Network Working GroupInternet Engineering Task Force                              B. Kothari
Internet-Draft                                             Cisco Systems
Internet Draft                                          Cohere Networks
Updates: 4761 (if approved)                                  K. Kompella
Intended status: Standards Track                        Juniper Networks                            K. Kompella
Expires: February 6, 2012 April 2013                                    Contrail Systems

                                                          W. Henderickx
                                                               F. Balus
                                                        S. Palislamovic
                                                         Alcatel-Lucent

                                                              J. Uttaro
                                                                   AT&T
                                                            July 6, 2011

                                                                 W. Lin
                                                       Juniper Networks

                                                       October 21, 2012

           BGP based Multi-homing in Virtual Private LAN Service
                draft-ietf-l2vpn-vpls-multihoming-03.txt
                 draft-ietf-l2vpn-vpls-multihoming-04.txt

Abstract

   Virtual Private LAN Service (VPLS) is a Layer 2 Virtual Private
   Network (VPN) that gives its customers the appearance that their
   sites are connected via a Local Area Network (LAN).  It is often
   required for the Service Provider (SP) to give the customer
   redundant connectivity to some sites, often called "multi-homing".
   This memo shows how BGP-based multi-homing can be offered in the
   context of LDP and BGP VPLS solutions.

Status of this Memo

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4 Introduction...................................................4
      1.1. General Terminology  . . . . . . . . . . . . . . . . . . .  4 Terminology.......................................4
      1.2. Conventions  . . . . . . . . . . . . . . . . . . . . . . .  4 used in this document.........................5
   2.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  6 Background.....................................................6
      2.1.  Scenarios  . . . . . . . . . . . . . . . . . . . . . . . .  6 Scenarios.................................................6
      2.2. VPLS Multi-homing Considerations . . . . . . . . . . . . .  7 Considerations..........................7
   3. Multi-homing Operation . . . . . . . . . . . . . . . . . . . .  8 Operations........................................8
      3.1. Provisioning Model . . . . . . . . . . . . . . . . . . . .  8 Model........................................8
      3.2. Multi-homing NLRI  . . . . . . . . . . . . . . . . . . . .  8 NLRI.........................................8
      3.3. Designated Forwarder Election  . . . . . . . . . . . . . .  9 Election.............................9
         3.3.1.  Attributes . . . . . . . . . . . . . . . . . . . . . .  9 Attributes...........................................9
         3.3.2. Variables Used . . . . . . . . . . . . . . . . . . . .  9
       3.3.3.  Election Procedures  . . . . . . . . . . . . . . . . . 11 Used.......................................9
            3.3.2.1. RD.............................................10
            3.3.2.2. MH-ID..........................................10
            3.3.2.3. VBO............................................10
            3.3.2.4. DOM............................................10
            3.3.2.5. ACS............................................10
            3.3.2.6. PREF...........................................11
            3.3.2.7. PE-ID..........................................11
      3.4. VPLS DF Election on PEs . . . . . . . . . . . . . . . . . . . . 13 PEs..................................14
      3.5. Pseudowire binding and traffic forwarding rules..........14
         3.5.1. Site-ID Binding Properties..........................14
         3.5.2. Standby Pseudowire Properties.......................15
   4. Multi-AS VPLS  . . . . . . . . . . . . . . . . . . . . . . . . 14 VPLS.................................................16
      4.1. Route Origin Extended Community  . . . . . . . . . . . . . 14 Community..........................16
      4.2.  VPLS Preference  . . . . . . . . . . . . . . . . . . . . . 14 Preference...............................................16
      4.3. Use of BGP-MH attributes in Inter-AS Methods . . . . . . . 15 Methods.............17
         4.3.1. Inter-AS Method (b): EBGP Redistribution of VPLS
         Information between ASBRs  . . . . . . . . . . . . . . 15                            ......18
         4.3.2. Inter-AS Method (c): Multi-Hop EBGP Redistribution of
         VPLS Information between ASes . . . . . . . . . . . 16                ..............19
   5. MAC Flush Operations . . . . . . . . . . . . . . . . . . . . . 18
     5.1.  MAC List FLush . . . . . . . . . . . . . . . . . . . . . . 18
     5.2.  Implicit MAC Flush . . . . . . . . . . . . . . . . . . . . 18
     5.3.  Minimizing the effects of fast link transitions  . . . . . 20
   6.  Backwards Compatibility  . . . . . . . . . . . . . . . . . . . 21
     6.1.  BGP based VPLS . . . . . . . . . . . . . . . . . . . . . . 21 Flush Operations..........................................20
      5.1. MAC List FLush...........................................20
      5.2. Implicit MAC Flush.......................................21
      5.3. Minimizing the effects of fast link transitions..........22
   6. Backwards Compatibility.......................................23
      6.1. BGP based VPLS...........................................23
      6.2. LDP VPLS with BGP Auto-discovery . . . . . . . . . . . . . 21 Auto-discovery.........................23
   7. Security Considerations  . . . . . . . . . . . . . . . . . . . 22 Considerations.......................................24
   8. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23 Considerations...........................................25
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 24 Acknowledgments...............................................26
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25 References...................................................27
      10.1. Normative References . . . . . . . . . . . . . . . . . . . 25 References....................................27
      10.2. Informative References . . . . . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27 References..................................27

1. Introduction

   Virtual Private LAN Service (VPLS) is a Layer 2 Virtual Private
   Network (VPN) that gives its customers the appearance that their
   sites are connected via a Local Area Network (LAN).  It is often
   required for a Service Provider (SP) to give the customer redundant
   connectivity to one or more sites, often called "multi-homing".
   [RFC4761] explains how VPLS can be offered using BGP for auto-
   discovery and signaling; section 3.5 of that document describes how
   multi-homing can be achieved in this context. [RFC6074] explains how
   VPLS can be offered using BGP for auto- discovery, (BGP-AD) and
   [RFC4762] explains how VPLS can be offered using LDP for signaling.
   This document provides a BGP-based multi-homing solution applicable
   to both BGP and LDP VPLS technologies. Note that BGP MH can be used
   for LDP VPLS without the use of the BGP- AD solution.

   Section 2 lays out some of the scenarios for multi-homing, other
   ways that this can be achieved, and some of the expectations of BGP-based BGP-
   based multi-homing.  Section 3 defines the components of BGP-based multi-
   homing,
   multi-homing, and the procedures required to achieve this.  Section
   7 may someday discuss security considerations.

1.1. General Terminology

   Some general terminology is defined here; most is from [RFC4761],
   [RFC4762] or [RFC4364].  Terminology specific to this memo is
   introduced as needed in later sections.

   A "Customer Edge" (CE) device, typically located on customer
   premises, connects to a "Provider Edge" (PE) device, which is owned
   and operated by the SP.  A "Provider" (P) device is also owned and
   operated by the SP, but has no direct customer connections.  A "VPLS
   Edge" (VE) device is a PE that offers VPLS services.

   A VPLS domain represents a bridging domain per customer.  A Route
   Target community as described in [RFC4360] is typically used to
   identify all the PE routers participating in a particular VPLS
   domain.  A VPLS site is a grouping of ports on a PE that belong to
   the same VPLS domain.  A Multi-homed (MH) site is uniquely
   identified by a MH site ID (MH-ID).  Sites are referred to as local
   or remote depending on whether they are configured on the PE router
   in context or on one of the remote PE routers (network peers).

1.2. Conventions used 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 RFC-2119 [RFC2119].

2. Background

   This section describes various scenarios where multi-homing may be
   required, and the implications thereof.  It also describes some of
   the singular properties of VPLS multi-homing, and what that means
   from both an operational point of view and an implementation point
   of view.  There are other approaches for providing multi-homing such
   as Spanning Tree Protocol, and this document specifies use of BGP
   for multi-homing.  Comprehensive comparison among the approaches is
   outside the scope of this document.

2.1. Scenarios

   CE1 is a VPLS CE that is dual-homed to both PE1 and PE2 for
   redundant connectivity.

                                ...............
                               .               .    ___ CE2
                         ___ PE1                .  /
                        /    :                  PE3
                     __/    :       Service      :
                 CE1 __     :       Provider    PE4
                       \     :                   : \___ CE3
                        \___ PE2                .
                               .               .
                                ...............

                              Figure 1: Scenario 1

   CE1 is a VPLS CE that is dual-homed to both PE1 and PE2 for
   redundant connectivity.  However, CE4, which is also in the same
   VPLS domain, is single-homed to just PE1.

                 CE4 -------    ...............
                            \  .               .    ___ CE2
                         ___ PE1                .  /
                        /    :                  PE3
                     __/    :       Service      :
                 CE1 __     :       Provider    PE4
                       \     :                   : \___ CE3
                        \___ PE2                .
                               .               .
                                ...............

                              Figure 2: Scenario 2

2.2. VPLS Multi-homing Considerations

   The first (perhaps obvious) fact about a multi-homed VPLS CE, such
   as CE1 in Figure 1 is that if CE1 is an Ethernet switch or bridge, a
   loop has been created in the customer VPLS.  This is a dangerous
   situation for an Ethernet network, and the loop must be broken.
   Even if CE1 is a router, it will get duplicates every time a packet
   is flooded, which is clearly undesirable.

   The next is that (unlike the case of IP-based multi-homing) only one
   of PE1 and PE2 can be actively sending traffic, either towards CE1
   or into the SP cloud.  That is to say, load balancing techniques
   will not work.  All other PEs MUST choose the same designated
   forwarder for a multi-homed site.  Call the PE that is chosen to
   send traffic to/from CE1 the "designated forwarder".

   In Figure 2, CE1 and CE4 must be dealt with independently, since CE1
   is dual-homed, but CE4 is not.

3. Multi-homing Operation Operations

   This section describes procedures for electing a designated
   forwarder among the set of PEs that are multi-homed to a customer
   site.  The procedures described in this section are applicable to
   BGP based VPLS, LDP based VPLS with BGP-AD or a VPLS that contains a
   mix of both BGP and LDP signaled PWs.

3.1. Provisioning Model

   Figure 1 shows a customer site, CE1, multi-homed to two VPLS PEs,
   PE1 and PE2.  In order for all VPLS PEs within the same VPLS domain
   to elect one of the multi-homed PEs as the designated forwarder, an
   indicator that the PEs are multi-homed to the same customer site is
   required.  This is achieved by assigning the same multi-homed site
   ID (MH-ID) on PE1 and PE2 for CE1.  When remote VPLS PEs receive
   NLRI advertisement from PE1 and PE2 for CE1, the two NLRI
   advertisements for CE1 are identified as candidates for designated
   forwarder selection due to the same MH-ID.  Thus, same MH-ID SHOULD
   be assigned on all VPLS PEs that are multi-homed to the same
   customer site. Note that a MH-ID=0 is invalid and a PE should
   discard such an advertisement.

3.2. Multi-homing NLRI

   Section 3.2.2 in [RFC4761] describes the encoding of the BGP VPLS
   NLRI.  This
   NLRI contains fields with the following fields: VE-ID, VE block offset, VE block
   size and label base.  For multi-homing operation, the same  While this NLRI MAY be used for multi-homing,
   a modified version of it, as detailed in this paragraph, is used for
   identifying the multi-homed customers sites.  The VE-ID field in the
   NLRI is set to MH-ID; the VE block offset, VE block size and label
   base are set to zero.  Thus, the NLRI contains 2 octets indicating
   the length, 8 octets for Route Distinguisher, 2 octets for MH-ID and
   7 octets with value zero.

   Figure 2 shows two customer sites, CE1 and CE4, connected to PE1
   with CE1 multi-homed to PE1 and PE2.  CE4 does not require special
   addressing, being associated with the base VPLS instance identified
   by the VSI-ID for LDP VPLS and VE-ID for BGP VPLS.  However, CE1
   which is multi-homed to PE1 and PE2 requires configuration of MH-ID
   and both PE1 and PE2 MUST be provisioned with the same MH-ID for
   CE1.

   It  As stated above, to ensure backward capabilities, it is valid
   to use BGP VPLS NLRI for multi-homing operations.  As such, it is
   valid to have non-zero VE block offset, VE block size and label base
   in the VPLS NLRI for a multi-homed site.  However, multi-homing
   operations in such a case are outside the scope of this document. site.

3.3. Designated Forwarder Election

   BGP-based multi-homing for VPLS relies on Standard BGP DF election best path
   selection and VPLS DF election.  The net result of doing both BGP and VPLS DF election
   elections is that of electing a single designated forwarder (DF)
   among the set of PEs to which a customer site is multi-homed.  All
   the PEs that are elected as non-designated forwarders MUST keep
   their attachment circuit to the multi-homed CE in blocked status (no
   forwarding).

   These election algorithms operate on VPLS advertisements, which
   include both the NLRI and attached BGP attributes.  In order to
   simplify the explanation  Given that
   semantics of these algorithms, we will use BGP VPLS NLRI does not necessarily follow a number standard IP
   Prefix form, a construct of
   variables derived from fields in advertisement (ADV) with the VPLS advertisement.  These variables
   of interest is defined.  The variables of interest are: RD, MH-ID,
   VBO, DOM, ACS, PREF and PE-ID.  The PE-ID, so the notation of:

               ADV -> = <RD, MH-ID, VBO, DOM, ACS, PREF, PE-ID> means that
   from a received VPLS advertisement ADV, the respective variables were
   derived.

   The following sections describe two attributes needed for standard
   BGP best path selection and VPLS DF election, then describe the variables and how they are derived
   from fields in VPLS advertisement ADV, and finally describe how DF
   election is done. elaborate on the
   selection processes.

3.3.1. Attributes

   The procedures below refer to two attributes: the Route Origin
   community (see Section 4.1) and the L2-info community (see Section
   4.2).  These attributes are required for inter-AS operation; for
   generality, the procedures below show how they are to be used. The
   procedures also say outline how to handle the case that either or both
   are not present.

3.3.2. Variables Used
3.3.2.1. RD

   RD is simply set to the Route Distinguisher field in the NLRI part
   of ADV.  Actual process of assigning Route Distinguisher values must
   guarantee its uniqueness per PE node.  Therefore, two multi-homed
   PEs offering the same VPLS service to a common set of CEs MUST
   allocate different RD values for this site respectively.

3.3.2.2. MH-ID

   MH-ID is simply set to the VE-ID field in the NLRI part of ADV.  The
   same MH-ID MUST be assigned to all PEs that are connected to the
   same multi-homed site.

3.3.2.3. VBO

   VBO is simply set to the VE Block Offset field in the NLRI part of
   ADV.  This field will typically be zero.

3.3.2.4. DOM

   This variable, indicating the VPLS domain to which ADV belongs, is
   derived by applying BGP policy to the Route Target extended
   communities in ADV.  The details of how this is done are outside the
   scope of this document.

3.3.2.5. ACS

   ACS is the status of the attachment circuits for a given site of a
   VPLS.  ACS = 1 if all attachment circuits for the site are down, and
   0 otherwise.

   For BGP-based Multi-homing, ADV MUST contain an L2-info extended
   community; within this community are control flags.  One of these
   flags is the 'D' bit, described in [I-D.kothari-l2vpn-auto-site-id].
   ACS is set to the value of the 'D' bit in ADV.

3.3.2.6. PREF

   PREF is derived from the Local Preference (LP) attribute in ADV as
   well as the VPLS Preference field (VP) in the L2-info extended
   community.  If the Local Preference attribute is missing, LP is set
   to 0; if the L2-info community is missing, VP is set to 0.  The
   following table shows how PREF is computed from LP and VP.

 +---------+---------------+----------+------------------------------+
 |    VP   |    LP Value   |   PREF   |            Comment           |
 |  Value  |               |   Value  |                              |
 +---------+---------------+----------+------------------------------+
 |    0    |       0       |     0    |   malformed advertisement,   |
 |         |               |          |         unless ACS=1         |
 |         |               |          |                              |
 |    0    | 1 to (2^16-1) |    LP    |    backwards compatibility   |
 |         |               |          |                              |
 |    0    |    2^16 to    | (2^16-1) |    backwards compatibility   |
 |         |    (2^32-1)   |          |                              |
 |         |               |          |                              |
 |    >0   | LP same as VP |    VP    |  Implementation supports VP  |
 |         |               |          |                              |
 |    >0   |    LP != VP   |     0    |    malformed advertisement   |
 +---------+---------------+----------+------------------------------+

                                Table 1

3.3.2.7. PE-ID

   If ADV contains a Route Origin (RO) community (see Section 4.1) with
   type 0x01, then PE-ID is set to the Global Administrator sub-field
   of the RO. Otherwise, if ADV has an ORIGINATOR_ID attribute, then
   PE-ID is set to the ORIGINATOR_ID.  Otherwise, PE-ID is set to the
   BGP Identifier.

3.3.3.  Election Procedures

   The election procedures described in this section apply equally to
   BGP VPLS and LDP VPLS.

   Election occurs in two stages.  The first stage divides all received
   VPLS advertisements into buckets  Subset of relevant these procedures documented in
   standard BGP best path selection deals with general IP Prefix BGP
   route selection processing as defined in RFC 4271 and comparable
   advertisements.  Distinction MUST NOT be made on whether RFC 4364.  A
   separate part of the NLRI algorithm defined under VPLS DF election is
   a multi-homing NLRI or not.  In this stage, advertisements may be
   discarded as not being relevant
   very specific to DF election.  The second stage
   picks a single "winner" from each bucket by repeatedly applying a
   tie-breaking algorithm designated forwarded election procedures performed
   on a pair of advertisements from that bucket.
   The tie-breaking rules are such per VPLS instance bases.  Given that the order in which notion of VPLS
   advertisement is not commonly used destination IP Prefix, a concept
   of bucketization is introduced.  By bucketizing advertisements and
   running them through two different sets of procedures based on
   variables of interest, we are picked from effectively adopting common sets of
   route selection rules to the bucket does not affect VPLS environment.  A distinction MUST
   NOT be made on whether the final
   result. NLRI is a multi-homing NLRI or not.  Note
   that this is a conceptual description of the process; an
   implementation MAY choose to realize this differently as long as the
   semantics are preserved.

   Note: these procedures supersede the tie breaking rules described in
   (Section 9.1.2.2) [RFC4271]

3.3.3.1.  Bucketization for and BGP DF Election

   An Best Path Selection

   From the advertisement

                 ADV -> <RD, MH-ID, VBO, ACS, PREF, PE-ID>

   is put into the bucket for <RD, MH-ID, VBO>.  In other words,

   we select variables of interest that satisfy a notion of the
   information in same
   route as it is applicable to BGP DF election consists of <RD, MH-ID, VBO> and only election.  As such, advertisements
   with the exact same RD, MH-ID and VBO are candidates for BGP
   Selection and put into BGP election bucket.

                          ADV -> <RD, MH-ID, VBO>

   A standard set of BGP path selection rules, as defined in RFC 4271
   and 4264 is applied as tie-breaking mechanism.  These tie-breaking
   rules are candidates applied to candidate advertisements by a BGP speaker
   responsible for DF
   election. processing and redistribution of BGP VPLS and MH
   NLRI.  If there is no winner and both ADVs are from the same PE, BGP
   path selection should simply consider this an update.

3.3.3.2.  Bucketization for and VPLS DF Election

   An advertisement

              ADV -> <RD, MH-ID, VBO, DOM, ACS, PREF, PE-ID>

   is discarded if DOM is not of interest to the VPLS PE.  Otherwise,
   ADV is put into the bucket for <DOM, MH-ID>.  In other words, all
   advertisements for a particular VPLS domain that have the same MH-ID
   and common DOM are candidates for VPLS DF election.

3.3.3.3.  Tie-breaking Rules

   This section describes the tie-breaking  Tie breaking
   rules for both BGP and VPLS DF election.  Tie-breaking rules for BGP DF election are applied to
   candidate advertisements by any BGP speaker.  Since RD must be same
   for advertisements to be candidates for different from standard BGP DF election, use of
   unique RDs will result best path
   selection.  As outlined in no candidate advertisements 3.3, the main reason for BGP tie-
   breaking rules and thus, that is the fact
   that only single VPLS PE can be a BGP speaker in such designated forwarder for a case will simply not
   do BGP DF election. given
   site.  Tie-breaking rules for VPLS DF election are applied to
   candidate advertisements by all VPLS PEs and the actions taken by
   VPLS PEs based on the VPLS DF election result are described
   in Section 3.4.

   Given two advertisements ADV1 and ADV2 from a given bucket, first
   compute the variables needed for DF election:

   ADV1 -> <RD1, MH-ID1, VBO1, DOM1, ACS1, PREF1, PE-ID1>
   ADV2 -> <RD2, MH-ID2, VBO2, DOM2, ACS2, PREF2, PE-ID2>

   Note that MH-ID1 = MH-ID2 and DOM1 = DOM2, since ADV1 and ADV2 came
   from the same bucket.  If this is for BGP DF election, RD1 = RD2 and
   VBO1 = VBO2 as well.  Then the following tie-breaking rules MUST be
   applied in the given order.

      1.  if (ACS1 != 1) AND (ACS2 == 1) ADV1 wins; stop
          if (ACS1 == 1) AND (ACS2 != 1) ADV2 wins; stop
          else continue

      2.  if (PREF1 > PREF2) ADV1 wins; stop;
          else if (PREF1 < PREF2) ADV2 wins; stop;
          else continue

      3.  if (PE-ID1 < PE-ID2) ADV1 wins; stop;
       else if (PE-ID1 > PE-ID2) ADV2 wins; stop;
       else ADV1 and ADV2 are from the same VPLS PE

   For BGP DF election, if there is no winner and
          else if (PE-ID1 > PE-ID2) ADV2 wins; stop;
          else ADV1 and ADV2 are from the same PE, BGP DF election should simply consider this as an
   update.

   For VPLS DF election, if PE

   If there is no winner and ADV1 and ADV2 are from the same PE, a VPLS
   PE MUST retain both ADV1 and ADV2.

3.4. VPLS DF Election on PEs

   DF election algorithm MUST be run by all multi-homed VPLS PEs.  In
   addition, all other PEs SHOULD also run the DF election algorithm.
   As a result of the DF election, multi-homed PEs that lose the DF
   election for a MH-ID MUST put the ACs associated with the MH-ID in
   non-forwarding state.  DF election result on the egress PEs can be
   used in traffic forwarding decision.

   Figure 2 shows two customer sites, CE1 and CE4, connected to PE1
   with CE1 multi-homed to PE1 and PE2.  If PE1 is the designated
   forwarder for CE1, based on the DF election result, PE3 can chose to
   not send unknown unicast and multicast traffic to PE2 as PE2 is not
   the designated forwarder for any customer site and it has no other
   single homed sites connected to it.

3.5. Pseudowire binding and traffic forwarding rules

3.5.1. Site-ID Binding Properties

   For the use case where a single PE provides connectivity to a set of
   CEs from which some on multi-homed and others are not, only single
   pseudowire MAY be established.  For example, if PE1 provides VPLS
   service to CE1 and CE4 which are both part of the same VPLS domain,
   but different sites, and CE1 is multi-homed, but CE4 is not (as
   described in figure 2), PE3 would establish only single pseudowire
   toward PE1.  A design needs to ensure that regardless of PE1's
   forwarding state in respect to DF or non-DF for multi-homed CE1,
   PE3s access to CE4 is established.  Since label allocation and
   pseudowire established is tied to site-ID, we need to ensure that
   proper pseudowire bindings are established.

   For set of given advertisements with the common DOM but with
   different Site-ID values, a VPLS PE speaker SHOULD instantiate and
   bind the pseudowire based on advertisement with the lowest Site-ID
   value.  Otherwise, binding would be completely random and during DF
   changes for multi-homed site, non-multi-homed CE might suffer
   traffic loss.

3.5.2. Standby Pseudowire Properties

   As the notion of the convergence is addressed for transport plane by
   use of RSVP FRR and LDP LFA, it is evident that similar solution is
   required at the service level plane as well.  This is most evident
   in large-scale deployments as it takes quite long time to converge.
   Ingress PE usually has to handle multiple relatively larger tasks
   and re-signal all pseudowires affected by egress PE or AC failure.
   Therefore, an implementation MAY choose to optimize the convergence
   by pre-signaling the second, standby, pseudowire toward non-DF end
   point for every active VPLS in 1:1 fashion.  This greatly improves
   the convergence times.  However, details of such implementation are
   still under research.

4. Multi-AS VPLS

   This section describes multi-homing in an inter-AS context.

4.1. Route Origin Extended Community

   Due to lack of information about the PEs that originate the VPLS
   NLRIs in inter-AS operations, Route Origin Extended Community
   [RFC4360] is used to carry the source PE's IP address.

   To use Route Origin Extended Community for carrying the originator
   VPLS PE's loopback address, the type field of the community MUST be
   set to 0x01 and the Global Administrator sub-field MUST be set to
   the PE's loopback IP address.

4.2.  VPLS Preference

   When multiple PEs are assigned the same site ID for multi-homing, it
   is often desired to be able to control the selection of a particular
   PE as the designated forwarder.  Section 3.5 in [RFC4761] describes
   the use of BGP Local Preference in path selection to choose a
   particular NLRI, where Local Preference indicates the degree of
   preference for a particular VE.  The use of Local Preference is
   inadequate when VPLS PEs are spread across multiple ASes as Local
   Preference is not carried across AS boundary.  A new field, VPLS
   preference (VP), is introduced in this document that can be used to
   accomplish this.  VPLS preference indicates a degree of preference
   for a particular customer site.  VPLS preference is not mandatory
   for intra-AS operation; the algorithm explained in Section 3.3 will
   work with or without the presence of VPLS preference.

   Section 3.2.4 in [RFC4761] describes the Layer2 Info Extended
   Community that carries control information about the pseudowires.
   The last two octets that were reserved now carries VPLS preference
   as shown in Figure 3.

                  +------------------------------------+
                  | Extended community type (2 octets) |
                  +------------------------------------+
                  |  Encaps Type (1 octet)             |
                  +------------------------------------+
                  |  Control Flags (1 octet)           |
                  +------------------------------------+
                  |  Layer-2 MTU (2 octet)             |
                  +------------------------------------+
                  |  VPLS Preference (2 octets)        |
                  +------------------------------------+

                 Figure 3: Layer2 Info Extended Community

   A VPLS preference is a 2-octets unsigned integer.  A value of zero
   indicates absence of a VP and is not a valid preference value.  This
   interpretation is required for backwards compatibility.
   Implementations using Layer2 Info Extended Community as described in
   (Section 3.2.4) [RFC4761] MUST set the last two octets as zero since
   it was a reserved field.

   For backwards compatibility, if VPLS preference is used, then BGP
   Local Preference MUST be set to the value of VPLS preference.  Note
   that a Local Preference value of zero for a MH-ID is not valid unless nless
   'D' bit in the control flags is set (see
   [I-D.kothari-l2vpn-auto-site-id]). [I-D.kothari-l2vpn-auto-
   site-id]).  In addition, Local Preference value greater than or
   equal to 2^16 for VPLS advertisements is not valid.

4.3. Use of BGP-MH attributes in Inter-AS Methods

   Section 3.4 in [RFC4761] and section 4 in [RFC6074] describe three
   methods (a, b and c) to connect sites in a VPLS to PEs that are
   across multiple AS.  Since VPLS advertisements in method (a) do not
   cross AS boundaries, multi-homing operations for method (a) remain
   exactly the same as they are within as AS.  However, for method (b)
   and (c), VPLS advertisements do cross AS boundary.  This section
   describes the VPLS operations for method (b) and method (c).
   Consider Figure 4 for inter-AS VPLS with multi-homed customer sites.

4.3.1.   Inter-AS Method (b): EBGP Redistribution of VPLS Information
         between ASBRs

                            AS1                    AS2
                           ........               ........
            CE2 _______   .        .             .        .
                    ___ PE1         .           .          PE3 --- CE3
                   /    :            .         .            :
                __/    :             :         :             :
            CE1 __     :           ASBR1 --- ASBR2           :
                  \     :            :         :            :
                   \___ PE2         .           .          PE4 ---- CE4
                          .        .             .         .
                           ........                ........

                        Figure 4: Inter-AS VPLS

   A customer has four sites, CE1, CE2, CE3 and CE4.  CE1 is multi-homed multi-
   homed to PE1 and PE2 in AS1.  CE2 is single-homed to PE1.  CE3 and
   CE4 are also single homed to PE3 and PE4 respectively in AS2.
   Assume that in addition to the base LDP/BGP VPLS addressing (VSI-IDs/VE-IDs), (VSI-
   IDs/VE-IDs), MH ID 1 is assigned for CE1.  After running DF election
   algorithm, all four VPLS PEs must elect the same designated
   forwarder for CE1 site. Since BGP Local Preference is not carried
   across AS boundary, VPLS preference as described in Section 4.2 MUST
   be used for carrying site preference in inter-AS VPLS operations.

   For Inter-AS method (b) ASBR1 will send a VPLS NLRI received from
   PE1 to ASBR2 with itself as the BGP nexthop.  ASBR2 will send the
   received NLRI from ASBR1 to PE3 and PE4 with itself as the BGP
   nexthop.  Since VPLS PEs use BGP Local Preference in DF election,
   for backwards compatibility, ASBR2 MUST set the Local Preference
   value in the VPLS advertisements it sends to PE3 and PE4 to the VPLS
   preference value contained in the VPLS advertisement it receives
   from ASBR1.  ASBR1 MUST do the same for the NLRIs it sends to PE1
   and PE2.  If ASBR1 receives a VPLS advertisement without a valid
   VPLS preference from a PE within its AS, then ASBR1 MUST set the
   VPLS preference in the advertisements to the Local Preference value
   before sending it to ASBR2.  Similarly, ASBR2 must do the same for
   advertisements without VPLS Preference it receives from PEs within
   its AS.  Thus, in method (b), ASBRs MUST update the VPLS and Local
   Preference based on the advertisements they receive either from an
   ASBR or a PE within their AS.

   In Figure 4, PE1 will send the VPLS advertisements with Route Origin
   Extended Community containing its loopback address.  PE2 will do the
   same.  Even though PE3 receives the VPLS advertisements for VE-ID 1
   and 2 from the same BGP nexthop, ASBR2, the source PE address
   contained in the Route Origin Extended Community is different for
   the CE1 and CE2 advertisements, and thus, PE3 creates two PWs, one
   for CE1 (for VE-ID 1) and another one for CE2 (for VE-ID 2).

4.3.2.   Inter-AS Method (c): Multi-Hop EBGP Redistribution of VPLS
         Information between ASes

   In this method, there is a multi-hop E-BGP peering between the PEs
   or Route Reflectors in AS1 and the PEs or Route Reflectors in AS2.
   There is no VPLS state in either control or data plane on the ASBRs.
   The multi-homing operations on the PEs in this method are exactly
   the same as they are in intra-AS scenario.  However, since Local
   Preference is not carried across AS boundary, the translation of LP
   to VP and vice versa MUST be done by RR, if RR is used to reflect
   VPLS advertisements to other ASes.  This is exactly the same as what
   a
   an ASBR does in case of method (b).  A RR must set the VP to the LP
   value in an advertisement before sending it to other ASes and must
   set the LP to the VP value in an advertisement that it receives from
   other ASes before sending to the PEs within the AS.

5. MAC Flush Operations

   In a service provider VPLS network, customer MAC learning is
   confined to PE devices and any intermediate nodes, such as a Route
   Reflector, do not have any state for MAC addresses.

   Topology changes either in the service provider's network or in
   customer's network can result in the movement of MAC addresses from
   one PE device to another.  Such events can result into traffic being
   dropped due to stale state of MAC addresses on the PE devices.  Age
   out timers that clear the stale state will resume the traffic
   forwarding, but age out timers are typically in minutes, and
   convergence of the order of minutes can severely impact customer's
   service.  To handle such events and expedite convergence of traffic,
   flushing of affected MAC addresses is highly desirable.

   This section describes the scenarios where VPLS flush is desirable
   and the specific VPLS Flush TLVs that provide capability to flush
   the affected MAC addresses on the PE devices.  All operations
   described in this section are in context of a particular VPLS domain
   and not across multiple VPLS domains.  Mechanisms for MAC flush are
   described in [I-D.kothari-l2vpn-vpls-flush] for BGP based VPLS and
   in [RFC4762] for LDP based VPLS.

5.1. MAC List FLush

   If multiple customer sites are connected to the same PE, PE1 as
   shown in Figure 2, and redundancy per site is desired when multi-homing multi-
   homing procedures described in this document are in effect, then it
   is desirable to flush just the relevant MAC addresses from a
   particular site when the site connectivity is lost.

   To flush particular set of MAC addresses, a PE SHOULD originate a
   flush message with MAC list that contains a list of MAC addresses
   that needs to be flushed.  In Figure 2, if connectivity between CE1
   and PE1 goes down and if PE1 was the designated forwarder for CE1,
   PE1 MAY send a list of MAC addresses that belong to CE1 to all its
   BGP peers.

   It is RECOMMENDED that in case of excessive link flap of customer
   attachment circuit in a short duration, a PE should have a means to
   throttle advertisements of flush messages so that excessive flooding
   of such advertisements do not occur.

5.2. Implicit MAC Flush

   Implicit MAC Flush refers to the use of BGP MH advertisements by the
   PEs to flush the MAC addresses learned from the previous designated
   forwarder.

   In case of a failure, when connectivity to a customer site is lost,
   remote PEs learn that a particular site is no longer reachable.  The
   local PE either withdraws the VPLS NLRI that it previously
   advertised for the site or it sends a BGP update message for the
   site's VPLS NLRI with the 'D' bit set.  In such cases, the remote
   PEs can flush all the MACs that were learned from the PE which
   reported the failure.

   However, in cases when a designated forwarder change occurs in
   absence of failures, such as when an attachment circuit comes up,
   the BGP MH advertisement from the PE reporting the change is not
   sufficient for MAC flush procedures.  Consider the case in Figure 2
   where PE1-CE1 link is non-operational and PE2 is the designated
   forwarder for CE1.  Also assume that Local Preference of PE1 is
   higher than PE2.  When PE1-CE1 link becomes operational, PE1 will
   send a BGP MH advertisement to all it's peers.  If PE3 elects PE1 as
   the new designated forwarder for CE1 and as a result flushes all the
   MACs learned from PE1 before PE2 elects itself as the non-designated
   forwarder, there is a chance that PE3 might learn MAC addresses from
   PE2 and as a result may black-hole traffic until those MAC addresses
   are deleted due to age out timers.

   A new flag 'F' is introduced in the Control Flags Bit Vector as a
   deterministic way to indicate when to flush.

                         Control Flags Bit Vector

                             0 1 2 3 4 5 6 7
                             +-+-+-+-+-+-+-+-+
                             |D|A|F|Z|Z|Z|C|S| (Z = MUST Be Zero)
                             +-+-+-+-+-+-+-+-+

                                 Figure 5

   A designatd designated forwarder must set the F bit and a non-designated
   forwarder must clear the F bit when sending BGP MH advertisements.
   A state transition from one to zero for the F bit can be used by a
   remote PE to flush all the MACs learned from the PE that is
   transitioning from designated forwarder to non-designated forwarder.

5.3. Minimizing the effects of fast link transitions

   Certain failure scenarios may result in fast transitions of the link
   towards the multi-homing CE which in turn will generate fast status
   transitions of one or multiple multi-homed sites reflected through
   multiple BGP MH advertisements and LDP MAC Flush messages.

   It is recommended that a timer to damp the link flaps be used for
   the port towards the multi-homed CE to minimize the number of MAC
   Flush events in the remote PEs and the occurrences of BGP state
   compressions for F bit transitions.  A timer value more than the
   time it takes BGP to converge in the network is recommended.

6. Backwards Compatibility

   No forwarding loops are formed when PEs or Route Reflectors that do
   not support procedures defined in this section co exist in the
   network with PEs or Route Reflectors that do support.

6.1. BGP based VPLS

   As explained in this section, multi-homed PEs to the same customer
   site MUST assign the same MH-ID and related NLRI SHOULD contain the
   block offset, block size and label base as zero.  Remote PEs that
   lack support of multi-homing operations specified in this document
   will fail to create any PWs for the multi-homed MH-IDs due to the
   label value of zero and thus, the multi-homing NLRI should have no
   impact on the operation of Remote PEs that lack support of multi-
   homing operations specified in this document.

6.2. LDP VPLS with BGP Auto-discovery

   The BGP-AD NLRI has a prefix length of 12 containing only a 8 bytes
   RD and a 4 bytes VSI-ID.  If a LDP VPLS PEs running BGP AD lacks
   support of multi-homing operations specified in this document, it
   SHOULD ignore a MH NLRI with the length field of 17.  As a result it
   will not ask LDP to create any PWs for the multi-homed Site-ID and
   thus, the multi-homing NLRI should have no impact on LDP VPLS
   operation.  MH PEs may use existing LDP MAC Flush to flush the
   remote LDP VPLS PEs or may use the implicit MAC Flush procedure.

7. Security Considerations

   No new security issues are introduced beyond those that are
   described in [RFC4761] and [RFC4762].

8. IANA Considerations

   At this time, this memo includes no request to IANA.

9. Acknowledgments

   The authors would like to thank Ian Cowburn, Yakov Rekhter, Nischal
   Sheth, and Mitali Singh for their insightful comments and probing
   questions.

   This document was prepared using 2-Word-v2.0.template.dot.

10. References

10.1. Normative References

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

   [RFC4761]  Kompella, K. and Y. Rekhter, "Virtual Private LAN
              Service (VPLS) Using BGP for Auto-Discovery and
              Signaling", RFC 4761, January 2007.

   [RFC6074]  Rosen, E., "Provisioning, Autodiscovery, and Signaling
              in L2VPNs", RFC 6074, January 2011.

10.2. Informative References

   [I-D.kothari-l2vpn-vpls-flush]
              Kothari, B. and R. Fernando, "VPLS Flush in BGP-based
              Virtual Private LAN Service",
              draft-kothari-l2vpn-vpls-flush-00 (work in progress),
              October 2008.

   [I-D.kothari-l2vpn-auto-site-id]
              Kothari, B., Kompella, K., and T. IV, "Automatic
              Generation of Site IDs for Virtual Private LAN Service",
              draft-kothari-l2vpn-auto-site-id-01 (work in progress),
              October 2008.

   [RFC4360]  Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
              Communities Attribute", RFC 4360, February 2006.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, February 2006.

   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, April 2006.

   [RFC4762]  Lasserre, M. and V. Kompella, "Virtual Private LAN
              Service (VPLS) Using Label Distribution Protocol (LDP)
              Signaling", RFC 4762, January 2007.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

Authors' Addresses

   Bhupesh Kothari
   Cisco Systems
   3750 Cisco Way
   San Jose, CA  95134, US
   Cohere Networks
   Email: bhupesh@cisco.com bhupesh@cohere.net

   Kireeti Kompella
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089 US
   Contrail Systems
   Email: kireeti@juniper.net kireeti.kompella@gmail.com

   Wim Henderickx
   Alcatel-Lucent
   Email: wim.henderickx@alcatel-lucent.be

   Florin Balus
   Alcatel-Lucent
   Email: florin.balus@alcatel-lucent.com

   Senad Palislamovic
   Alcatel-Lucent
   Email: senad.palislamovic@alcatel-lucent.com

   James Uttaro
   AT&T
   200 S. Laurel Avenue
   Middletown, NJ  07748, US
   Email: uttaro@att.com

   Wen Lin
   Juniper Networks
   Email: wlin@juniper.net