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rfc:rfc7782

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.

Zhang, et al. Standards Track [Page 1] RFC 7782 Multi-Attach for Active-Active Edge February 2016

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

Zhang, et al. Standards Track [Page 2] RFC 7782 Multi-Attach for Active-Active Edge February 2016

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.

Zhang, et al. Standards Track [Page 3] RFC 7782 Multi-Attach for Active-Active Edge February 2016

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.

Zhang, et al. Standards Track [Page 4] RFC 7782 Multi-Attach for Active-Active Edge February 2016

 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).

Zhang, et al. Standards Track [Page 5] RFC 7782 Multi-Attach for Active-Active Edge February 2016

 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].

Zhang, et al. Standards Track [Page 6] RFC 7782 Multi-Attach for Active-Active Edge February 2016

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.

Zhang, et al. Standards Track [Page 7] RFC 7782 Multi-Attach for Active-Active Edge February 2016

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.

Zhang, et al. Standards Track [Page 8] RFC 7782 Multi-Attach for Active-Active Edge February 2016

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.

Zhang, et al. Standards Track [Page 9] RFC 7782 Multi-Attach for Active-Active Edge February 2016

 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.

Zhang, et al. Standards Track [Page 10] RFC 7782 Multi-Attach for Active-Active Edge February 2016

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

Zhang, et al. Standards Track [Page 11] RFC 7782 Multi-Attach for Active-Active Edge February 2016

 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

Zhang, et al. Standards Track [Page 12] RFC 7782 Multi-Attach for Active-Active Edge February 2016

 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.

Zhang, et al. Standards Track [Page 13] RFC 7782 Multi-Attach for Active-Active Edge February 2016

 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.

Zhang, et al. Standards Track [Page 14] RFC 7782 Multi-Attach for Active-Active Edge February 2016

 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].

Zhang, et al. Standards Track [Page 15] RFC 7782 Multi-Attach for Active-Active Edge February 2016

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

Zhang, et al. Standards Track [Page 16] RFC 7782 Multi-Attach for Active-Active Edge February 2016

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>.

Zhang, et al. Standards Track [Page 17] RFC 7782 Multi-Attach for Active-Active Edge February 2016

 [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>.

Zhang, et al. Standards Track [Page 18] RFC 7782 Multi-Attach for Active-Active Edge February 2016

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.

Zhang, et al. Standards Track [Page 19] RFC 7782 Multi-Attach for Active-Active Edge February 2016

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>}.

Zhang, et al. Standards Track [Page 20] RFC 7782 Multi-Attach for Active-Active Edge February 2016

 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.

Zhang, et al. Standards Track [Page 21] RFC 7782 Multi-Attach for Active-Active Edge February 2016

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

Zhang, et al. Standards Track [Page 22]

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