





Internet Engineering Task Force (IETF)                          M. Zhang
Request for Comments: 7782                                        Huawei
Category: Standards Track                                     R. Perlman
ISSN: 2070-1721                                                      EMC
                                                                 H. Zhai
                                                       Astute Technology
                                                              M. Durrani
                                                           Cisco Systems
                                                                S. Gupta
                                                             IP Infusion
                                                           February 2016


          Transparent Interconnection of Lots of Links (TRILL)
           Active-Active Edge Using Multiple MAC Attachments

Abstract

   TRILL (Transparent Interconnection of Lots of Links) active-active
   service provides end stations with flow-level load balance and
   resilience against link failures at the edge of TRILL campuses, as
   described in RFC 7379.

   This document specifies a method by which member RBridges (also
   referred to as Routing Bridges or TRILL switches) in an active-active
   edge RBridge group use their own nicknames as ingress RBridge
   nicknames to encapsulate frames from attached end systems.  Thus,
   remote edge RBridges (who are not in the group) will see one host
   Media Access Control (MAC) address being associated with the multiple
   RBridges in the group.  Such remote edge RBridges are required to
   maintain all those associations (i.e., MAC attachments) and to not
   flip-flop among them (as would occur prior to the implementation of
   this specification).  The design goals of this specification are
   discussed herein.

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/rfc7782.



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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to 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.

Table of Contents

   1. Introduction ....................................................3
   2. Acronyms and Terminology ........................................4
   3. Overview ........................................................5
   4. Incremental Deployable Options ..................................6
      4.1. Details of Option B ........................................7
           4.1.1. Advertising Data Labels for Active-Active Edge ......7
           4.1.2. Discovery of Active-Active Edge Members .............8
           4.1.3. Advertising Learned MAC Addresses ...................9
      4.2. Extended RBridge Capability Flags APPsub-TLV ..............11
   5. Meeting the Design Goals .......................................12
      5.1. No MAC Address Flip-Flopping (Normal Unicast Egress) ......12
      5.2. Regular Unicast/Multicast Ingress .........................12
      5.3. Correct Multicast Egress ..................................12
           5.3.1. No Duplication (Single Exit Point) .................12
           5.3.2. No Echo (Split Horizon) ............................13
      5.4. No Black-Hole or Triangular Forwarding ....................14
      5.5. Load Balance towards the AAE ..............................14
      5.6. Scalability ...............................................14
   6. E-L1FS Backward Compatibility ..................................15
   7. Security Considerations ........................................15
   8. IANA Considerations ............................................16
      8.1. TRILL APPsub-TLVs .........................................16
      8.2. Extended RBridge Capabilities Registry ....................16
      8.3. Active-Active Flags .......................................17
   9. References .....................................................17
      9.1. Normative References ......................................17
      9.2. Informative References ....................................19
   Appendix A. Scenarios for Split Horizon ...........................20
   Acknowledgments ...................................................21
   Authors' Addresses ................................................22




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1.  Introduction

   As discussed in [RFC7379], in a TRILL (Transparent Interconnection of
   Lots of Links) Active-Active Edge (AAE) topology, a Local
   Active-Active Link Protocol (LAALP) -- for example, a Multi-Chassis
   Link Aggregation (MC-LAG) bundle -- is used to connect multiple
   RBridges (Routing Bridges or TRILL switches) to multi-port Customer
   Equipment (CE), such as a switch, virtual switch (vSwitch), or
   multi-port end station.  A set of end nodes is attached in the case
   of a switch or vSwitch.  It is required that data traffic within a
   specific VLAN from this end node set (including the multi-port
   end-station case) can be ingressed and egressed by any of these
   RBridges simultaneously.  End systems in the set can spread their
   traffic among these edge RBridges at the flow level.  When a link
   fails, end systems keep using the remaining links in the LAALP
   without waiting for the convergence of TRILL, which provides
   resilience to link failures.

   Since a frame from each end node can be ingressed by any RBridge in
   the local AAE group, a remote edge RBridge may observe multiple
   attachment points (i.e., egress RBridges) for this end node.  This
   issue is known as "MAC address flip-flopping"; see [RFC7379] for a
   discussion.

   Per this document, AAE member RBridges use their own nicknames to
   ingress frames into the TRILL campus.  Remote edge RBridges are
   required to keep multiple points of attachment per MAC address and
   Data Label (VLAN or Fine-Grained Label [RFC7172]) attached to the
   AAE.  This addresses the MAC flip-flopping issue.  Using this
   solution, as specified in this document, in an AAE group does not
   prohibit the use of other solutions in other AAE groups in the same
   TRILL campus.  For example, the specification in this document and
   the specification in [RFC7781] could be simultaneously deployed for
   different AAE groups in the same campus.

   The main body of this document is organized as follows:  Section 2
   lists acronyms and terms.  Section 3 describes the overview model.
   Section 4 provides options for incremental deployment.  Section 5
   describes how this approach meets the design goals.  Section 6
   discusses backward compatibility.  Section 7 covers security
   considerations.  Section 8 covers IANA considerations.










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2.  Acronyms and Terminology

   AAE: Active-Active Edge

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

   CE: Customer Equipment (end station or bridge).  The device can be
      either physical or virtual equipment.

   Data Label: VLAN or Fine-Grained Label (FGL)

   DRNI: Distributed Resilient Network Interconnect.  A link aggregation
      specified in [802.1AX] that can provide an LAALP between (a) one,
      two, or three CEs and (b) two or three RBridges.

   E-L1FS: Extended Level 1 Flooding Scope

   Edge RBridge: An RBridge providing end-station service on one or more
      of its ports.

   ESADI: End Station Address Distribution Information [RFC7357]

   FGL: Fine-Grained Label [RFC7172]

   FS-LSP: Flooding Scope Link State Protocol Data Unit

   IS: Intermediate System [IS-IS]

   IS-IS: Intermediate System to Intermediate System [IS-IS]

   LAALP: Local Active-Active Link Protocol [RFC7379].  Any protocol
      similar to MC-LAG (or DRNI) that runs in a distributed fashion on
      a CE, on the links from that CE to a set of edge group RBridges,
      and on those RBridges.

   LSP: Link State PDU

   MC-LAG: Multi-Chassis Link Aggregation.  Proprietary extensions of
      link aggregation [802.1AX] that can provide an LAALP between one
      CE and two or more RBridges.

   PDU: Protocol Data Unit

   RBridge: A device implementing the TRILL protocol.





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   TRILL: Transparent Interconnection of Lots of Links or Tunneled
      Routing in the Link Layer [RFC6325] [RFC7177].

   TRILL switch: An alternative name for an RBridge.

   vSwitch: A virtual switch, such as a hypervisor, that also simulates
      a bridge.

   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 RFC 2119 [RFC2119].

   Familiarity with [RFC6325], [RFC6439], and [RFC7177] is assumed in
   this document.

3.  Overview

                               +-----+
                               | RB4 |
                    +----------+-----+----------+
                    |                           |
                    |                           |
                    |       Rest of campus      |
                    |                           |
                    |                           |
                    +-+-----+--+-----+--+-----+-+
                      | RB1 |  | RB2 |  | RB3 |
                      +-----\  +-----+  /-----+
                              \   |   /
                                \ | /
                                 |||LAALP1
                                 |||
                                +---+
                                | B |
                                +---+
                             H1 H2 H3 H4: VLAN 10

        Figure 1: An Example Topology for TRILL Active-Active Edge

   Figure 1 shows an example network for TRILL AAE (see also Figure 1 in
   [RFC7379]).  In this figure, end nodes (H1, H2, H3, and H4) are
   attached to a bridge (B) that communicates with multiple RBridges
   (RB1, RB2, and RB3) via the LAALP.  Suppose that RB4 is a "remote"
   RBridge not in the AAE group in the TRILL campus.  This connection
   model is also applicable to the virtualized environment where the
   physical bridge can be replaced with a vSwitch while those bare metal
   hosts are replaced with virtual machines (VMs).




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   For a frame received from its attached end node sets, a member
   RBridge of the AAE group conforming to this document always
   encapsulates that frame using its own nickname as the ingress
   nickname, regardless of whether it is unicast or multicast.

   With the two options specified below, even though remote RBridge RB4
   will see multiple attachments for each MAC address from one of the
   end nodes, MAC address flip-flopping will not cause any problems.

4.  Incremental Deployable Options

   This section specifies two options.  Option A requires new hardware
   support.  Option B can be incrementally implemented throughout a
   TRILL campus with common existing TRILL "fast path" hardware.
   Further details on Option B are given in Section 4.1.

   Option A:
      A new capability announcement would appear in LSPs: "I can cope
      with data-plane learning of multiple attachments for an end node."
      This mode of operation is generally not supported by existing
      TRILL fast path hardware.  Only if all edge RBridges to which the
      group has data connectivity -- and that are interested in any of
      the Data Labels in which the AAE is interested -- announce this
      capability can the AAE group safely use this approach.  If all
      such RBridges do not announce this "Option A" capability, then a
      fallback would be needed, such as reverting from active-active to
      active-standby operation or isolating the RBridges that would need
      to support this capability but do not support it.  Further details
      for Option A are beyond the scope of this document, except that,
      as described in Section 4.2, a bit is reserved to indicate support
      for Option A, because a remote RBridge supporting Option A is
      compatible with an AAE group using Option B.

   Option B:
      As pointed out in Section 4.2.6 of [RFC6325] and Section 5.3 of
      [RFC7357], one MAC address may be persistently claimed to be
      attached to multiple RBridges within the same Data Label in the
      TRILL ESADI-LSPs.  For Option B, AAE member RBridges make use of
      the TRILL ESADI protocol to distribute multiple attachments of a
      MAC address.  Remote RBridges SHOULD disable data-plane MAC
      learning for such multi-attached MAC addresses from TRILL Data
      packet decapsulation, unless they also support Option A.  The
      ability to configure an RBridge to disable data-plane learning is
      provided by the base TRILL protocol [RFC6325].







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4.1.  Details of Option B

   With Option B, the receiving edge RBridges MUST avoid flip-flop
   errors for MAC addresses learned from the TRILL Data packet
   decapsulation for the originating RBridge within these Data Labels.
   It is RECOMMENDED that the receiving edge RBridge disable data-plane
   MAC learning from TRILL Data packet decapsulation within those
   advertised Data Labels for the originating RBridge, unless the
   receiving RBridge also supports Option A.  Alternative
   implementations that produce the same expected behavior, i.e., the
   receiving edge RBridge does not flip-flop among multiple MAC
   attachments, are acceptable.  For example, the confidence-level
   mechanism as specified in [RFC6325] can be used.  Let the receiving
   edge RBridge give a prevailing confidence value (e.g., 0x21) to the
   first MAC attachment learned from the data plane over others from the
   TRILL Data packet decapsulation.  The receiving edge RBridge will
   stick to this MAC attachment until it is overridden by one learned
   from the ESADI protocol [RFC7357].  The MAC attachment learned from
   ESADI is set to have a higher confidence value (e.g., 0x80) to
   override any alternative learning from the decapsulation of received
   TRILL Data packets [RFC6325].

4.1.1.  Advertising Data Labels for Active-Active Edge

   An RBridge in an AAE group MUST participate in ESADI in Data Labels
   enabled for its attached LAALPs.  This document further registers two
   data flags, which are used to advertise that the originating RBridge
   supports and participates in an AAE.  These two flags are allocated
   from the Interested VLANs Flag Bits that appear in the Interested
   VLANs and Spanning Tree Roots sub-TLV and the Interested Labels Flag
   Bits that appear in the Interested Labels and Spanning Tree Roots
   sub-TLV [RFC7176] (see Section 8.3).  When these flags are set to 1,
   the originating RBridge is advertising Data Labels for LAALPs rather
   than plain LAN links.

















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4.1.2.  Discovery of Active-Active Edge Members

   Remote edge RBridges need to discover RBridges in an AAE.  This is
   achieved by listening to the following "AA LAALP Group RBridges"
   TRILL APPsub-TLV included in the TRILL GENINFO TLV in FS-LSPs
   [RFC7780]:

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Type = AA-LAALP-GROUP-RBRIDGES| (2 bytes)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Length                        | (2 bytes)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Sender Nickname               | (2 bytes)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | LAALP ID Size |                 (1 byte)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+-+
      | LAALP ID                        (k bytes)       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+-+

   o  Type: AA LAALP Group RBridges (TRILL APPsub-TLV type 252)

   o  Length: 3 + k

   o  Sender Nickname: The nickname the originating RBridge will use as
      the ingress nickname.  This field is useful because the
      originating RBridge might own multiple nicknames.

   o  LAALP ID Size: The length, k, of the LAALP ID in bytes.

   o  LAALP ID: The ID of the LAALP, which is k bytes long.  If the
      LAALP is an MC-LAG or DRNI, it is the 8-byte ID, as specified in
      Clause 6.3.2 of [802.1AX].

   This APPsub-TLV is expected to rarely change, as it only does so in
   cases of the creation or elimination of an AAE group, or of link
   failure or restoration to the CE in such a group.















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4.1.3.  Advertising Learned MAC Addresses

   Whenever MAC addresses from the LAALP of this AAE are learned through
   ingress or configuration, the originating RBridge MUST advertise
   these MAC addresses using the MAC-Reachability TLV [RFC6165] via the
   ESADI protocol [RFC7357].  The MAC-Reachability TLVs are composed in
   a way that each TLV only contains MAC addresses of end nodes attached
   to a single LAALP.  Each such TLV is enclosed in a TRILL APPsub-TLV,
   defined as follows:

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Type = AA-LAALP-GROUP-MAC     | (2 bytes)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Length                        | (2 bytes)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | LAALP ID Size |                 (1 byte)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+-+
      | LAALP ID                        (k bytes)       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+-+
      | MAC-Reachability TLV            (7 + 6*n bytes) |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+-+

   o  Type: AA LAALP Group MAC (TRILL APPsub-TLV type 253)

   o  Length: The MAC-Reachability TLV [RFC6165] is contained in the
      value field as a sub-TLV.  The total number of bytes contained in
      the value field is given by k + 8 + 6*n.

   o  LAALP ID Size: The length, k, of the LAALP ID in bytes.

   o  LAALP ID: The ID of the LAALP, which is k bytes long.  Here, it
      also serves as the identifier of the AAE.  If the LAALP is an
      MC-LAG (or DRNI), it is the 8-byte ID, as specified in
      Clause 6.3.2 of [802.1AX].

   o  MAC-Reachability sub-TLV: The AA-LAALP-GROUP-MAC APPsub-TLV value
      contains the MAC-Reachability TLV as a sub-TLV (see [RFC6165];
      n is the number of MAC addresses present).  As specified in
      Section 2.2 of [RFC7356], the Type and Length fields of the
      MAC-Reachability TLV are encoded as unsigned 16-bit integers.  The
      1-byte unsigned confidence value, along with these TLVs, SHOULD be
      set to prevail over those MAC addresses learned from TRILL Data
      decapsulation by remote edge RBridges.

   This AA-LAALP-GROUP-MAC APPsub-TLV MUST be included in a TRILL
   GENINFO TLV [RFC7357] in the ESADI-LSP.  There may be more than one
   occurrence of such TRILL APPsub-TLVs in one ESADI-LSP fragment.




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   For those MAC addresses contained in an AA-LAALP-GROUP-MAC
   APPsub-TLV, this document applies.  Otherwise, [RFC7357] applies.
   For example, an AAE member RBridge continues to enclose MAC addresses
   learned from TRILL Data packet decapsulation in MAC-Reachability TLVs
   as per [RFC6165] and advertise them using the ESADI protocol.

   When the remote RBridge learns MAC addresses contained in the
   AA-LAALP-GROUP-MAC APPsub-TLV via the ESADI protocol [RFC7357], it
   sends the packets destined to these MAC addresses to the closest one
   (the one to which the remote RBridge has the least-cost forwarding
   path) of those RBridges in the AAE identified by the LAALP ID in the
   AA-LAALP-GROUP-MAC APPsub-TLV.  If there are multiple equal
   least-cost member RBridges, the ingress RBridge is required to select
   one of them in a pseudorandom way, as specified in Section 5.3 of
   [RFC7357].

   When another RBridge in the same AAE group receives an ESADI-LSP with
   the AA-LAALP-GROUP-MAC APPsub-TLV, it also learns MAC addresses of
   those end nodes served by the corresponding LAALP.  These MAC
   addresses SHOULD be learned as if those end nodes are locally
   attached to this RBridge itself.

   An AAE member RBridge MUST use the AA-LAALP-GROUP-MAC APPsub-TLV to
   advertise in ESADI the MAC addresses learned from a plain local link
   (a non-LAALP link) with Data Labels that happen to be covered by the
   Data Labels of any attached LAALP.  The reason is that MAC learning
   from TRILL Data packet decapsulation within these Data Labels at the
   remote edge RBridge has normally been disabled for this RBridge.

   This APPsub-TLV changes whenever the MAC reachability situation for
   the LAALP changes.




















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4.2.  Extended RBridge Capability Flags APPsub-TLV

   The following Extended RBridge Capability Flags APPsub-TLV will be
   included in E-L1FS FS-LSP fragment zero [RFC7780] as an APPsub-TLV of
   the TRILL GENINFO TLV:

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Type = EXTENDED-RBRIDGE-CAP   | (2 bytes)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Length                        | (2 bytes)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Topology                      | (2 bytes)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |E|H|     Reserved                                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Reserved (continued)                                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o  Type: Extended RBridge Capability (TRILL APPsub-TLV type 254)

   o  Length: Set to 10.

   o  Topology: Indicates the topology to which the capabilities apply.
      When this field is set to zero, either topologies are not in use
      or the capabilities apply to all topologies [TRILL-MT].

   o  E: Bit 0 of the capability bits.  When this bit is set, it
      indicates that the originating RBridge acts as specified in
      Option B above.

   o  H: Bit 1 of the capability bits.  When this bit is set, it
      indicates that the originating RBridge keeps multiple MAC
      attachments learned from TRILL Data packet decapsulation with fast
      path hardware; that is, it acts as specified in Option A above.

   o  Reserved: Flags extending from bit 2 through bit 63 of the
      capability bits.  Reserved for future use.  These MUST be sent as
      zero and ignored on receipt.

   The Extended RBridge Capability Flags TRILL APPsub-TLV is used to
   notify other RBridges as to whether the originating RBridge supports
   the capability indicated by the E and H bits.  For example, if the
   E bit is set, it indicates that the originating RBridge will act as
   defined in Option B.  That is, it will disable the MAC learning from
   TRILL Data packet decapsulation within Data Labels advertised by AAE
   RBridges while waiting for the TRILL ESADI-LSPs to distribute the
   {MAC, Nickname, Data Label} association.  Meanwhile, this RBridge is
   able to act as an AAE RBridge.  It is required that MAC addresses



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   learned from local LAALPs be advertised in TRILL ESADI-LSPs, using
   the AA-LAALP-GROUP-MAC APPsub-TLV, which is defined in Section 4.1.3.
   If an RBridge in an AAE group, as specified herein, observes a remote
   RBridge interested in one or more of that AAE group's Data Labels and
   the remote RBridge does not support, as indicated by its extended
   capabilities, either Option A or Option B, then the AAE group MUST
   fall back to active-standby mode.

   This APPsub-TLV is expected to rarely change, as it only needs to be
   updated when RBridge capabilities change, e.g., due to an upgrade or
   reconfiguration.

5.  Meeting the Design Goals

   This section explores how this specification meets the major design
   goals of AAE.

5.1.  No MAC Address Flip-Flopping (Normal Unicast Egress)

   Since all RBridges talking with the AAE RBridges in the campus are
   able to see multiple attachments for one MAC address in ESADI
   [RFC7357], a MAC address learned from one AAE member will not be
   overwritten by the same MAC address learned from another AAE member.
   Although multiple entries for this MAC address will be created, for
   return traffic the remote RBridge is required to consistently use one
   of the attachments for each MAC address rather than flip-flopping
   among them (see Section 4.2.6 of [RFC6325] and Section 5.3 of
   [RFC7357]).

5.2.  Regular Unicast/Multicast Ingress

   LAALP guarantees that each frame will be sent to the AAE via exactly
   one uplink.  RBridges in the AAE simply follow the process per
   [RFC6325] to ingress the frame.  For example, each RBridge uses its
   own nickname as the ingress nickname to encapsulate the frame.  In
   such a scenario, each RBridge takes for granted that it is the
   Appointed Forwarder for the VLANs enabled on the uplink of the LAALP.

5.3.  Correct Multicast Egress

   A fundamental design goal of AAE is that there must be no duplication
   or forwarding loop.

5.3.1.  No Duplication (Single Exit Point)

   When multi-destination TRILL Data packets for a specific Data Label
   are received from the campus, it is important that exactly one
   RBridge out of the AAE group let through each multi-destination



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   packet so that no duplication will happen.  The LAALP will have
   defined its selection function (using hashing or an election
   algorithm) to designate a forwarder for a multi-destination frame.
   Since AAE member RBridges support the LAALP, they are able to utilize
   that selection function to determine the single exit point.  If the
   output of the selection function points to the port attached to the
   receiving RBridge itself (i.e., the packet should be egressed out of
   this node), the receiving RBridge MUST egress this packet for that
   AAE group.  Otherwise, the packet MUST NOT be egressed for that AAE
   group.  (For ports that lead to non-AAE links, the receiving RBridge
   determines whether to egress the packet or not, according to
   [RFC6325], which is updated by [RFC7172].)

5.3.2.  No Echo (Split Horizon)

   When a multi-destination frame originated from an LAALP is ingressed
   by an RBridge of an AAE group, distributed to the TRILL network, and
   then received by another RBridge in the same AAE group, it is
   important that this receiving RBridge does not egress this frame back
   to this LAALP.  Otherwise, it will cause a forwarding loop (echo).
   The well-known "split horizon" technique (as discussed in
   Section 2.2.1 of [RFC1058]) is used to eliminate the echo issue.

   RBridges in the AAE group need to perform split horizon based on the
   ingress RBridge nickname plus the VLAN of the TRILL Data packet.
   They need to set up per-port filtering lists consisting of the tuple
   of <ingress nickname, VLAN>.  Packets with information matching any
   entry in the filtering list MUST NOT be egressed out of that port.
   The information for such filters is obtained by listening to the
   AA-LAALP-GROUP-RBRIDGES TRILL APPsub-TLVs, as defined in
   Section 4.1.2.  Note that all enabled VLANs MUST be consistent on all
   ports connected to an LAALP.  So, the enabled VLANs need not be
   included in these TRILL APPsub-TLVs.  They can be locally obtained
   from the port attached to that LAALP.  By parsing these APPsub-TLVs,
   the receiving RBridge discovers all other RBridges connected to the
   same LAALP.  The Sender Nickname of the originating RBridge will be
   added to the filtering list of the port attached to the LAALP.  For
   example, RB3 in Figure 1 will set up a filtering list that looks like
   {<RB1, VLAN 10>, <RB2, VLAN 10>} on its port attached to LAALP1.
   According to split horizon, TRILL Data packets within VLAN 10
   ingressed by RB1 or RB2 will not be egressed out of this port.

   When there are multiple LAALPs connected to the same RBridge, these
   LAALPs may have VLANs that overlap.  Here, a VLAN overlap means that
   this VLAN ID is enabled by multiple LAALPs.  A customer may require
   that hosts within these overlapped VLANs communicate with each other.





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   Appendix A provides several scenarios to explain how hosts
   communicate within the overlapped VLANs and how split horizon
   happens.

5.4.  No Black-Hole or Triangular Forwarding

   If a sub-link of the LAALP fails while remote RBridges continue to
   send packets towards the failed port, a black-hole happens.  If the
   AAE member RBridge with that failed port starts to redirect the
   packets to other member RBridges for delivery, triangular forwarding
   occurs.

   The member RBridge attached to the failed sub-link makes use of the
   ESADI protocol to flush those MAC addresses affected by the failure,
   as defined in Section 5.2 of [RFC7357].  After doing that, no packets
   will be sent towards the failed port, and hence no black-hole will
   happen.  Nor will the member RBridge need to redirect packets to
   other member RBridges; thus, triangular forwarding is avoided.

5.5.  Load Balance towards the AAE

   Since a remote RBridge can see multiple attachments of one MAC
   address in ESADI, this remote RBridge can choose to spread the
   traffic towards the AAE members on a per-flow basis.  Each of them is
   able to act as the egress point.  In doing this, the forwarding paths
   need not be limited to the least-cost path selection from the ingress
   RBridge to the AAE RBridges.  The traffic load from the remote
   RBridge towards the AAE RBridges can be balanced based on a
   pseudorandom selection method (see Section 4.1.3).

   Note that the load-balance method adopted at a remote ingress RBridge
   is not to replace the load-balance mechanism of LAALP.  These two
   load-spreading mechanisms should take effect separately.

5.6.  Scalability

   With Option A, multiple attachments need to be recorded for a MAC
   address learned from AAE RBridges.  More entries may be consumed in
   the MAC learning table.  However, MAC addresses attached to an LAALP
   are usually only a small part of all MAC addresses in the whole TRILL
   campus.  As a result, the extra table memory space required by
   multi-attached MAC addresses can usually be accommodated in an
   RBridge's unused MAC table space.








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   With Option B, remote RBridges will keep the multiple attachments of
   a MAC address in the ESADI link-state databases, which are usually
   maintained by software.  In the MAC table, which is normally
   implemented in hardware, an RBridge still establishes only one entry
   for each MAC address.

6.  E-L1FS Backward Compatibility

   The Extended TLVs defined in Sections 4.1.2, 4.1.3, and 4.2 of this
   document are to be used in an Extended Level 1 Flooding Scope
   (E-L1FS) PDU [RFC7356] [RFC7780].  For those RBridges that do not
   support E-L1FS, the EXTENDED-RBRIDGE-CAP TRILL APPsub-TLV will not be
   sent out either, and MAC multi-attach active-active is not supported.

7.  Security Considerations

   For security considerations pertaining to extensions transported by
   TRILL ESADI, see the Security Considerations section in [RFC7357].

   For extensions not transported by TRILL ESADI, RBridges may be
   configured to include the IS-IS Authentication TLV (10) in the IS-IS
   PDUs to use IS-IS security [RFC5304] [RFC5310].

   Since currently deployed LAALPs [RFC7379] are proprietary, security
   over membership in, and internal management of, AAE groups is
   proprietary.  In environments where the above authentication is not
   adopted, a rogue RBridge that insinuates itself into an AAE group can
   disrupt end-station traffic flowing into or out of that group.  For
   example, if there are N RBridges in the group, it could typically
   control 1/Nth of the traffic flowing out of that group and a similar
   amount of unicast traffic flowing into that group.

   For general TRILL security considerations, see [RFC6325].


















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8.  IANA Considerations

8.1.  TRILL APPsub-TLVs

   IANA has allocated three new types under the TRILL GENINFO TLV
   [RFC7357] for the TRILL APPsub-TLVs defined in Sections 4.1.2, 4.1.3,
   and 4.2 of this document.  The following entries have been added to
   the "TRILL APPsub-TLV Types under IS-IS TLV 251 Application
   Identifier 1" registry on the TRILL Parameters IANA web page.

      Type   Name                     Reference
      ----   ----                     ---------
      252    AA-LAALP-GROUP-RBRIDGES  RFC 7782
      253    AA-LAALP-GROUP-MAC       RFC 7782
      254    EXTENDED-RBRIDGE-CAP     RFC 7782

8.2.  Extended RBridge Capabilities Registry

   IANA has created a registry under the "Transparent
   Interconnection of Lots of Links (TRILL) Parameters" registry
   as follows:

   Name: Extended RBridge Capabilities

   Registration Procedure: Expert Review

   Reference: RFC 7782

      Bit   Mnemonic  Description       Reference
      ----  --------  -----------       ---------
      0     E         Option B Support  RFC 7782
      1     H         Option A Support  RFC 7782
      2-63  -         Unassigned


















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8.3.  Active-Active Flags

   IANA has allocated two flag bits, with mnemonic "AA", as follows:

   One flag bit is allocated from the Interested VLANs Flag Bits.

      Bit   Mnemonic  Description              Reference
      ---   --------  -----------              ---------
      16    AA        VLANs for Active-Active  RFC 7782

   One flag bit is allocated from the Interested Labels Flag Bits.

      Bit   Mnemonic  Description               Reference
      ---   --------  -----------               ---------
      4     AA        FGLs for Active-Active    RFC 7782

9.  References

9.1.  Normative References

   [802.1AX]  IEEE, "IEEE Standard for Local and metropolitan area
              networks - Link Aggregation", IEEE Std 802.1AX-2014,
              DOI 10.1109/IEEESTD.2014.7055197, December 2014.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC6165]  Banerjee, A. and D. Ward, "Extensions to IS-IS for Layer-2
              Systems", RFC 6165, DOI 10.17487/RFC6165, April 2011,
              <http://www.rfc-editor.org/info/rfc6165>.

   [RFC6325]  Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
              Ghanwani, "Routing Bridges (RBridges): Base Protocol
              Specification", RFC 6325, DOI 10.17487/RFC6325, July 2011,
              <http://www.rfc-editor.org/info/rfc6325>.

   [RFC6439]  Perlman, R., Eastlake, D., Li, Y., Banerjee, A., and F.
              Hu, "Routing Bridges (RBridges): Appointed Forwarders",
              RFC 6439, DOI 10.17487/RFC6439, November 2011,
              <http://www.rfc-editor.org/info/rfc6439>.

   [RFC7172]  Eastlake 3rd, D., Zhang, M., Agarwal, P., Perlman, R., and
              D. Dutt, "Transparent Interconnection of Lots of Links
              (TRILL): Fine-Grained Labeling", RFC 7172,
              DOI 10.17487/RFC7172, May 2014,
              <http://www.rfc-editor.org/info/rfc7172>.



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   [RFC7176]  Eastlake 3rd, D., Senevirathne, T., Ghanwani, A., Dutt,
              D., and A. Banerjee, "Transparent Interconnection of Lots
              of Links (TRILL) Use of IS-IS", RFC 7176,
              DOI 10.17487/RFC7176, May 2014,
              <http://www.rfc-editor.org/info/rfc7176>.

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

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

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

   [RFC7780]  Eastlake 3rd, D., Zhang, M., Perlman, R., Banerjee, A.,
              Ghanwani, A., and S. Gupta, "Transparent Interconnection
              of Lots of Links (TRILL): Clarifications, Corrections, and
              Updates", RFC 7780, DOI 10.17487/RFC7780, February 2016,
              <http://www.rfc-editor.org/info/rfc7780>.
























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9.2.  Informative References

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

   [RFC1058]  Hedrick, C., "Routing Information Protocol", RFC 1058,
              DOI 10.17487/RFC1058, June 1988,
              <http://www.rfc-editor.org/info/rfc1058>.

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304,
              October 2008, <http://www.rfc-editor.org/info/rfc5304>.

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

   [RFC7379]  Li, Y., Hao, W., Perlman, R., Hudson, J., and H. Zhai,
              "Problem Statement and Goals for Active-Active Connection
              at the Transparent Interconnection of Lots of Links
              (TRILL) Edge", RFC 7379, DOI 10.17487/RFC7379,
              October 2014, <http://www.rfc-editor.org/info/rfc7379>.

   [RFC7781]  Zhai, H., Senevirathne, T., Perlman, R., Zhang, M., and Y.
              Li, "Transparent Interconnection of Lots of Links (TRILL):
              Pseudo-Nickname for Active-Active Access", RFC 7781,
              DOI 10.17487/RFC7781, February 2016,
              <http://www.rfc-editor.org/info/rfc7781>.

   [TRILL-MT] Eastlake 3rd, D., Zhang, M., Banerjee, A., and V. Manral,
              "TRILL: Multi-Topology", Work in Progress,
              draft-ietf-trill-multi-topology-00, September 2015.












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Appendix A.  Scenarios for Split Horizon

   +------------------+   +------------------+   +------------------+
   |        RB1       |   |        RB2       |   |        RB3       |
   +------------------+   +------------------+   +------------------+
   L1       L2       L3   L1       L2       L3   L1       L2       L3
   VL10-20  VL15-25  VL15 VL10-20  VL15-25  VL15 VL10-20  VL15-25  VL15
   LAALP1   LAALP2   LAN  LAALP1   LAALP2   LAN  LAALP1   LAALP2   LAN
   B1       B2       B10  B1       B2       B20  B1       B2       B30

          Figure 2: An Example Topology to Explain Split Horizon

   Suppose that RB1, RB2, and RB3 are the active-active group connecting
   LAALP1 and LAALP2.  LAALP1 and LAALP2 are connected to B1 and B2 at
   their other ends.  Suppose that all these RBridges use port L1 to
   connect LAALP1 while they use port L2 to connect LAALP2.  Assume that
   all three L1 ports enable VLANs 10-20 while all three L2 ports enable
   VLANs 15-25, so that there is an overlap of VLANs 15-20.  A customer
   may require that hosts within these overlapped VLANs communicate with
   each other.  That is, hosts attached to B1 in VLANs 15-20 need to
   communicate with hosts attached to B2 in VLANs 15-20.  Assume that
   the remote plain RBridge RB4 also has hosts attached in VLANs 15-20
   that need to communicate with those hosts in these VLANs attached to
   B1 and B2.

   There are two major requirements:

   1. Frames ingressed from RB1-L1-VLANs 15-20 MUST NOT be egressed out
      of ports RB2-L1 and RB3-L1.

   2. At the same time, frames coming from B1-VLANs 15-20 should reach
      B2-VLANs 15-20.

   RB3 stores the information for split horizon on its ports L1 and L2.

      On L1: {<ingress_nickname_RB1, VLANs 10-20>,
         <ingress_nickname_RB2, VLANs 10-20>}.

      On L2: {<ingress_nickname_RB1, VLANs 15-25>,
         <ingress_nickname_RB2, VLANs 15-25>}.











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   Five clarification scenarios follow:

   a. Suppose that RB2 or RB3 receives a TRILL multi-destination data
      packet with VLAN 15 and ingress_nickname_RB1.  RB3 is the single
      exit point (selected according to the hashing function of LAALP)
      for this packet.  On ports L1 and L2, RB3 has covered
      <ingress_nickname_RB1, VLAN 15>, so that RB3 will not egress this
      packet out of either L1 or L2.  Here, "split horizon" happens.

      Beforehand, RB1 obtains a native frame on port L1 from B1 in
      VLAN 15.  RB1 determines that it should be forwarded as a
      multi-destination packet across the TRILL campus.  Also, RB1
      replicates this frame without TRILL encapsulation and sends it out
      of port L2, so that B2 will get this frame.

   b. Suppose that RB2 or RB3 receives a TRILL multi-destination data
      packet with VLAN 15 and ingress_nickname_RB4.  RB3 is the single
      exit point.  On ports L1 and L2, since RB3 has not stored any
      tuple with ingress_nickname_RB4, RB3 will decapsulate the packet
      and egress it out of both ports L1 and L2.  So, both B1 and B2
      will receive the frame.

   c. Suppose that there is a plain LAN link port L3 on RB1, RB2, and
      RB3, connecting to B10, B20, and B30, respectively.  These L3
      ports happen to be configured with VLAN 15.  On port L3, RB2 and
      RB3 store no information for split horizon for AAE (since this
      port has not been configured to be in any LAALP).  They will
      egress the packet ingressed from RB1-L1 in VLAN 15.

   d. If a packet is ingressed from RB1-L1 or RB1-L2 with VLAN 15,
      port RB1-L3 will not egress packets with ingress_nickname_RB1.
      RB1 needs to replicate this frame without encapsulation and sends
      it out of port L3.  This kind of "bounce" behavior for
      multi-destination frames is just as specified in paragraph 3 of
      Section 4.6.1.2 of [RFC6325].

   e. If a packet is ingressed from RB1-L3, since RB1-L1 and RB1-L2
      cannot egress packets with VLAN 15 and ingress_nickname_RB1, RB1
      needs to replicate this frame without encapsulation and sends it
      out of ports L1 and L2.  (Also see paragraph 3 of Section 4.6.1.2
      of [RFC6325].)

Acknowledgments

   The authors would like to thank the following people for their
   comments and suggestions: Andrew Qu, Donald Eastlake, Erik Nordmark,
   Fangwei Hu, Liang Xia, Weiguo Hao, Yizhou Li, and Mukhtiar Shaikh.




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Authors' Addresses

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

   Email: zhangmingui@huawei.com


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

   Email: radia@alum.mit.edu


   Hongjun Zhai
   Nanjing Astute Software Technology Co. Ltd
   57 Andemen Avenue, Yuhuatai District
   Nanjing, Jiangsu  210016
   China

   Email: honjun.zhai@tom.com


   Muhammad Durrani
   Cisco Systems
   170 West Tasman Dr.
   San Jose, CA  95134
   United States

   Email: mdurrani@cisco.com


   Sujay Gupta
   IP Infusion
   RMZ Centennial
   Mahadevapura Post
   Bangalore  560048
   India

   Email: sujay.gupta@ipinfusion.com





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