L2VPN
Network Working Group                                         B. Kothari
Internet Draft
Internet-Draft                                             Cisco Systems
Updates: 4761 (if approved)                                  K. Kompella
Intended status: Standards Track                        Juniper Networks
Expires: January April 28, 2011                                    W. Henderickx
                                                                F. Balus
                                                          Alcatel-Lucent
                                                               J. Uttaro
                                                                    AT&T
                                                          July 12,
                                                        October 25, 2010

         BGP based Multi-homing in Virtual Private LAN Service
                   draft-ietf-l2vpn-vpls-multihoming-01
                draft-ietf-l2vpn-vpls-multihoming-02.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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   document may not be modified outside the IETF Standards Process, and
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Abstract

   Virtual Private LAN Service (VPLS) is a Layer 2 Virtual Private
   Network (VPN) that gives its customers

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the appearance that their
   sites are connected via a Local Area Network (LAN). It is often
   required for copyright in some of this
   material may not have granted the Service Provider (SP) to give IETF Trust the customer redundant
   connectivity right to some sites, often called "multi-homing". This memo
   shows how BGP-based multi-homing can be offered allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the context of LDP IETF Standards Process, and BGP VPLS solutions. derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1. Introduction...................................................3  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  General Terminology.......................................4 Terminology  . . . . . . . . . . . . . . . . . . .  4
     1.2.  Conventions used in this document.........................4  . . . . . . . . . . . . . . . . . . . . . . .  4
   2. Background.....................................................4  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1. Scenarios.................................................4  Scenarios  . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.2.  VPLS Multi-homing Considerations..........................5 Considerations . . . . . . . . . . . . .  7
   3.  Multi-homing Operation.........................................6 Operation . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Provisioning Model........................................6 Model . . . . . . . . . . . . . . . . . . . .  8
     3.2.  Multi-homing NLRI.........................................6 NLRI  . . . . . . . . . . . . . . . . . . . .  8
     3.3.  Designated Forwarder Election.............................7 Election  . . . . . . . . . . . . . .  9
       3.3.1. Attributes...........................................7
         3.3.2. Variables Used.......................................8
            3.3.2.1. RD..............................................8
            3.3.2.2. MH-ID...........................................8
            3.3.2.3. VBO.............................................8
            3.3.2.4. DOM.............................................8
            3.3.2.5. ACS.............................................8
            3.3.2.6. PREF............................................8
            3.3.2.7. PE-ID...........................................9  Attributes . . . . . . . . . . . . . . . . . . . . . .  9
       3.3.2.  Variables Used . . . . . . . . . . . . . . . . . . . .  9
       3.3.3.  Election Procedures..................................9
            3.3.3.1. Bucketization for BGP DF Election..............10
            3.3.3.2. Bucketization for VPLS DF Election.............10
            3.3.3.3. Tie-breaking Rules.............................10 Procedures  . . . . . . . . . . . . . . . . . 11
     3.4.  DF Election on PEs.......................................11 PEs . . . . . . . . . . . . . . . . . . . . 13
   4.  Multi-AS VPLS.................................................12 VPLS  . . . . . . . . . . . . . . . . . . . . . . . . 14
     4.1.  Route Origin Extended Community..........................12 Community  . . . . . . . . . . . . . 14
     4.2.  VPLS Preference..........................................12 Preference  . . . . . . . . . . . . . . . . . . . . . 14
     4.3.  Use of BGP-MH attributes in Inter-AS Methods.............13 Methods . . . . . . . 15
       4.3.1.  Inter-AS Method (b): EBGP Redistribution of VPLS
               Information between ASBRs..................................14 ASBRs  . . . . . . . . . . . . . . 15
       4.3.2.  Inter-AS Method (c): Multi-Hop EBGP Redistribution
               of VPLS Information between ASes..............................15 ASes . . . . . . . . . . . 16
   5.  MAC Flush Operations..........................................15 Operations . . . . . . . . . . . . . . . . . . . . . 18
     5.1.  MAC List Flush...........................................16 FLush . . . . . . . . . . . . . . . . . . . . . . 18
     5.2.  Implicit MAC Flush.......................................16 Flush . . . . . . . . . . . . . . . . . . . . 18
     5.3.  Minimizing the effects of fast link transitions  . . . . . 20
   6.  Backwards Compatibility.......................................16 Compatibility  . . . . . . . . . . . . . . . . . . . 21
     6.1.  BGP based VPLS...........................................17 VPLS . . . . . . . . . . . . . . . . . . . . . . 21
     6.2.  LDP VPLS with BGP Auto-discovery.........................17 Auto-discovery . . . . . . . . . . . . . 21
   7.  Security Considerations.......................................17 Considerations  . . . . . . . . . . . . . . . . . . . 22
   8.  IANA Considerations...........................................17 Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   9. References....................................................17
      9.1.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 24
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     10.1. Normative References.....................................17
      9.2. References . . . . . . . . . . . . . . . . . . . 25
     10.2. Informative References...................................18
   10. Acknowledgments..............................................18 References . . . . . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 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. [I-D.ietf-l2vpn-
   signaling]
   [I-D.ietf-l2vpn-signaling] 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
   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
   multi-homing.  Section 3 defines the components of BGP-based 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 [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

The most basic scenario is shown in Figure 1.

   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 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 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

   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 VE-ID, VE block offset, VE block
   size and label base.  For multi-homing operation, the same NLRI 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 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.

3.3.  Designated Forwarder Election

   BGP-based multi-homing for VPLS relies on BGP DF election and VPLS DF
   election.  The net result of doing both BGP and VPLS DF election 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 of these algorithms, we will use a number of
   variables derived from fields in the VPLS advertisement.  These
   variables are: RD, MH-ID, VBO, DOM, ACS, PREF and PE-ID.  The
   notation 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
   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.

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
   4.2.1). 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 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.

3.3.2.2.  MH-ID

   MH-ID is simply set to the VE-ID field in the NLRI part of ADV.

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   |
   +---------+---------------+----------+------------------------------+
                            Figure 3 PREF table

                                  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 of relevant and comparable
   advertisements.  Distinction MUST NOT be made on whether the NLRI is
   a multi-homing NLRI or not.  In this stage, advertisements may be
   discarded as not being relevant to DF election.  The second stage
   picks a single "winner" from each bucket by repeatedly applying a
   tie-breaking algorithm on a pair of advertisements from that bucket.
   The tie-breaking rules are such that the order in which
   advertisements are picked from the bucket does not affect the final
   result.  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 BGP DF Election

   An advertisement

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

   is discarded if DOM is not of interest to the BGP speaker. Otherwise,
   ADV is put into the bucket for <DOM, RD, <RD, MH-ID, VBO>.  In other words, the
   information in BGP DF election consists of <DOM, RD, <RD, MH-ID, VBO> and only
   advertisements with exact same <DOM, RD, <RD, MH-ID, VBO, DOM> VBO> are candidates for DF
   election.

3.3.3.2.  Bucketization for 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
   are candidates for VPLS DF election.

3.3.3.3.  Tie-breaking Rules

   This section describes the 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 BGP DF election, use of
   unique RDs will result in no candidate advertisements for BGP tie-
   breaking rules and thus, a BGP speaker in such a case will simply not
   do BGP DF election.  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 ADV1 and ADV2 are from
   the same PE, BGP DF election should simply consider this as an
   update.

   For VPLS DF election, 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.  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 loose 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.

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 4 Layer 2 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 VP VPLS preference is used, then BGP
   Local Preference MUST be set to the value of VP. VPLS preference.  Note
   that a Local Preference value of zero for a MH-Site MH-ID is not valid unless
   'D' bit in the control flags is set (see
   [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 [I-D.ietf-l2vpn-signaling]
   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 5 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 5 4: Inter-AS VPLS

   A customer has four sites, CE1, CE2, CE3 and CE4.  CE1 is 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), 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 5, 4, PE1 will send the VPLS advertisements, including the
   ones for MH site CE1, 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 VPLS advertisements received from PE1
   CE1 and PE2, CE2 advertisements, and thus, PE3 can apply correctly the DF Election algorithm as the
   resulting PE-IDs are different. 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 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 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
   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 SHOULD 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

   When

   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 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 forwarder must set the F bit and a BGP update message for the site's VPLS NLRI with non-designated
   forwarder must clear the 'D' F bit set.

   If a remote PE detects that a multi-homed PE has transitioned when sending BGP MH advertisements.  A
   state transition from
   being a DF one to a non-DF, then zero for the F bit can be used by a
   remote PE can choose to flush all
   MAC addresses that it the MACs learned from the multi-homed PE that is
   transitioning from DF designated forwarder to non-DF. Alternatively non-designated forwarder.

5.3.  Minimizing the remote PE effects of fast link transitions

   Certain failure scenarios may chose to react
   when detecting result in fast transitions of the link
   towards the non-DF 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 DF transition damp the link flaps be used for a the
   port towards the multi-homed PE by
   flushing in CE to minimize the related VPLS context all number of MAC Flush
   events in the MACs learned with remote PEs and the
   exception occurrences of BGP state
   compressions for F bit transitions.  A timer value more than the MACs associated with time
   it takes BGP to converge in the new DF PE. 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 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 Yakov Rekhter, Nischal Sheth, and
   Mitali Singh for their insightful comments and probing questions.

10.  References

9.1.

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.

   [RFC4447]  Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
              Heron, "Pseudowire Setup and Maintenance Using the Label
              Distribution Protocol (LDP)", RFC 4447, April 2006.

   [RFC4446]  Martini, L., "IANA Allocations for Pseudowire Edge to Edge
              Emulation (PWE3)", BCP 116, RFC 4446, April 2006.

   [I-D.ietf-l2vpn-signaling]
              Rosen, E., "Provisioning, Autodiscovery, and Signaling in
              L2VPNs", draft-ietf-l2vpn-signaling-08 (work in progress),
              May 2006.

   [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
              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.

   [I-D.ietf-pwe3-redundancy-bit]
              Muley, P., Bocci, M., and L. Martini,
             "Preferential "May 14, 2010 Pseudowire Preferential
              Forwarding Status bit definition", draft-
             ietf-pwe3-redundancy-bit-01 Bit", draft-ietf-pwe3-redundancy-bit-03
              (work in progress), September
             2008.

9.2. May 2010.

10.2.  Informative References

   [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.

10. Acknowledgments

   The authors would like to thank Yakov Rekhter, Nischal Sheth, Mitali
   Singh, Deven Raut and Nehal Bhau for their insightful comments and
   probing questions.

Authors' Addresses

   Bhupesh Kothari
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale,
   Cisco Systems
   3750 Cisco Way
   San Jose, CA  94089  95134
   US

   Email: bhupesh@juniper.net bhupesh@cisco.com

   Kireeti Kompella
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   US

   Email: kireeti@juniper.net

   Wim Henderickx
   Alcatel-Lucent
   Copernicuslaan 50
   2018 Antwerp, Belgium

   Email: wim.henderickx@alcatel-lucent.be

   Florin Balus
   Alcatel-Lucent
   701 E. Middlefield Road
   Mountain View, CA, USA 94043

   Email: florin.balus@alcatel-lucent.com

   James Uttaro
   AT&T
   200 S. Laurel Avenue
   Middletown, NJ  07748
   USA
   US

   Email: uttaro@att.com