on that link.  (This may cause the suppression
 of certain IGMP membership report messages from end stations, but
 that is not significant because any multicast traffic that such
 reports would be requesting will be sent to such end stations under
 these circumstances.)

Perlman, et al. Standards Track [Page 58] RFC 6325 RBridge Protocol July 2011

 A campus may contain a mixture of Rbridges with different levels of
 IP-derived multicast optimization.  An RBridge may receive IP-derived
 multicast frames that should have been pruned earlier in the tree
 distribution.  It silently discards such frames.
 See also "Considerations for Internet Group Management Protocol
 (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches"
 [RFC4541].

4.5.5. Forwarding Using a Distribution Tree

 With full optimization, forwarding a multi-destination data frame is
 done as follows.  References to adjacencies below do not include the
 adjacency on which a frame was received.
 o  The RBridge RBn receives a multi-destination TRILL Data frame with
    inner VLAN-x and a TRILL header indicating that the selected tree
    is the nickname1 tree;
 o  if the source from which the frame was received is not one of the
    adjacencies in the nickname1 tree for the specified ingress
    RBridge, the frame is dropped (see Section 4.5.1);
 o  else, if the frame is an IGMP or MLD announcement message or an
    MRD query message, then the encapsulated frame is forwarded onto
    adjacencies in the nickname1 tree that indicate there are
    downstream VLAN-x IPv4 or IPv6 multicast routers as appropriate;
 o  else, if the frame is for a Layer 2 multicast address derived from
    an IP multicast group, but its IP address is not the range of IP
    multicast addresses that must be treated as broadcast, the frame
    is forwarded onto adjacencies in the nickname1 tree that indicate
    there are downstream VLAN-x IP multicast routers of the
    corresponding type (IPv4 or IPv6), as well as adjacencies that
    indicate there are downstream VLAN-x receivers for that group
    address;
 o  else (the inner frame is for a Layer 2 multicast address not
    derived from an IP multicast group or an unknown destination or
    broadcast or an IP multicast address that is required to be
    treated as broadcast), the frame is forwarded onto an adjacency if
    and only if that adjacency is in the nickname1 tree, and marked as
    reaching VLAN-x links.
 For each link for which RBn is appointed forwarder, RBn additionally
 checks to see if it should decapsulate the frame and send it to the
 link in native form, or process the frame locally.

Perlman, et al. Standards Track [Page 59] RFC 6325 RBridge Protocol July 2011

 TRILL ESADI frames will be delivered only to RBridges that are
 appointed forwarders for their VLAN.  Such frames will be multicast
 throughout the campus, like other non-IP-derived multicast data
 frames, on the distribution tree chosen by the RBridge that created
 the TRILL ESADI frame, and pruned according to the Inner.VLAN ID.
 Thus, all the RBridges that are appointed forwarders for a link in
 that VLAN receive them.

4.6. Frame Processing Behavior

 This section describes RBridge behavior for all varieties of received
 frames, including how they are forwarded when appropriate.  Section
 4.6.1 covers native frames, Section 4.6.2 covers TRILL frames, and
 Section 4.6.3 covers Layer 2 control frames.  Processing may be
 organized or sequenced in a different way than described here as long
 as the result is the same.  See Section 1.4 for frame type
 definitions.
 Corrupt frames, for example, frames that are not a multiple of 8
 bits, are too short or long for the link protocol/hardware in use, or
 have a bad FCS are discarded on receipt by an RBridge port as they
 are discarded on receipt at an IEEE 802.1 bridge port.
 Source address information ( { VLAN, Outer.MacSA, port } ) is learned
 by default from any frame with a unicast source address (see Section
 4.8).

4.6.1. Receipt of a Native Frame

 If the port is configured as disabled or if end-station service is
 disabled on the port by configuring it as a trunk port or configuring
 it to use P2P Hellos, the frame is discarded.
 The ingress Rbridge RB1 determines the VLAN ID for a native frame
 according to the same rules as IEEE [802.1Q-2005] bridges do (see
 Appendix D and Section 4.9.2).  Once the VLAN is determined, if RB1
 is not the appointed forwarder for that VLAN on the port where the
 frame was received or is inhibited, the frame is discarded.  If it is
 appointed forwarder for that VLAN and is not inhibited (see Section
 4.2.4.3), then the native frame is forwarded according to Section
 4.6.1.1 if it is unicast and according to Section 4.6.1.2 if it is
 multicast or broadcast.

4.6.1.1. Native Unicast Case

 If the destination MAC address of the native frame is a unicast
 address, the following steps are performed.

Perlman, et al. Standards Track [Page 60] RFC 6325 RBridge Protocol July 2011

 The Layer 2 destination address and VLAN are looked up in the ingress
 RBridge's database of MAC addresses and VLANs to find the egress
 RBridge RBm or the local egress port or to discover that the
 destination is the receiving RBridge or is unknown.  One of the
 following four cases will apply:
 1. If the destination is the receiving RBridge, the frame is locally
    processed.
 2. If the destination is known to be on the same link from which the
    native frame was received but is not the receiving RBridge, the
    RBridge silently discards the frame, since the destination should
    already have received it.
 3. If the destination is known to be on a different local link for
    which RBm is the appointed forwarder, then RB1 converts the native
    frame to a TRILL Data frame with an Outer.MacDA of the next hop
    RBridge towards RBm, a TRILL header with M = 0, an ingress
    nickname for RB1, and the egress nickname for RBm.  If ingress RB1
    has multiple nicknames, it SHOULD use the same nickname in the
    ingress nickname field whenever it encapsulates a native frame
    from any particular source MAC address and VLAN.  This simplifies
    end node learning.  If RBm is RB1, processing then proceeds as in
    Section 4.6.2.4; otherwise, the Outer.MacSA is set to the MAC
    address of the RB1 port on the path to the next hop RBridge
    towards RBm and the frame is queued for transmission out of that
    port.
 4. If a unicast destination MAC is unknown in the frame's VLAN, RB1
    handles the frame as described in Section 4.6.1.2 for a broadcast
    frame except that the Inner.MacDA is the original native frame's
    unicast destination address.

4.6.1.2. Native Multicast and Broadcast Frames

 If the RBridge has multiple ports attached to the same link, all but
 one received copy of a native multicast or broadcast frame is
 discarded to avoid duplication.  All such frames that are part of the
 same flow must be accepted on the same port to avoid re-ordering.
 If the frame is a native IGMP [RFC3376], MLD [RFC2710], or MRD
 [RFC4286] frame, then RB1 SHOULD analyze it, learn any group
 membership or IP multicast router presence indicated, and announce
 that information for the appropriate VLAN in its LSP (see Section
 4.7).
 For all multi-destination native frames, RB1 forwards the frame in
 native form to its links where it is appointed forwarder for the

Perlman, et al. Standards Track [Page 61] RFC 6325 RBridge Protocol July 2011

 frame's VLAN, subject to further pruning and inhibition.  In
 addition, it converts the native frame to a TRILL Data frame with the
 All-RBridges multicast address as Outer.MacDA, a TRILL header with
 the multi-destination bit M = 1, the ingress nickname for RB1, and
 the egress nickname for the distribution tree it decides to use.  It
 then forwards the frame on the pruned distribution tree (see Section
 4.5) setting the Outer.MacSA of each copy sent to the MAC address of
 the RB1 port on which it is sent.
 The default is for RB1 to write into the egress nickname field the
 nickname for a distribution tree, from the set of distribution trees
 RB1 has announced it might use, whose root is least cost from RB1.
 RB1 MAY choose different distribution trees for different frames if
 RB1 has been configured to path-split multicast.  In that case, RB1
 MUST select a tree by specifying a nickname that is a distribution
 tree root (see Section 4.5).  Also, RB1 MUST select a nickname that
 RB1 has announced (in RB1's own LSP) to be one of those that RB1
 might use.  The strategy RB1 uses to select distribution trees in
 multipathing multi-destination frames is beyond the scope of this
 document.

4.6.2. Receipt of a TRILL Frame

 A TRILL frame either has the TRILL or L2-IS-IS Ethertype or has a
 multicast Outer.MacDA allocated to TRILL (see Section 7.2).  The
 following tests are performed sequentially, and the first that
 matches controls the handling of the frame:
 1. If the Outer.MacDA is All-IS-IS-RBridges and the Ethertype is
    L2-IS-IS, the frame is handled as described in Section 4.6.2.1.
 2. If the Outer.MacDA is a multicast address allocated to TRILL other
    than All-RBridges, the frame is discarded.
 3. If the Outer.MacDA is a unicast address other than the receiving
    Rbridge port MAC address, the frame is discarded.  (Such discarded
    frames are most likely addressed to another RBridge on a multi-
    access link and that other Rbridge will handle them.)
 4. If the Ethertype is not TRILL, the frame is discarded.
 5. If the Version field in the TRILL header is greater than 0, the
    frame is discarded.
 6. If the hop count is 0, the frame is discarded.
 7. If the Outer.MacDA is multicast and the M bit is zero or if the
    Outer.MacDA is unicast and M bit is one, the frame is discarded.

Perlman, et al. Standards Track [Page 62] RFC 6325 RBridge Protocol July 2011

 8. By default, an RBridge MUST NOT forward TRILL-encapsulated frames
    from a neighbor with which it does not have a TRILL IS-IS
    adjacency.  RBridges MAY be configured per port to accept these
    frames for forwarding in cases where it is known that a non-
    peering device (such as an end station) is configured to originate
    TRILL-encapsulated frames that can be safely forwarded.
 9. The Inner.MacDA is then tested.  If it is the All-ESADI-RBridges
    multicast address and RBn implements the ESADI protocol,
    processing proceeds as in Section 4.6.2.2 below.  If it is any
    other address or RBn does not implement ESADI, processing proceeds
    as in Section 4.6.2.3.

4.6.2.1. TRILL Control Frames

 The frame is processed by the TRILL IS-IS instance on RBn and is not
 forwarded.

4.6.2.2. TRILL ESADI Frames

 If M == 0, the frame is silently discarded.
 The egress nickname designates the distribution tree.  The frame is
 forwarded as described in Section 4.6.2.5.  In addition, if the
 forwarding Rbridge is an appointed forwarder for a link in the
 specified VLAN and implements the TRILL ESADI protocol and ESADI is
 enabled at the forwarding Rbridge for that VLAN, the inner frame is
 decapsulated and provided to that local ESADI protocol.

4.6.2.3. TRILL Data Frames

 The M flag is then checked.  If it is zero, processing continues as
 described in Section 4.6.2.4, if it is one, processing continues as
 described in Section 4.6.2.5.

4.6.2.4. Known Unicast TRILL Data Frames

 The egress nickname in the TRILL header is examined, and if it is
 unknown or reserved, the frame is discarded.
 If RBn is a transit RBridge, the hop count is decremented by one and
 the frame is forwarded to the next hop RBridge towards the egress
 RBridge.  (The provision permitting RBridges to decrease the hop
 count by more than one under some circumstances (see Section 3.6)
 applies only to multi-destination frames, not to the known unicast
 frames considered in this subsection.)  The Inner.VLAN is not
 examined by a transit RBridge when it forwards a known unicast TRILL
 Data frame.  For the forwarded frame, the Outer.MacSA is the MAC

Perlman, et al. Standards Track [Page 63] RFC 6325 RBridge Protocol July 2011

 address of the transmitting port, the Outer.MacDA is the unicast
 address of the next hop RBridge, and the VLAN is the Designated VLAN
 on the link onto which the frame is being transmitted.
 If RBn is not a transit RBridge, that is, if the egress RBridge
 indicated is the RBridge performing the processing, the Inner.MacSA
 and Inner.VLAN ID are, by default, learned as associated with the
 ingress nickname unless that nickname is unknown or reserved or the
 Inner.MacSA is not unicast.  Then the frame being forwarded is
 decapsulated to native form, and the following checks are performed:
 o  The Inner.MacDA is checked.  If it is not unicast, the frame is
    discarded.
 o  If the Inner.MacDA corresponds to the RBridge doing the
    processing, the frame is locally delivered.
 o  The Inner.VLAN ID is checked.  If it is 0x0 or 0xFFF, the frame is
    discarded.
 o  The Inner.MacDA and Inner.VLAN ID are looked up in RBn's local
    address cache and the frame is then either sent onto the link
    containing the destination, if the RBridge is appointed forwarder
    for that link for the frame's VLAN and is not inhibited (or
    discarded if it is inhibited), or processed as in one of the
    following two paragraphs.
 A known unicast TRILL Data frame can arrive at the egress Rbridge
 only to find that the combination of Inner.MacDA and Inner.VLAN is
 not actually known by that RBridge.  One way this can happen is that
 the address information may have timed out in the egress RBridge MAC
 address cache.  In this case, the egress RBridge sends the native
 frame out on all links that are in the frame's VLAN for which the
 RBridge is appointed forwarder and has not been inhibited, except
 that it MAY refrain from sending the frame on links where it knows
 there cannot be an end station with the destination MAC address, for
 example, the link port is configured as a trunk (see Section 4.9.1).
 If, due to manual configuration or learning from Layer 2
 registration, the destination MAC and VLAN appear in RBn's local
 address cache for two or more links for which RBn is an uninhibited
 appointed forwarder for the frame's VLAN, RBn sends the native frame
 on all such links.

4.6.2.5. Multi-Destination TRILL Data Frames

 The egress and ingress nicknames in the TRILL header are examined
 and, if either is unknown or reserved, the frame is discarded.

Perlman, et al. Standards Track [Page 64] RFC 6325 RBridge Protocol July 2011

 The Outer.MacSA is checked and the frame discarded if it is not a
 tree adjacency for the tree indicated by the egress RBridge nickname
 on the port where the frame was received.  The Reverse Path
 Forwarding Check is performed on the ingress and egress nicknames and
 the frame discarded if it fails.  If there are multiple TRILL-Hello
 pseudonode suppressed parallel links to the previous hop RBridge, the
 frame is discarded if it has been received on the wrong one.  If the
 RBridge has multiple ports connected to the link, the frame is
 discarded unless it was received on the right one.  For more
 information on the checks in this paragraph, see Section 4.5.2.
 If the Inner.VLAN ID of the frame is 0x0 or 0xFFF, the frame is
 discarded.
 If the RBridge is an appointed forwarder for the Inner.VLAN ID of the
 frame, the Inner.MacSA and Inner.VLAN ID are, by default, learned as
 associated with the ingress nickname unless that nickname is unknown
 or reserved or the Inner.MacSA is not unicast.  A copy of the frame
 is then decapsulated, sent in native form on those links in its VLAN
 for which the RBridge is appointed forwarder subject to additional
 pruning and inhibition as described in Section 4.2.4.3, and/or
 locally processed as appropriate.
 The hop count is decreased (possibly by more than one; see Section
 3.6), and the frame is forwarded down the tree specified by the
 egress RBridge nickname pruned as described in Section 4.5.
 For the forwarded frame, the Outer.MacSA is set to that of the port
 on which the frame is being transmitted, the Outer.MacDA is the
 All-RBridges multicast address, and the VLAN is the Designated VLAN
 of the link on which the frame is being transmitted.

4.6.3. Receipt of a Layer 2 Control Frame

 Low-level control frames received by an RBridge are handled within
 the port where they are received as described in Section 4.9.
 There are two types of high-level control frames, distinguished by
 their destination address, which are handled as described in the
 sections referenced below.
    Name   Section   Destination Address
    BPDU   4.9.3     01-80-C2-00-00-00
    VRP    4.9.4     01-80-C2-00-00-21

Perlman, et al. Standards Track [Page 65] RFC 6325 RBridge Protocol July 2011

4.7. IGMP, MLD, and MRD Learning

 Ingress RBridges SHOULD learn, based on native IGMP [RFC3376], MLD
 [RFC2710], and MRD [RFC4286] frames they receive in VLANs for which
 they are an uninhibited appointed forwarder, which IP-derived
 multicast messages should be forwarded onto which links.  Such frames
 are also, in general, encapsulated as TRILL Data frames and
 distributed as described below and in Section 4.5.
 An IGMP or MLD membership report received in native form from a link
 indicates a multicast group listener for that group on that link.  An
 IGMP or MLD query or an MRD advertisement received in native form
 from a link indicates the presence of an IP multicast router on that
 link.
 IP multicast group membership reports have to be sent throughout the
 campus and delivered to all IP multicast routers, distinguishing IPv4
 and IPv6.  All IP-derived multicast traffic must also be sent to all
 IP multicast routers for the same version of IP.
 IP multicast data SHOULD only be sent on links where there is either
 an IP multicast router for that IP type (IPv4 or IPv6) or an IP
 multicast group listener for that IP-derived multicast MAC address,
 unless the IP multicast address is in the range required to be
 treated as broadcast.
 RBridges do not need to announce themselves as listeners to the IPv4
 All-Snoopers multicast group (the group used for MRD reports
 [RFC4286]), because the IPv4 multicast address for that group is in
 the range where all frames sent to that IP multicast address must be
 broadcast (see [RFC4541], Section 2.1.2).  However, RBridges that are
 performing IPv6-derived multicast optimization MUST announce
 themselves as listeners to the IPv6 All-Snoopers multicast group.
 See also "Considerations for Internet Group Management Protocol
 (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches"
 [RFC4541].

4.8. End-Station Address Details

 RBridges have to learn the MAC addresses and VLANs of their locally
 attached end stations for link/VLAN pairs for which they are the
 appointed forwarder.  Learning this enables them to do the following:
 o  Forward the native form of incoming known unicast TRILL Data
    frames onto the correct link.

Perlman, et al. Standards Track [Page 66] RFC 6325 RBridge Protocol July 2011

 o  Decide, for an incoming native unicast frame from a link, where
    the RBridge is the appointed forwarder for the frame's VLAN,
    whether the frame is
  1. known to have been destined for another end station on the same

link, so the RBridge need do nothing, or

  1. has to be converted to a TRILL Data frame and forwarded.
 RBridges need to learn the MAC addresses, VLANs, and remote RBridges
 of remotely attached end stations for VLANs for which they and the
 remote RBridge are an appointed forwarder, so they can efficiently
 direct native frames they receive that are unicast to those addresses
 and VLANs.

4.8.1. Learning End-Station Addresses

 There are five independent ways an RBridge can learn end-station
 addresses as follows:
 1. From the observation of VLAN-x frames received on ports where it
    is appointed VLAN-x forwarder, learning the { source MAC, VLAN,
    port } triplet of received frames.
 2. The { source MAC, VLAN, ingress RBridge nickname } triplet of any
    native frames that it decapsulates.
 3. By Layer 2 registration protocols learning the { source MAC, VLAN,
    port } of end stations registering at a local port.
 4. By running the TRILL ESADI protocol for one or more VLANs and
    thereby receiving remote address information and/or transmitting
    local address information.
 5. By management configuration.
 RBridges MUST implement capabilities 1 and 2 above.  RBridges use
 these capabilities unless configured, for one or more particular
 VLANs and/or ports, not to learn from either received frames or from
 decapsulating native frames to be transmitted or both.
 RBridges MAY implement capabilities 3 and 4 above.  If capability 4
 is implemented, the ESADI protocol is run only when the RBridge is
 configured to do so on a per-VLAN basis.
 RBridges SHOULD implement capability 5.

Perlman, et al. Standards Track [Page 67] RFC 6325 RBridge Protocol July 2011

 Entries in the table of learned MAC and VLAN addresses and associated
 information also have a one-octet unsigned confidence level
 associated with each entry whose rationale is given below.  Such
 information learned from the observation of data has a confidence of
 0x20 unless configured to have a different confidence.  This
 confidence level can be configured on a per-RBridge basis separately
 for information learned from local native frames and that learned
 from remotely originated encapsulated frames.  Such information
 received via the TRILL ESADI protocol is accompanied by a confidence
 level in the range 0 to 254.  Such information configured by
 management defaults to a confidence level of 255 but may be
 configured to have another value.
 The table of learned MAC addresses includes (1) { confidence, VLAN,
 MAC address, local port } for addresses learned from local native
 frames and local registration protocols, (2) { confidence, VLAN, MAC
 address, egress RBridge nickname } for addresses learned from remote
 encapsulated frames and ESADI link state databases, and (3)
 additional information to implement timeout of learned addresses,
 statically configured addresses, and the like.
 When a new address and related information learned from observing
 data frames are to be entered into the local database, there are
 three possibilities:
 A. If this is a new { address, VLAN } pair, the information is
    entered accompanied by the confidence level.
 B. If there is already an entry for this { address, VLAN } pair with
    the same accompanying delivery information, the confidence level
    in the local database is set to the maximum of its existing
    confidence level and the confidence level with which it is being
    learned.  In addition, if the information is being learned with
    the same or a higher confidence level than its existing confidence
    level, timer information is reset.
 C. If there is already an entry for this { address, VLAN } pair with
    different information, the learned information replaces the older
    information only if it is being learned with higher or equal
    confidence than that in the database entry.  If it replaces older
    information, timer information is also reset.

4.8.2. Learning Confidence Level Rationale

 The confidence level mechanism allows an RBridge campus manager to
 cause certain address learning sources to prevail over others.  In a
 default configuration, without the optional ESADI protocol, addresses
 are only learned from observing local native frames and the

Perlman, et al. Standards Track [Page 68] RFC 6325 RBridge Protocol July 2011

 decapsulation of received TRILL Data frames.  Both of these sources
 default to confidence level 0x20 so, since learning at an equal or
 high confidence overrides previous learning, the learning in such a
 default case mimics default 802.1 bridge learning.
 While RBridge campus management policies are beyond the scope of this
 document, here are some example types of policies that can be
 implemented with the confidence mechanism and the rationale for each:
 o  Locally received native frames might be considered more reliable
    than decapsulated frames received from remote parts of the campus.
    To stop MAC addresses learned from such local frames from being
    usurped by remotely received forged frames, the confidence in
    locally learned addresses could be increased or that in addresses
    learned from remotely sourced decapsulated frames decreased.
 o  MAC address information learned through a cryptographically
    authenticated Layer 2 registration protocol, such as 802.1X with a
    cryptographically based EAP method, might be considered more
    reliable than information learned through the mere observation of
    data frames.  When such authenticated learned address information
    is transmitted via the ESADI protocol, the use of authentication
    in the TRILL ESADI LSP frames could make tampering with it in
    transit very difficult.  As a result, it might be reasonable to
    announce such authenticated information via the ESADI protocol
    with a high confidence, so it would override any alternative
    learning from data observation.
 Manually configured address information is generally considered
 static and so defaults to a confidence of 0xFF while no other source
 of such information can be configured to a confidence any higher than
 0xFE.  However, for other cases, such as where the manual
 configuration is just a starting point that the Rbridge campus
 manager wishes to be dynamically overridable, the confidence of such
 manually configured information may be configured to a lower value.

4.8.3. Forgetting End-Station Addresses

 While RBridges need to learn end-station addresses as described
 above, it is equally important that they be able to forget such
 information.  Otherwise, frames for end stations that have moved to a
 different part of the campus could be indefinitely black-holed by
 RBridges with stale information as to the link to which the end
 station is attached.
 For end-station address information locally learned from frames
 received, the time out from the last time a native frame was received
 or decapsulated with the information conforms to the recommendations

Perlman, et al. Standards Track [Page 69] RFC 6325 RBridge Protocol July 2011

 of [802.1D].  It is referred to as the "Ageing Time" and is
 configurable per RBridge with a range of from 10 seconds to 1,000,000
 seconds and a default value of 300 seconds.
 The situation is different for end-station address information
 acquired via the TRILL ESADI protocol.  It is up to the originating
 RBridge to decide when to remove such information from its ESADI LSPs
 (or up to ESADI protocol timeouts if the originating RBridge becomes
 inaccessible).
 When an RBridge ceases to be appointed forwarder for VLAN-x on a
 port, it forgets all end-station address information learned from the
 observation of VLAN-x native frames received on that port.  It also
 increments a per-VLAN counter of the number of times it lost
 appointed forwarder status on one of its ports for that VLAN.
 When, for all of its ports, RBridge RBn is no longer appointed
 forwarder for VLAN-x, it forgets all end-station address information
 learned from decapsulating VLAN-x native frames.  Also, if RBn is
 participating in the TRILL ESADI protocol for VLAN-x, it ceases to so
 participate after sending a final LSP nulling out the end-station
 address information for the VLAN that it had been originating.  In
 addition, all other RBridges that are VLAN-x forwarder on at least
 one of their ports notice that the link state data for RBn has
 changed to show that it no longer has a link on VLAN-x.  In response,
 they forget all end-station address information they have learned
 from decapsulating VLAN-x frames that show RBn as the ingress
 RBridge.
 When the appointed forwarder lost counter for RBridge RBn for VLAN-x
 is observed to increase via the TRILL IS-IS link state database but
 RBn continues to be an appointed forwarder for VLAN-x on at least one
 of its ports, every other RBridge that is an appointed forwarder for
 VLAN-x modifies the aging of all the addresses it has learned by
 decapsulating native frames in VLAN-x from ingress RBridge RBn as
 follows: the time remaining for each entry is adjusted to be no
 larger than a per-RBridge configuration parameter called (to
 correspond to [802.1D]) "Forward Delay".  This parameter is in the
 range of 4 to 30 seconds with a default value of 15 seconds.

4.8.4. Shared VLAN Learning

 RBridges can map VLAN IDs into a smaller number of identifiers for
 purposes of address learning, as [802.1Q-2005] bridges can.  Then,
 when a lookup is done in learned address information, this identifier
 is used for matching in place of the VLAN ID.  If the ID of the VLAN
 on which the address was learned is not retained, then there are the
 following consequences:

Perlman, et al. Standards Track [Page 70] RFC 6325 RBridge Protocol July 2011

 o  The RBridge no longer has the information needed to participate in
    the TRILL ESADI protocol for the VLANs whose ID is not being
    retained.
 o  In cases where Section 4.8.3 above requires the discarding of
    learned address information based on a particular VLAN, when the
    VLAN ID is not available for entries under a shared VLAN
    identifier, instead the time remaining for each entry under that
    shared VLAN identifier is adjusted to be no longer than the
    RBridge's "Forward Delay".
 Although outside the scope of this specification, there are some
 Layer 2 features in which a set of VLANs has shared learning, where
 one of the VLANs is the "primary" and the other VLANs in the group
 are "secondaries".  An example of this is where traffic from
 different communities is separated using VLAN tags, and yet some
 resource (such as an IP router or DHCP server) is to be shared by all
 the communities.  A method of implementing this feature is to give a
 VLAN tag, say, Z, to a link containing the shared resource, and have
 the other VLANs, say, A, C, and D, be part of the group { primary =
 Z, secondaries = A, C, D }.  An RBridge, aware of this grouping,
 attached to one of the secondary VLANs in the group also claims to be
 attached to the primary VLAN.  So an RBridge attached to A would
 claim to also be attached to Z.  An RBridge attached to the primary
 would claim to be attached to all the VLANs in the group.
 This document does not specify how VLAN groups might be used.  Only
 RBridges that participate in a VLAN group will be configured to know
 about the VLAN group.  However, to detect misconfiguration, an
 RBridge configured to know about a VLAN group SHOULD report the VLAN
 group in its LSP.

4.9. RBridge Ports

 Section 4.9.1 below describes the several RBridge port configuration
 bits, Section 4.9.2 gives a logical port structure in terms of frame
 processing, and Sections 4.9.3 and 4.9.4 describe the handling of
 high-level control frames.

4.9.1. RBridge Port Configuration

 There are four per-port configuration bits as follows:
 o  Disable port bit.  When this bit is set, all frames received or to
    be transmitted are discarded, with the possible exception of some
    Layer 2 control frames (see Section 1.4) that may be generated and
    transmitted or received and processed within the port.  By
    default, ports are enabled.

Perlman, et al. Standards Track [Page 71] RFC 6325 RBridge Protocol July 2011

 o  End-station service disable (trunk port) bit.  When this bit is
    set, all native frames received on the port and all native frames
    that would have been sent on the port are discarded.  (See
    Appendix B.)  (Note that, for this document, "native frames" does
    not include Layer 2 control frames.)  By default, ports are not
    restricted to being trunk ports.
    If a port with end-station service disabled reports, in a TRILL-
    Hello frame it sends out that port, which VLANs it provides end-
    station support for, it reports that there are none.
 o  TRILL traffic disable (access port) bit.  If this bit is set, the
    goal is to avoid sending any TRILL frames, except TRILL-Hello
    frames, on the port since it is intended only for native end-
    station traffic.  By default, ports are not restricted to being
    access ports.  This bit is reported in TRILL-Hello frames.  If RB1
    is the DRB and has this bit set in its TRILL-Hello, the DRB still
    appoints VLAN forwarders.  However, usually no pseudonode is
    reported, and none of the inter-RBridge links associated with that
    link are reported in LSPs.
    If the DRB RB1 does not have this bit set, but neighbor RB2 on the
    link does have the bit set, then RB1 does not appoint RB2 as
    appointed forwarder for any VLAN, and none of the RBridges
    (including the pseudonode) report RB2 as a neighbor in LSPs.
    In some cases even though the DRB has the "access port" flag set,
    the DRB MAY choose to create a pseudonode for the access port.  In
    this case, the other RBridges report connectivity to the
    pseudonode in their LSP, but the DRB sets the "overload" flag in
    the pseudonode LSP.
 o  Use P2P Hellos bit.  If this bit is set, Hellos sent on this port
    are IS-IS P2P Hellos.  By default TRILL-Hellos are used.  See
    Section 4.2.4.1 for more information on P2P links.
 The dominance relationship of these four configuration bits is as
 follows, where configuration bits to the left dominate those to the
 right.  That is to say, when any pair of bits is asserted,
 inconsistencies in behavior they mandate are resolved in favor of
 behavior mandated by the bit to the left of the other bit in this
 list.
       Disable > P2P > Access > Trunk

Perlman, et al. Standards Track [Page 72] RFC 6325 RBridge Protocol July 2011

4.9.2. RBridge Port Structure

 An RBridge port can be modeled as having a lower-level structure
 similar to that of an [802.1Q-2005] bridge port as shown in Figure
 11.  In this figure, the double lines represent the general flow of
 the frames and information while single lines represent information
 flow only.  The dashed lines in connection with VRP (GVRP/MVRP) are
 to show that VRP support is optional.  An actual RBridge port
 implementation may be structured in any way that provides the correct
 behavior.

Perlman, et al. Standards Track [Page 73] RFC 6325 RBridge Protocol July 2011

                   +----------------------------------------------
                   |                RBridge
                   |
                   | Interport Forwarding, IS-IS.  Management, ...
                   |
                   +----++----------------------+-------------++--
                        ||                      |             ||
                  TRILL || Data                 |             ||
                        ||                   +--+---------+   ||
          +-------------++-----+             |   TRILL    |   ||
          |    Encapsulation   |      +------+ IS-IS Hello|   ||
          |    Decapsulation   |      |      | Processing |   ||
          |     Processing     |      |      +-----++-----+   ||
          +--------------------+      |            ||         ||
          |  RBridge Appointed +------+            ||         ||
      +---+   Forwarder and    |                   ||         ||
      |   |  Inhibition Logic  +==============\\   ||   //====++
      |   +---------+--------+-+   Native       \\ ++ //
      |             |        |     Frames         \++/
      |             |        |                     ||
 +----+-----+  +- - + - - +  |                     ||
 |  RBridge |  |  RBridge |  |                     || All TRILL and
 |   BPDU   |  |    VRP   |  |                     || Native Frames
 |Processing|  |Processing|  |                     ||
 +-----++---+  + - - -+- -+  |            +--------++--+ <- EISS
       ||             |      |            |   802.1Q   |
       ||            |       |            | Port VLAN  |
       ||             |      |            |and Priority|
       ||            |       |            | Processing |
   +---++------------++------+------------+------------+ <-- ISS
   |        802.1/802.3 Low-Level Control Frame        |
   |        Processing, Port/Link Control Logic        |
   +------------++-------------------------------------+
                ||
                ||        +------------+
                ||        | 802.3 PHY  |
                ++========+ (Physical  +======== 802.3
                          | Interface) |         Link
                          +------------+
                Figure 11: Detailed RBridge Port Model
 Low-level control frames are handled in the lower-level port/link
 control logic in the same way as in an [802.1Q-2005] bridge port.
 This can optionally include a variety of 802.1 or link specific
 protocols such as PAUSE (Annex 31B of [802.3]), link layer discovery
 [802.1AB], link aggregation [802.1AX], MAC security [802.1AE], or
 port-based access control [802.1X].  While handled at a low level,

Perlman, et al. Standards Track [Page 74] RFC 6325 RBridge Protocol July 2011

 these frames may affect higher-level processing.  For example, a
 Layer 2 registration protocol may affect the confidence in learned
 addresses.  The upper interface to this lower-level port control
 logic corresponds to the Internal Sublayer Service (ISS) in
 [802.1Q-2005].
 High-level control frames (BPDUs and, if supported, VRP frames) are
 not VLAN tagged.  Although they extend through the ISS interface,
 they are not subject to port VLAN processing.  Behavior on receipt of
 a VLAN tagged BPDU or VLAN tagged VRP frame is unspecified.  If a VRP
 is not implemented, then all VRP frames are discarded.  Handling of
 BPDUs is described in Section 4.9.3.  Handling of VRP frames is
 described in Section 4.9.4.
 Frames other than Layer 2 control frames, that is, all TRILL and
 native frames, are subject to port VLAN and priority processing that
 is the same as for an [802.1Q-2005] bridge.  The upper interface to
 the port VLAN and priority processing corresponds to the Extended
 Internal Sublayer Service (EISS) in [802.1Q-2005].
 In this model, RBridge port processing below the EISS layer is
 identical to an [802.1Q-2005] bridge except for (1) the handling of
 high-level control frames and (2) that the discard of frames that
 have exceeded the Maximum Transit Delay is not mandatory but MAY be
 done.
 As described in more detail elsewhere in this document, incoming
 native frames are only accepted if the RBridge is an uninhibited
 appointed forwarder for the frame's VLAN, after which they are
 normally encapsulated and forwarded; outgoing native frames are
 usually obtained by decapsulation and are only output if the RBridge
 is an uninhibited appointed forwarder for the frame's VLAN.
 TRILL-Hellos, MTU-probes, and MTU-acks are handled per port and, like
 all TRILL IS-IS frames, are never forwarded.  They can affect the
 appointed forwarder and inhibition logic as well as the RBridge's
 LSP.
 Except TRILL-Hellos, MTU-probes, and MTU-acks, all TRILL control as
 well as TRILL data and ESADI frames are passed up to higher-level
 RBridge processing on receipt and passed down for transmission on
 creation or forwarding.  Note that these frames are never blocked due
 to the appointed forwarder and inhibition logic, which affects only
 native frames, but there are additional filters on some of them such
 as the Reverse Path Forwarding Check.

Perlman, et al. Standards Track [Page 75] RFC 6325 RBridge Protocol July 2011

4.9.3. BPDU Handling

 If RBridge campus topology were static, RBridges would simply be end
 stations from a bridging perspective, terminating but not otherwise
 interacting with spanning tree.  However, there are reasons for
 RBridges to listen to and sometimes to transmit BPDUs as described
 below.  Even when RBridges listen to and transmit BPDUs, this is a
 local RBridge port activity.  The ports of a particular RBridge never
 interact so as to make the RBridge as a whole a spanning tree node.

4.9.3.1. Receipt of BPDUs

 Rbridges MUST listen to spanning tree configuration BPDUs received on
 a port and keep track of the root bridge, if any, on that link.  If
 MSTP is running on the link, this is the CIST root.  This information
 is reported per VLAN by the RBridge in its LSP and may be used as
 described in Section 4.2.4.3.  In addition, the receipt of spanning
 tree configuration BPDUs is used as an indication that a link is a
 bridged LAN, which can affect the RBridge transmission of BPDUs.
 An RBridge MUST NOT encapsulate or forward any BPDU frame it
 receives.
 RBridges discard any topology change BPDUs they receive, but note
 Section 4.9.3.3.

4.9.3.2. Root Bridge Changes

 A change in the root bridge seen in the spanning tree BPDUs received
 at an RBridge port may indicate a change in bridged LAN topology,
 including the possibility of the merger of two bridged LANs or the
 like, without any physical indication at the port.  During topology
 transients, bridges may go into pre-forwarding states that block
 TRILL-Hello frames.  For these reasons, when an RBridge sees a root
 bridge change on a port for which it is appointed forwarder for one
 or more VLANs, it is inhibited for a period of time between zero and
 30 seconds.  (An inhibited appointed forwarder discards all native
 frames received from or that it would otherwise have sent to the
 link.)  This time period is configurable per port and defaults to 30
 seconds.
 For example, consider two bridged LANs carrying multiple VLANs, each
 with various RBridge appointed forwarders.  Should they become
 merged, due to a cable being plugged in or the like, those RBridges
 attached to the original bridged LAN with the lower priority root
 will see a root bridge change while those attached to the other
 original bridged LAN will not.  Thus, all appointed forwarders in the

Perlman, et al. Standards Track [Page 76] RFC 6325 RBridge Protocol July 2011

 lower priority set will be inhibited for a time period while things
 are sorted out by BPDUs within the merged bridged LAN and TRILL-Hello
 frames between the RBridges involved.

4.9.3.3. Transmission of BPDUs

 When an RBridge ceases to be appointed forwarder for one or more
 VLANs out a particular port, it SHOULD, as long as it continues to
 receive spanning tree BPDUs on that port, send topology change BPDUs
 until it sees the topology change acknowledged in a spanning tree
 configuration BPDU.
 RBridges MAY support a capability for sending spanning tree BPDUs for
 the purpose of attempting to force a bridged LAN to partition as
 discussed in Appendix A.3.3.

4.9.4. Dynamic VLAN Registration

 Dynamic VLAN registration provides a means for bridges (and less
 commonly end stations) to request that VLANs be enabled or disabled
 on ports leading to the requestor.  This is done by VLAN registration
 protocol (VRP) frames: GVRP or MVRP.  RBridges MAY implement GVRP
 and/or MVRP as described below.
 VRP frames are never encapsulated as TRILL frames between RBridges or
 forwarded in native form by an RBridge.  If an RBridge does not
 implement a VRP, it discards any VRP frames received and sends none.
 RBridge ports may have dynamically enabled VLANs.  If an RBridge
 supports a VRP, the actual enablement of dynamic VLANs is determined
 by GVRP/MVRP frames received at the port as it would be for an
 [802.1Q-2005] / [802.1ak] bridge.
 An RBridge that supports a VRP sends GVRP/MVRP frames as an
 [802.1Q-2005] / [802.1ak] bridge would send on each port that is not
 configured as an RBridge trunk port or P2P port.  For this purpose,
 it sends VRP frames to request traffic in the VLANs for which it is
 appointed forwarder and in the Designated VLAN, unless the Designated
 VLAN is disabled on the port, and to not request traffic in any other
 VLAN.

5. RBridge Parameters

 This section lists parameters for RBridges.  It is expected that the
 TRILL MIB will include many of the items listed in this section plus
 additional Rbridge status and data including traffic and error
 counts.

Perlman, et al. Standards Track [Page 77] RFC 6325 RBridge Protocol July 2011

 The default value and range are given for parameters added by TRILL.
 Where a parameter is defined as a 16-bit unsigned integer and an
 explicit maximum is not given, that maximum is 2**16-1.  For
 parameters imported from [802.1Q-2005], [802.1D], or IS-IS [ISO10589]
 [RFC1195], see those standards for default and range if not given
 here.

5.1. Per RBridge

 The following parameters occur per RBridge:
 o  Number of nicknames, which defaults to 1 and may be configured in
    the range of 1 to 256.
 o  The desired number of distribution trees to be calculated by every
    RBridge in the campus and a desired number of distribution trees
    for the advertising RBridge to use, both of which are unsigned
    16-bit integers that default to 1 (see Section 4.5).
 o  The maximum number of distribution trees the RBridge can compute.
    This is a 16-bit unsigned integer that is implementation and
    environment dependent and not subject to management configuration.
 o  Two lists of nicknames, one designating the distribution trees to
    be computed and one designating distribution trees to be used as
    discussed in Section 4.5.  By default, these lists are empty.
 o  The parameters Ageing Timer and Forward Delay with the default and
    range specified in [802.1Q-2005].
 o  Two unsigned octets that are, respectively, the confidence in
    { MAC, VLAN, local port } triples learned from locally received
    native frames and the confidence in { MAC, VLAN, remote RBridge }
    triples learned from decapsulating frames.  These each default to
    0x20 and may each be configured to values from 0x00 to 0xFE.
 o  The desired minimum acceptable inter-RBridge link MTU for the
    campus, that is, originatingLSPBufferSize.  This is a 16-bit
    unsigned integer number of octets that defaults to 1470 bytes,
    which is the minimum valid value.  Any lower value being
    advertised by an RBridge is ignored.
 o  The number of failed MTU-probes before the RBridge concludes that
    a particular MTU is not supported, which defaults to 3 and may be
    configured between 1 and 255.

Perlman, et al. Standards Track [Page 78] RFC 6325 RBridge Protocol July 2011

 Static end-station address information and confidence in such end
 station information statically configured can also be configured with
 a default confidence of 0xFF and range of 0x00 to 0xFF.  By default,
 there is no such static address information.  The quantity of such
 information that can be configured is implementation dependent.

5.2. Per Nickname Per RBridge

 The following is configuration information per nickname at each
 RBridge:
 o  Priority to hold the nickname, which defaults to 0x40 if no
    specific value has been configured or 0xC0 if it is configured
    (see Section 3.7.3).
 o  Nickname priority to be selected as a distribution tree root, a
    16-bit unsigned integer that defaults to 0x8000.
 o  Nickname value, an unsigned 16-bit quantity that defaults to the
    configured value if configured, else to the last value held if the
    RBridge coming up after a reboot and that value is remembered,
    else to a random value; however, in all cases the reserved values
    0x0000 and 0xFFC0 through 0xFFFF are excluded (see Section 3.7.3).

5.3. Per Port Per RBridge

 An RBridge has the following per-port configuration parameters:
 o  The same parameters as an [802.1Q-2005] port in terms of C-VLAN
    IDs.  In addition, there is an Announcing VLANs set that defaults
    to the enabled VLANs on the port (see Section 4.4.3) and ranges
    from the null set to the set of all legal VLAN IDs.
 o  The same parameters as an [802.1Q-2005] port in terms of frame
    priority code point mapping (see [802.1Q-2005]).
 o  The inhibition time for the port when it observed a change in the
    root bridge of an attached bridged LAN.  This is in units of
    seconds, defaults to 30, and can be configured to any value from 0
    to 30.
 o  The Desired Designated VLAN that the RBridge will advertise in its
    TRILL Hellos if it is the DRB for the link via that port.  This
    defaults to the lowest VLAN ID enabled on the port and may be
    configured to any valid VLAN ID that is enabled on the port (0x001
    through 0xFFE).

Perlman, et al. Standards Track [Page 79] RFC 6325 RBridge Protocol July 2011

 o  Four per-port configuration bits: disable port (default 0 ==
    enabled), disable end-station service (trunk, default 0 ==
    enabled), access port (default 0 == not restricted to being an
    access port), and use P2P Hellos (default 0 == use TRILL Hellos).
    (See Section 4.9.1.)
 o  One bit per port such that, if the bit is set, it disables
    learning { MAC address, VLAN, port } triples from locally received
    native frames on the port.  Default value is 0 == learning
    enabled.
 o  The priority of the RBridge to be DRB and its Holding Time via
    that port with defaults and range as specified in IS-IS [ISO10589]
    [RFC1195].
 o  A bit that, when set, enables the receipt of TRILL-encapsulated
    frames from an Outer.MacSA with which the RBridges does not have
    an IS-IS adjacency.  Default value is 0 == disabled.
 o  Configuration for the optional send-BPDUs solution to the wiring
    closet topology problem as described in Appendix A.3.3.  Default
    Bridge Address is the System ID of the RBridge with the lowest
    System ID.  If RB1 and RB2 are part of a wiring closet topology,
    both need to be configured to know about this, and know the ID
    that should be used in the spanning tree protocol on the specified
    port.

5.4. Per VLAN Per RBridge

 An RBridge has the following per-VLAN configuration parameters:
 o  Per-VLAN ESADI protocol participation flag, 7-bit priority, and
    Holding Time.  Default participation flag is 0 == not
    participating.  Default and range of priority and Holding Time as
    specified in IS-IS [ISO10589] [RFC1195].
 o  One bit per VLAN that, if set, disables learning { MAC address,
    VLAN, remote RBridge } triples from frames decapsulated in the
    VLAN.  Defaults to 0 == learning enabled.

6. Security Considerations

 Layer 2 bridging is not inherently secure.  It is, for example,
 subject to spoofing of source addresses and bridging control
 messages.  A goal for TRILL is that RBridges do not add new issues
 beyond those existing in current bridging technology.

Perlman, et al. Standards Track [Page 80] RFC 6325 RBridge Protocol July 2011

 Countermeasures are available such as to configure the TRILL IS-IS
 and ESADI protocols to use IS-IS security [RFC5304] [RFC5310] and
 ignore unauthenticated TRILL control and ESADI frames received.
 RBridges using IS-IS security will need configuration.
 IEEE 802.1 port admission and link security mechanisms, such as
 [802.1X] and [802.1AE], can also be used.  These are best thought of
 as being implemented below TRILL (see Section 4.9.2) and are outside
 the scope of TRILL (just as they are generally out of scope for
 bridging standards [802.1D] and 802.1Q); however, TRILL can make use
 of secure registration through the confidence level communicated in
 the optional TRILL ESADI protocol (see Section 4.8).
 TRILL encapsulates native frames inside the RBridge campus while they
 are in transit between ingress RBridge and egress RBridge(s) as
 described in Sections 2.3 and 4.1.  Thus, TRILL ignorant devices with
 firewall features that cannot be detected by RBridges as end stations
 will generally not be able to inspect the content of such frames for
 security checking purposes.  This may render them ineffective.  Layer
 3 routers and hosts appear to RBridges to be end stations, and native
 frames will be decapsulated before being sent to such devices.  Thus,
 they will not see the TRILL Ethertype.  Firewall devices that do not
 appear to an RBridge to be an end station, for example, bridges with
 co-located firewalls, should be modified to understand TRILL
 encapsulation.
 RBridges do not prevent nodes from impersonating other nodes, for
 instance, by issuing bogus ARP/ND replies.  However, RBridges do not
 interfere with any schemes that would secure neighbor discovery.

6.1. VLAN Security Considerations

 TRILL supports VLANs.  These provide logical separation of traffic,
 but care should be taken in using VLANs for security purposes.  To
 have reasonable assurance of such separation, all the RBridges and
 links (including bridged LANs) in a campus must be secured and
 configured so as to prohibit end stations from using dynamic VLAN
 registration frames or otherwise gaining access to any VLAN carrying
 traffic for which they are not authorized to read and/or inject.
 Furthermore, if VLANs were used to keep some information off links
 where it might be observed in a bridged LAN, this will no longer
 work, in general, when bridges are replaced with RBridges; with
 encapsulation and a different outer VLAN tag, the data will travel
 the least cost transit path regardless of VLAN.  Appropriate counter
 measures are to use end-to-end encryption or an appropriate TRILL
 security option should one be specified.

Perlman, et al. Standards Track [Page 81] RFC 6325 RBridge Protocol July 2011

6.2. BPDU/Hello Denial-of-Service Considerations

 The TRILL protocol requires that an appointed forwarder at an RBridge
 port be temporarily inhibited if it sees a TRILL-Hello from another
 RBridge claiming to be the appointed forwarder for the same VLAN or
 sees a root bridge change out that port.  Thus, it would seem that
 forged BPDUs showing repeated root bridge changes and forged TRILL-
 Hello frames with the Appointed Forwarder flag set could represent a
 significant denial-of-service attack.  However, the situation is not
 as bad as it seems.
 The best defense against forged TRILL-Hello frames or other IS-IS
 messages is the use of IS-IS security [RFC5304] [RFC5310].  Rogue end
 stations would not normally have access to the required IS-IS keying
 material needed to forge authenticatible messages.
 Authentication similar to IS-IS security is usually unavailable for
 BPDUs.  However, it is also the case that in typical modern wired
 LANs, all the links are point-to-point.  If you have an all-RBridged
 point-to-point campus, then the worst that an end-station can do by
 forging BPDUs or TRILL-Hello frames is to deny itself service.  This
 could be either through falsely inhibiting the forwarding of native
 frames by the RBridge to which it is connected or by falsely
 activating the optional decapsulation check (see Section 4.2.4.3).
 However, when an RBridge campus contains bridged LANs, those bridged
 LANs appear to any connected RBridges to be multi-access links.  The
 forging of BPDUs by an end-station attached to such a bridged LAN
 could affect service to other end-stations attached to the same
 bridged LAN.  Note that bridges never forward BPDUs but process them,
 although this processing may result in the issuance of further BPDUs.
 Thus, for an end-station to forge BPDUs to cause continuing changes
 in the root bridge as seen by an RBridge through intervening bridges
 would typically require it to cause root bridge thrashing throughout
 the bridged LAN that would be disruptive even in the absence of
 RBridges.
 Some bridges can be configured to not send BPDUs and/or to ignore
 BPDUs on particular ports, and RBridges can be configured not to
 inhibit appointed forwarding on a port due to root bridge changes;
 however, such configuration should be used with caution as it can be
 unsafe.

7. Assignment Considerations

 This section discuses IANA and IEEE 802 assignment considerations.
 See [RFC5226].

Perlman, et al. Standards Track [Page 82] RFC 6325 RBridge Protocol July 2011

7.1. IANA Considerations

 A new IANA registry has been created for TRILL Parameters with two
 subregistries as below.
 The initial contents of the TRILL Nicknames subregistry are as
 follows:
    0x0000 Reserved to indicate no nickname specified
    0x0001-0xFFBF Dynamically allocated by the RBridges within each
        RBridge campus
    0xFFC0-0xFFFE Available for allocation by RFC Required (single
        value) or IETF Review (single or multiple values)
    0xFFFF Permanently reserved
 The initial contents of the TRILL Multicast Address subregistry are
 as follows:
    01-80-C2-00-00-40  Assigned as All-RBridges
    01-80-C2-00-00-41  Assigned as All-IS-IS-RBridges
    01-80-C2-00-00-42  Assigned as All-ESADI-RBridges
    01-80-C2-00-00-43 to 01-80-C2-00-00-4F  Available for allocation
                       by IETF Review

7.2. IEEE Registration Authority Considerations

 The Ethertype 0x22F3 is assigned by the IEEE Registration Authority
 to the TRILL Protocol.
 The Ethertype 0x22F4 is assigned by the IEEE Registration Authority
 for L2-IS-IS.
 The block of 16 multicast MAC addresses from <01-80-C2-00-00-40> to
 <01-80-C2-00-00-4F> is assigned by the IEEE Registration Authority
 for IETF TRILL protocol use.

8. Normative References

 [802.1ak]  "IEEE Standard for Local and metropolitan area networks /
            Virtual Bridged Local Area Networks / Multiple
            Registration Protocol", IEEE Standard 802.1ak-2007, 22
            June 2007.
 [802.1D]   "IEEE Standard for Local and metropolitan area networks /
            Media Access Control (MAC) Bridges", 802.1D-2004, 9 June
            2004.

Perlman, et al. Standards Track [Page 83] RFC 6325 RBridge Protocol July 2011

 [802.1Q-2005]
            "IEEE Standard for Local and metropolitan area networks /
            Virtual Bridged Local Area Networks", 802.1Q-2005, 19 May
            2006.
 [802.3]    "IEEE Standard for Information technology /
            Telecommunications and information exchange between
            systems / Local and metropolitan area networks / Specific
            requirements Part 3: Carrier sense multiple access with
            collision detection (CSMA/CD) access method and physical
            layer specifications", 802.3-2008, 26 December 2008.
 [ISO10589] ISO/IEC, "Intermediate system to Intermediate system
            routeing information exchange protocol for use in
            conjunction with the Protocol for providing the
            Connectionless-mode Network Service (ISO 8473)", ISO/IEC
            10589:2002.
 [RFC1112]  Deering, S., "Host extensions for IP multicasting", STD 5,
            RFC 1112, August 1989.
 [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
            dual environments", RFC 1195, December 1990.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2464]  Crawford, M., "Transmission of IPv6 Packets over Ethernet
            Networks", RFC 2464, December 1998.
 [RFC2710]  Deering, S., Fenner, W., and B. Haberman, "Multicast
            Listener Discovery (MLD) for IPv6", RFC 2710, October
            1999.
 [RFC3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
            Thyagarajan, "Internet Group Management Protocol, Version
            3", RFC 3376, October 2002.
 [RFC3417]  Presuhn, R., Ed., "Transport Mappings for the Simple
            Network Management Protocol (SNMP)", STD 62, RFC 3417,
            December 2002.
 [RFC3419]  Daniele, M. and J. Schoenwaelder, "Textual Conventions for
            Transport Addresses", RFC 3419, December 2002.
 [RFC4286]  Haberman, B. and J. Martin, "Multicast Router Discovery",
            RFC 4286, December 2005.

Perlman, et al. Standards Track [Page 84] RFC 6325 RBridge Protocol July 2011

 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.
 [RFC5303]  Katz, D., Saluja, R., and D. Eastlake 3rd, "Three-Way
            Handshake for IS-IS Point-to-Point Adjacencies", RFC 5303,
            October 2008.
 [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
            Engineering", RFC 5305, October 2008.
 [RFC6165]  Banerjee, A. and D. Ward, "Extensions to IS-IS for Layer-2
            Systems", RFC 6165, April 2011.
 [RFC6326]  Eastlake, D., Banerjee, A., Dutt, D., Perlman, R., and A.
            Ghanwani, "Transparent Interconnection of Lots of Links
            (TRILL) Use of IS-IS", RFC 6326, July 2011.

9. Informative References

 [802.1AB]  "IEEE Standard for Local and Metropolitan Networks /
            Station and Media Access Control Connectivity Discovery",
            802.1AB-2009, 17 September 2009.
 [802.1ad]  "IEEE Standard for and metropolitan area networks /
            Virtual Bridged Local Area Networks / Provider Bridges",
            802.1ad-2005, 26 May 2005.
 [802.1ah]  "IEEE Standard for Local and Metropolitan Area Networks /
            Virtual Bridged Local Area Networks / Provider Backbone
            Bridges", 802.1ah-2008, 1 January 2008.
 [802.1aj]  Virtual Bridged Local Area Networks / Two-Port Media
            Access Control (MAC) Relay, 802.1aj Draft 4.2, 24
            September 2009.
 [802.1AE]  "IEEE Standard for Local and metropolitan area networks /
            Media Access Control (MAC) Security", 802.1AE-2006, 18
            August 2006.
 [802.1AX]  "IEEE Standard for Local and metropolitan area networks /
            Link Aggregation", 802.1AX-2008, 1 January 2008.
 [802.1X]  "IEEE Standard for Local and metropolitan area networks /
            Port Based Network Access Control", 802.1X-REV Draft 2.9,
            3 September 2008.

Perlman, et al. Standards Track [Page 85] RFC 6325 RBridge Protocol July 2011

 [RBridges] Perlman, R., "RBridges: Transparent Routing", Proc.
            Infocom 2005, March 2004.
 [RFC1661]  Simpson, W., Ed., "The Point-to-Point Protocol (PPP)", STD
            51, RFC 1661, July 1994.
 [RFC3411]  Harrington, D., Presuhn, R., and B. Wijnen, "An
            Architecture for Describing Simple Network Management
            Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
            December 2002.
 [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
            "Randomness Requirements for Security", BCP 106, RFC 4086,
            June 2005.
 [RFC4541]  Christensen, M., Kimball, K., and F. Solensky,
            "Considerations for Internet Group Management Protocol
            (IGMP) and Multicast Listener Discovery (MLD) Snooping
            Switches", RFC 4541, May 2006.
 [RFC4789]  Schoenwaelder, J. and T. Jeffree, "Simple Network
            Management Protocol (SNMP) over IEEE 802 Networks", RFC
            4789, November 2006.
 [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
            Authentication", RFC 5304, October 2008.
 [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
            and M. Fanto, "IS-IS Generic Cryptographic
            Authentication", RFC 5310, February 2009.
 [RFC5342]  Eastlake 3rd, D., "IANA Considerations and IETF Protocol
            Usage for IEEE 802 Parameters", BCP 141, RFC 5342,
            September 2008.
 [RFC5556]  Touch, J. and R. Perlman, "Transparent Interconnection of
            Lots of Links (TRILL): Problem and Applicability
            Statement", RFC 5556, May 2009.
 [RP1999]   Perlman, R., "Interconnection: Bridges, Routers, Switches,
            and Internetworking Protocols, 2nd Edition", Addison
            Wesley Longman, Chapter 3, 1999.
 [VLAN-MAPPING]
            Perlman, R., Dutt, D., Banerjee, A., Rijhsinghani, A., and
            D. Eastlake 3rd, "RBridges: Campus VLAN and Priority
            Regions", Work in Progress, April 2011.

Perlman, et al. Standards Track [Page 86] RFC 6325 RBridge Protocol July 2011

Appendix A. Incremental Deployment Considerations

 Some aspects of partial RBridge deployment are described below for
 link cost determination (Appendix A.1) and possible congestion due to
 appointed forwarder bottlenecks (Appendix A.2).  A particular example
 of a problem related to the TRILL use of a single appointed forwarder
 per link per VLAN (the "wiring closet topology") is explored in
 detail in Appendix A.3.

A.1. Link Cost Determination

 With an RBridged campus having no bridges or repeaters on the links
 between RBridges, the RBridges can accurately determine the number of
 physical hops involved in a path and the line speed of each hop,
 assuming this is reported by their port logic.  With intervening
 devices, this is no longer possible.  For example, as shown in Figure
 12, the two bridges B1 and B2 can completely hide a slow link so that
 both Rbridges RB1 and RB2 incorrectly believe the link is faster.
          +-----+        +----+        +----+        +-----+
          |     |  Fast  |    |  Slow  |    |  Fast  |     |
          | RB1 +--------+ B1 +--------+ B2 +--------+ RB2 |
          |     |  Link  |    |  Link  |    |  Link  |     |
          +-----+        +----+        +----+        +-----+
                Figure 12: Link Cost of a Bridged Link
 Even in the case of a single intervening bridge, two RBridges may
 know they are connected but each sees the link as a different speed
 from how it is seen by the other.
 However, this problem is not unique to RBridges.  Bridges can
 encounter similar situations due to links hidden by repeaters, and
 routers can encounter similar situations due to links hidden by
 bridges, repeaters, or Rbridges.

A.2. Appointed Forwarders and Bridged LANs

 With partial RBridge deployment, the RBridges may partition a bridged
 LAN into a relatively small number of relatively large remnant
 bridged LANs, or possibly not partition it at all so a single bridged
 LAN remains.  Such configuration can result in the following problem:
 The requirement that native frames enter and leave a link via the
 link's appointed forwarder for the VLAN of the frame can cause
 congestion or suboptimal routing.  (Similar problems can occur within
 a bridged LAN due to the spanning tree algorithm.)  The extent to
 which such a problem will occur is highly dependent on the network

Perlman, et al. Standards Track [Page 87] RFC 6325 RBridge Protocol July 2011

 topology.  For example, if a bridged LAN had a star-like structure
 with core bridges that connected only to other bridges and peripheral
 bridges that connected to end stations and are connected to core
 bridges, the replacement of all of the core bridges by RBridges
 without replacing the peripheral bridges would generally improve
 performance without inducing appointed forwarder congestion.
 Solutions to this problem are discussed below and a particular
 example explored in Appendix A.3.
 Inserting RBridges so that all the bridged portions of the LAN stay
 connected to each other and have multiple RBridge connections is
 generally the least efficient arrangement.
 There are four techniques that may help if the problem above occurs
 and that can, to some extent, be used in combination:
 1. Replace more IEEE 802.1 customer bridges with RBridges so as to
    minimize the size of the remnant bridged LANs between RBridges.
    This requires no configuration of the RBridges unless the bridges
    they replace required configuration.
 2. Re-arrange network topology to minimize the problem.  If the
    bridges and RBridges involved are configured, this may require
    changes in their configuration.
 3. Configure the RBridges and bridges so that end stations on a
    remnant bridged LAN are separated into different VLANs that have
    different appointed forwarders.  If the end stations were already
    assigned to different VLANs, this is straightforward (see Section
    4.2.4.2).  If the end stations were on the same VLAN and have to
    be split into different VLANs, this technique may lead to
    connectivity problems between end stations.
 4. Configure the RBridges such that their ports that are connected to
    the bridged LAN send spanning tree configuration BPDUs (see
    Section A.3.3) in such a way as to force the partition of the
    bridged LAN.  (Note: A spanning tree is never formed through an
    RBridge but always terminates at RBridge ports.)  To use this
    technique, the RBridges must support this optional feature, and
    would need to be configured to use it, but the bridges involved
    would rarely have to be configured.  This technique makes the
    bridged LAN unavailable for TRILL through traffic because the
    bridged LAN partitions.
 Conversely to item 3 above, there may be bridged LANs that use VLANs,
 or use more VLANs than would otherwise be necessary, to support the
 Multiple Spanning Tree Protocol or otherwise reduce the congestion

Perlman, et al. Standards Track [Page 88] RFC 6325 RBridge Protocol July 2011

 that can be caused by a single spanning tree.  Replacing the IEEE
 802.1 bridges in such LANs with RBridges may enable a reduction in or
 elimination of VLANs and configuration complexity.

A.3. Wiring Closet Topology

 If 802.1 bridges are present and RBridges are not properly
 configured, the bridge spanning tree or the DRB may make
 inappropriate decisions.  Below is a specific example of the more
 general problem that can occur when a bridged LAN is connected to
 multiple RBridges.
 In cases where there are two (or more) groups of end nodes, each
 attached to a bridge (say, B1 and B2), and each bridge is attached to
 an RBridge (say, RB1 and RB2, respectively), with an additional link
 connecting B1 and B2 (see Figure 13), it may be desirable to have the
 B1-B2 link only as a backup in case one of RB1 or RB2 or one of the
 links B1-RB1 or B2-RB2 fails.
                +-------------------------------+
                |             |          |      |
                |  Data    +-----+    +-----+   |
                | Center  -| RB1 |----| RB2 |-  |
                |          +-----+    +-----+   |
                |             |          |      |
                +-------------------------------+
                              |          |
                              |          |
                +-------------------------------+
                |             |          |      |
                |          +----+     +----+    |
                | Wiring   | B1 |-----| B2 |    |
                | Closet   +----+     +----+    |
                | Bridged                       |
                | LAN                           |
                +-------------------------------+
                   Figure 13: Wiring Closet Topology
 For example, B1 and B2 may be in a wiring closet and it may be easy
 to provide a short, high-bandwidth, low-cost link between them while
 RB1 and RB2 are at a distant data center such that the RB1-B1 and
 RB2-B2 links are slower and more expensive.
 Default behavior might be that one of RB1 or RB2 (say, RB1) would
 become DRB for the bridged LAN including B1 and B2 and appoint itself
 forwarder for the VLANs on that bridged LAN.  As a result, RB1 would
 forward all traffic to/from the link, so end nodes attached to B2

Perlman, et al. Standards Track [Page 89] RFC 6325 RBridge Protocol July 2011

 would be connected to the campus via the path B2-B1-RB1, rather than
 the desired B2-RB2.  This wastes the bandwidth of the B2-RB2 path and
 cuts available bandwidth between the end stations and the data center
 in half.  The desired behavior would be to make use of both the
 RB1-B1 and RB2-B2 links.
 Three solutions to this problem are described below.

A.3.1. The RBridge Solution

 Of course, if B1 and B2 are replaced with RBridges, the right thing
 will happen without configuration (other than VLAN support), but this
 may not be immediately practical if bridges are being incrementally
 replaced by RBridges.

A.3.2. The VLAN Solution

 If the end stations attached to B1 and B2 are already divided among a
 number of VLANs, RB1 and RB2 could be configured so that whichever
 becomes DRB for this link will appoint itself forwarder for some of
 these VLANs and appoint the other RBridge for the remaining VLANs.
 Should either of the RBridges fail or become disconnected, the other
 will have only itself to appoint as forwarder for all the VLANs.
 If the end stations are all on a single VLAN, then it would be
 necessary to assign them between at least two VLANs to use this
 solution.  This may lead to connectivity problems that might require
 further measures to rectify.

A.3.3. The Spanning Tree Solution

 Another solution is to configure the relevant ports on RB1 and RB2 to
 be part of a "wiring closet group", with a configured per-RBridge
 port "Bridge Address" Bx (which may be RB1 or RB2's System ID).  Both
 RB1 and RB2 emit spanning tree BPDUs on their configured ports as
 highest priority root Bx.  This causes the spanning tree to logically
 partition the bridged LAN as desired by blocking the B1-B2 link at
 one end or the other (unless one of the bridges is configured to also
 have highest priority and has a lower ID, which we consider to be a
 misconfiguration).  With the B1-B2 link blocked, RB1 and RB2 cannot
 see each other's TRILL-Hellos via that link and each acts as
 Designated RBridge and appointed forwarder for its respective
 partition.  Of course, with this partition, no TRILL through traffic
 can flow through the RB1-B1-B2-RB2 path.
 In the spanning tree configuration BPDU, the Root is "Bx" with
 highest priority, cost to Root is 0, Designated Bridge ID is "RB1"
 when RB1 transmits and "RB2" when RB2 transmits, and port ID is a

Perlman, et al. Standards Track [Page 90] RFC 6325 RBridge Protocol July 2011

 value chosen independently by each of RB1 and RB2 to distinguish each
 of its own ports.  The topology change flag is zero, and the topology
 change acknowledgement flag is set if and only if a topology change
 BPDU has been received on the port since the last configuration BPDU
 was transmitted on the port.  (If RB1 and RB2 were actually bridges
 on the same shared medium with no bridges between them, the result
 would be that the one with the larger ID sees "better" BPDUs (because
 of the tiebreaker on the third field: the ID of the transmitting
 bridge), and would turn off its port.)
 Should either RB1 or the RB1-B1 link or RB2 or the RB2-B2 link fail,
 the spanning tree algorithm will stop seeing one of the RBx roots and
 will unblock the B1-B2 link maintaining connectivity of all the end
 stations with the data center.
 If the link RB1-B1-B2-RB2 is on the cut set of the campus and RB2 and
 RB1 have been configured to believe they are part of a wiring closet
 group, the campus becomes partitioned as the link is blocked.

A.3.4. Comparison of Solutions

 Replacing all 802.1 customer bridges with RBridges is usually the
 best solution with the least amount of configuration required,
 possibly none.
 The VLAN solution works well with a relatively small amount of
 configuration if the end stations are already divided among a number
 of VLANs.  If they are not, it becomes more complex and problematic.
 The spanning tree solution does quite well in this particular case.
 But it depends on both RB1 and RB2 having implemented the optional
 feature of being able to configure a port to emit spanning tree BPDUs
 as described in Appendix A.3.3 above.  It also makes the bridged LAN
 whose partition is being forced unavailable for through traffic.
 Finally, while in this specific example it neatly breaks the link
 between the two bridges B1 and B2, if there were a more complex
 bridged LAN, instead of exactly two bridges, there is no guarantee
 that it would partition into roughly equal pieces.  In such a case,
 you might end up with a highly unbalanced load on the RB1-B1 link and
 the RB2-B2 link although this is still better than using only one of
 these links exclusively.

Perlman, et al. Standards Track [Page 91] RFC 6325 RBridge Protocol July 2011

Appendix B. Trunk and Access Port Configuration

 Many modern bridged LANs are organized into a core and access model,
 The core bridges have only point-to-point links to other bridges
 while the access bridges connect to end stations, core bridges, and
 possibly other access bridges.  It seems likely that some RBridge
 campuses will be organized in a similar fashion.
 An RBridge port can be configured as a trunk port, that is, a link to
 another RBridge or RBridges, by configuring it to disable end-station
 support.  There is no reason for such a port to have more than one
 VLAN enabled and in its Announcing Set on the port.  Of course, the
 RBridge (or RBridges) to which it is connected must have the same
 VLAN enabled.  There is no reason for this VLAN to be other than the
 default VLAN 1 unless the link is actually over carrier Ethernet or
 other facilities that only provide some other specific VLAN or the
 like.  Such configuration minimizes wasted TRILL-Hellos and
 eliminates useless decapsulation and transmission of multi-
 destination traffic in native form onto the link (see Sections 4.2.4
 and 4.9.1).
 An RBridge access port would be expected to lead to a link with end
 stations and possibly one or more bridges.  Such a link might also
 have more than one RBridge connected to it to provide more reliable
 service to the end stations.  It would be a goal to minimize or
 eliminate transit traffic on such a link as it is intended for end-
 station native traffic.  This can be accomplished by turning on the
 access port configuration bit for the RBridge port or ports connected
 to the link as further detailed in Section 4.9.1.
 When designing RBridge configuration user interfaces, consideration
 should be given to making it convenient to configure ports as trunk
 and access ports.

Appendix C. Multipathing

 Rbridges support multipathing of both known unicast and multi-
 destination traffic.  Implementation of multipathing is optional.
 Multi-destination traffic can be multipathed by using different
 distribution tree roots for different frames.  For example, assume
 that in Figure 14 end stations attached to RBy are the source of
 various multicast streams each of which has multiple listeners
 attached to various of RB1 through RB9.  Assuming equal bandwidth
 links, a distribution tree rooted at RBy will predominantly use the
 vertical links among RB1 through RB9 while one rooted at RBz will
 predominantly use the horizontal.  If RBy chooses its nickname as the
 distribution tree root for half of this traffic and an RBz nickname

Perlman, et al. Standards Track [Page 92] RFC 6325 RBridge Protocol July 2011

 as the root for the other half, it may be able to substantially
 increase the aggregate bandwidth by making use of both the vertical
 and horizontal links among RB1 through RB9.
 Since the distribution trees an RBridge must calculate are the same
 for all RBridges and transit RBridges MUST respect the tree root
 specified by the ingress RBridge, a campus will operate correctly
 with a mix of RBridges some of which use different roots for
 different multi-destination frames they ingress and some of which use
 a single root for all such frames.
                            +---+
                            |RBy|---------------+
                            +---+               |
                           /  |  \              |
                         /    |    \            |
                       /      |      \          |
                    +---+   +---+   +---+       |
                    |RB1|---|RB2|---|RB3|       |
                    +---+   +---+   +---+\      |
                      |       |       |    \    |
                    +---+   +---+   +---+    \+---+
                    |RB4|---|RB5|---|RB6|-----|RBz|
                    +---+   +---+   +---+    /+---+
                      |       |       |    /
                    +---+   +---+   +---+/
                    |RB7|---|RB8|---|RB9|
                    +---+   +---+   +---+
                Figure 14: Multi-Destination Multipath
 Known unicast Equal Cost Multipathing (ECMP) can occur at an RBridge
 if, instead of using a tiebreaker criterion when building SPF paths,
 information is retained about ports through which equal cost paths
 are available.  Different unicast frames can then be sent through
 those different ports and will be forwarded by equal cost paths.  For
 example, in Figure 15, which shows only RBridges and omits any
 bridges present, there are three equal cost paths between RB1 and RB2
 and two equal cost paths between RB2 and RB5.  Thus, for traffic
 transiting this part of the campus from left to right, RB1 may be
 able to perform three way ECMP and RB2 may be able to perform two-way
 ECMP.
 A transit RBridge receiving a known unicast frame forwards it towards
 the egress RBridge and is not concerned with whether it believes
 itself to be on any particular path from the ingress RBridge or a

Perlman, et al. Standards Track [Page 93] RFC 6325 RBridge Protocol July 2011

 previous transit RBridge.  Thus, a campus will operate correctly with
 a mix of RBridges some of which implement ECMP and some of which do
 not.
 There are actually three possibilities for the parallel paths between
 RB1 and RB2 as follows:
 1. If two or three of these paths have pseudonodes, then all three
    will be distinctly visible in the campus-wide link state and ECMP
    as described above is applicable.
 2. If the paths use P2P Hellos or otherwise do not have pseudonodes,
    these three paths would appear as a single adjacency in the link
    state.  In this case, multipathing across them would be an
    entirely local matter for RB1 and RB2.  It can be freely done for
    known unicast frames but not for multi-destination frames as
    described in Section 4.5.2.
 3. If and only if the three paths between RB1 and RB2 are single hop
    equal bandwidth links with no intervening bridges, then it would
    be permissible to combine them into one logical link through the
    [802.1AX] "link aggregation" feature.  Rbridges MAY implement link
    aggregation since that feature operates below TRILL (see Section
    4.9.2).
                             +---+       double line = 10 Gbps
               -----      ===|RB3|---     single line = 1 Gbps
              /     \   //   +---+   \
          +---+     +---+            +---+
       ===|RB1|-----|RB2|            |RB5|===
          +---+     +---+            +---+
              \     /   \    +---+   //
               -----     ----|RB4|===
                             +---+
                  Figure 15: Known Unicast Multipath
 When multipathing is used, frames that follow different paths will be
 subject to different delays and may be re-ordered.  While some
 traffic may be order/delay insensitive, typically most traffic
 consists of flows of frames where re-ordering within a flow is
 damaging.  How to determine flows or what granularity flows should
 have is beyond the scope of this document.  (This issue is discussed
 in [802.1AX].)

Perlman, et al. Standards Track [Page 94] RFC 6325 RBridge Protocol July 2011

Appendix D. Determination of VLAN and Priority

 A high-level, informative summary of how VLAN ID and priority are
 determined for incoming native frames, omitting some details, is
 given in the bulleted items below.  For more detailed information,
 see [802.1Q-2005].
 o  When an untagged native frame arrives, an unconfigured RBridge
    associates the default priority zero and the VLAN ID 1 with it.
    It actually sets the VLAN for the untagged frame to be the "port
    VLAN ID" associated with that port.  The port VLAN ID defaults to
    VLAN ID 1 but may be configured to be any other VLAN ID.  An
    Rbridge may also be configured on a per-port basis to discard such
    frames or to associate a different priority code point with them.
    Determination of the VLAN ID associated with an incoming untagged
    non-control frame may also be made dependent on the Ethertype or
    NSAP (referred to in 802.1 as the Protocol) of the arriving frame,
    the source MAC address, or other local rules.
 o  When a priority tagged native frame arrives, an unconfigured
    RBridge associates with it both the port VLAN ID, which defaults
    to 1, and the priority code point provided in the priority tag in
    the frame.  An Rbridge may be configured on a per-port basis to
    discard such frames or to associate them with a different VLAN ID
    as described in the point immediately above.  It may also be
    configured to map the priority code point provided in the frame by
    specifying, for each of the eight possible values that might be in
    the frame, what actual priority code point will be associated with
    the frame by the RBridge.
 o  When a C-tagged (formerly called Q-tagged) native frame arrives,
    an unconfigured RBridge associates with it the VLAN ID and
    priority in the C-tag.  An RBridge may be configured on a per-port
    per-VLAN basis to discard such frames.  It may also be configured
    on a per-port basis to map the priority value as specified above
    for priority tagged frames.
 In 802.1, the process of associating a priority code point with a
 frame, including mapping a priority provided in the frame to another
 priority, is referred to as priority "regeneration".

Appendix E. Support of IEEE 802.1Q-2005 Amendments

 This informational appendix briefly comments on RBridge support for
 completed and in-process amendments to IEEE [802.1Q-2005].  There is
 no assurance that existing RBridge protocol specifications or
 existing bridges will support not yet specified future [802.1Q-2005]
 amendments just as there is no assurance that existing bridge

Perlman, et al. Standards Track [Page 95] RFC 6325 RBridge Protocol July 2011

 protocol specifications or existing RBridges will support not yet
 specified future TRILL amendments.
 The information below is frozen as of 25 October 2009.  For the
 latest status, see the IEEE 802.1 working group
 (http://grouper.ieee.org/groups/802/1/).

E.1. Completed Amendments

 802.1ad-2005 Provider Bridges - Sometimes called "Q-in-Q", because
       VLAN tags used to be called "Q-tags", 802.1ad specifies
       Provider Bridges that tunnel customer bridge traffic within
       service VLAN tags (S-tags).  If the customer LAN is an RBridge
       campus, that traffic will be bridged by Provider Bridges.
       Customer bridge features involving Provider Bridge awareness,
       such as the ability to configure a customer bridge port to add
       an S-tag to a frame before sending it to a Provider Bridge, are
       below the EISS layer and can be supported in RBridge ports
       without modification to the TRILL protocol.
 802.1ag-2007 Connectivity Fault Management (CFM) - This 802.1 feature
       is at least in part dependent on the symmetric path and other
       characteristics of spanning tree.  The comments provided to the
       IETF TRILL working group by the IEEE 802.1 working group stated
       that "TRILL weakens the applicability of CFM".
 802.1ak-2007 Multiple Registration Protocol - Supported to the extent
       described in Section 4.9.4.
 802.1ah-2008 Provider Backbone Bridges - Sometimes called "MAC-in-
       MAC", 802.1ah provides for Provider Backbone Bridges that
       tunnel customer bridge traffic within different outer MAC
       addresses and using a tag (the "I-tag") to preserve the
       original MAC addresses and signal other information.  If the
       customer LAN is an RBridge campus, that traffic will be bridged
       by Provider Backbone Bridges.  Customer bridge features
       involving Provider Backbone Bridge awareness, such as the
       ability to configure a customer bridge port to add an I-tag to
       a frame before sending it to a Provider Backbone Bridge, are
       below the EISS layer and can be supported in RBridge ports
       without modification to the TRILL protocol.
 802.1Qaw-2009 Management of Data-Driven and Data-Dependent
       Connectivity Fault - Amendment building on 802.1ag.  See
       comments on 802.1ag-2007 above.

Perlman, et al. Standards Track [Page 96] RFC 6325 RBridge Protocol July 2011

 802.1Qay-2009 Provider Backbone Bridge Traffic Engineering -
       Amendment building on 802.1ah to configure traffic engineered
       routing.  See comments on 802.1ah-2008 above.

E.2. In-Process Amendments

 The following are amendments to IEEE [802.1Q-2005] that are in
 process.  As such, the brief comments below are based on drafts and
 may be incorrect for later versions or any final amendment.
 802.1aj Two-port MAC Relay [802.1aj] - This amendment specifies a MAC
       relay that will be transparent to RBridges.  RBridges are
       compatible with IEEE 802.1aj devices as currently specified, in
       the same sense that IEEE 802.1Q-2005 bridges are compatible
       with such devices.
 802.1aq Shortest Path Bridging - This amendment provides for improved
       routing in bridged LANs.
 802.1Qat Stream Reservation Protocol - Modification to 802.1Q to
       support the 802.1 Timing and Synchronization.  This protocol
       reserves resources for streams at supporting bridges.
 802.1Qau Congestion Notification - It currently appears that
       modifications to RBridge behavior above the EISS level would be
       needed to support this amendment.  Such modifications are
       beyond the scope of this document.
 802.1Qav Forwarding and Queuing Enhancements for Time-Sensitive
       Streams - Modification to 802.1Q to support the 802.1 Timing
       and Synchronization protocol.  This amendment specifies methods
       to support the resource reservations made through the 802.1Qat
       protocol (see above).
 802.1Qaz Enhanced Transmission Selection - It appears that this
       amendment will be below the EISS layer and can be supported in
       RBridge ports without modification to the TRILL protocol.
 802.1Qbb Priority-based Flow Control - Commonly called "per-priority
       pause", it appears that this amendment will be below the EISS
       layer and can be supported in RBridge ports without
       modification to the TRILL protocol.
 802.1bc Remote Customer Service Interfaces.  This is an extension to
       802.1Q provider bridging.  See 802.1ad-2005 above.

Perlman, et al. Standards Track [Page 97] RFC 6325 RBridge Protocol July 2011

 802.1Qbe Multiple Backbone Service Instance Identifier (I-SID)
       Registration Protocol (MIRP).  This is an extension to 802.1Q
       provider backbone bridging.  See 802.1ah-2008 above.
 802.1Qbf Provider Backbone Bridge Traffic Engineering (PBB-TE)
       Infrastructure Segment Protection.  This amendment extends
       802.1Q to support certain types of failover between provider
       backbone bridges.  See 802.1ah-2008 above.

Appendix F. Acknowledgements

 Many people have contributed to this design, including the following,
 in alphabetic order:
    Bernard Aboba, Alia Atlas, Ayan Banerjee, Caitlin Bestler, Suresh
    Boddapati, David Michael Bond, Stewart Bryant, Ross Callon, James
    Carlson, Pasi Eronen, Dino Farinacci, Adrian Farrell, Don Fedyk,
    Bill Fenner, Eric Gray, Sujay Gupta, Joel Halpern, Andrew Lange,
    Acee Lindem, Vishwas Manral, Peter McCann, Israel Meilik, David
    Melman, Nandakumar Natarajan, Erik Nordmark, Jeff Pickering, Tim
    Polk, Dan Romascanu, Sanjay Sane, Pekka Savola, Matthew R. Thomas,
    Joe Touch, Mark Townsley, Kate Zebrose.

Perlman, et al. Standards Track [Page 98] RFC 6325 RBridge Protocol July 2011

Authors' Addresses

 Radia Perlman
 Intel Labs
 2200 Mission College Blvd.
 Santa Clara, CA 95054-1549 USA
 Phone: +1-408-765-8080
 EMail: Radia@alum.mit.edu
 Donald E. Eastlake, 3rd
 Huawei Technologies
 155 Beaver Street
 Milford, MA 01757 USA
 Phone: +1-508-333-2270
 EMail: d3e3e3@gmail.com
 Dinesh G. Dutt
 Cisco Systems
 170 Tasman Drive
 San Jose, CA 95134-1706 USA
 Phone: +1-408-527-0955
 EMail: ddutt@cisco.com
 Silvano Gai
 Cisco Systems
 170 Tasman Drive
 San Jose, CA 95134-1706 USA
 EMail: silvano@ip6.com
 Anoop Ghanwani
 Brocade
 130 Holger Way
 San Jose, CA 95134 USA
 Phone: +1-408-333-7149
 EMail: anoop@alumni.duke.edu

Perlman, et al. Standards Track [Page 99]

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