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Versions: 00 01 02 03 04 05 06 07 08 RFC 4761

Network Working Group                                   K. Kompella, Ed.
Internet-Draft                                           Y. Rekhter, Ed.
Expires: October 10, 2005                               Juniper Networks
                                                           April 8, 2005


                      Virtual Private LAN Service
                      draft-ietf-l2vpn-vpls-bgp-05

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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   Virtual Private LAN Service (VPLS), also known as Transparent LAN
   Service, and Virtual Private Switched Network service, is a useful
   Service Provider offering.  The service offers a Layer 2 Virtual
   Private Network (VPN); however, in the case of VPLS, the customers in
   the VPN are connected by a multipoint network, in contrast to the
   usual Layer 2 VPNs, which are point-to-point in nature.

   This document describes the functions required to offer VPLS, a



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   mechanism for signaling a VPLS, and rules for forwarding VPLS frames
   across a packet switched network.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1   Scope of this Document . . . . . . . . . . . . . . . . . .  3
     1.2   Conventions used in this document  . . . . . . . . . . . .  4
     1.3   Changes from version 04 to 05  . . . . . . . . . . . . . .  4
     1.4   Changes from version 03 to 04  . . . . . . . . . . . . . .  5
   2.  Functional Model . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1   Terminology  . . . . . . . . . . . . . . . . . . . . . . .  6
     2.2   Assumptions  . . . . . . . . . . . . . . . . . . . . . . .  7
     2.3   Interactions . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Control Plane  . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.1   Autodiscovery  . . . . . . . . . . . . . . . . . . . . . .  9
       3.1.1   Functions  . . . . . . . . . . . . . . . . . . . . . .  9
       3.1.2   Protocol Specification . . . . . . . . . . . . . . . . 10
     3.2   Signaling  . . . . . . . . . . . . . . . . . . . . . . . . 10
       3.2.1   Concepts . . . . . . . . . . . . . . . . . . . . . . . 10
       3.2.2   PW Setup and Teardown  . . . . . . . . . . . . . . . . 11
       3.2.3   Signaling PE Capabilities  . . . . . . . . . . . . . . 12
     3.3   BGP VPLS Operation . . . . . . . . . . . . . . . . . . . . 13
     3.4   Multi-AS VPLS  . . . . . . . . . . . . . . . . . . . . . . 14
       3.4.1   a) VPLS-to-VPLS connections at the AS border
               routers. . . . . . . . . . . . . . . . . . . . . . . . 15
       3.4.2   b) EBGP redistribution of VPLS information between
               ASBRs. . . . . . . . . . . . . . . . . . . . . . . . . 15
       3.4.3   c) Multi-hop EBGP redistribution of VPLS
               information between ASes.  . . . . . . . . . . . . . . 16
       3.4.4   Allocation of VE IDs Across Multiple ASes  . . . . . . 17
     3.5   Multi-homing and Path Selection  . . . . . . . . . . . . . 17
   4.  Data Plane . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     4.1   Encapsulation  . . . . . . . . . . . . . . . . . . . . . . 19
     4.2   Forwarding . . . . . . . . . . . . . . . . . . . . . . . . 19
       4.2.1   MAC address learning . . . . . . . . . . . . . . . . . 19
       4.2.2   Flooding . . . . . . . . . . . . . . . . . . . . . . . 19
       4.2.3   "Split Horizon" Forwarding . . . . . . . . . . . . . . 20
   5.  Deployment Options . . . . . . . . . . . . . . . . . . . . . . 21
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     8.1   Normative References . . . . . . . . . . . . . . . . . . . 24
     8.2   Informative References . . . . . . . . . . . . . . . . . . 24
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 25
   A.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 26
   B.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
       Intellectual Property and Copyright Statements . . . . . . . . 28



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1.  Introduction

   Virtual Private LAN Service (VPLS), also known as Transparent LAN
   Service, and Virtual Private Switched Network service, is a useful
   service offering.  A Virtual Private LAN appears in (almost) all
   respects as a LAN to customers of a Service Provider.  However, in a
   VPLS, the customers are not all connected to a single LAN; the
   customers may be spread across a metro or wide area.  In essence, a
   VPLS glues several individual LANs across a packet-switched network
   to appear and function as a single LAN ([6]).

   This document describes the functions needed to offer VPLS, and goes
   on to describe a mechanism for signaling a VPLS, as well as a
   mechanism for transport of VPLS frames over tunnels across a packet
   switched network.  The signaling mechanism uses BGP as the control
   plane protocol.  This document also briefly discusses deployment
   options, in particular, the notion of decoupling functions across
   devices.

   Alternative approaches include: [11], which allows one to build a
   Layer 2 VPN with Ethernet as the interconnect; and [10]), which
   allows one to set up an Ethernet connection across a packet-switched
   network.  Both of these, however, offer point-to-point Ethernet
   services.  What distinguishes VPLS from the above two is that a VPLS
   offers a multipoint service.  A mechanism for setting up pseudowires
   for VPLS using the Label Distribution Protocol (LDP) is defined in
   [7].

1.1  Scope of this Document

   This document has four major parts: defining a VPLS functional model;
   defining a control plane for setting up VPLS; defining the data plane
   for VPLS (encapsulation and forwarding of data); and defining various
   deployment options.

   The functional model underlying VPLS is laid out in Section 2.  This
   describes the service being offered, the network components that
   interact to provide the service, and at a high level their
   interactions.

   The control plane described in this document uses Multiprotocol BGP
   [3] to establish VPLS service, i.e., for the autodiscovery of VPLS
   members and for the setup and teardown of the pseudowires that
   constitute a given VPLS instance.  Section 3 focuses on this, and
   also describes how a VPLS that spans Autonomous System boundaries is
   set up, as well as how multi-homing is handled.  Using BGP as the
   control plane for VPNs is not new (see [11], [9] and [8]): what is
   described here is based on the mechanisms proposed in [9].



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   The forwarding plane and the actions that a participating PE must
   take is described in Section 4.

   In Section 5, the notion of 'decoupled' operation is defined, and the
   interaction of decoupled and non-decoupled PEs is described.
   Decoupling allows for more flexible deployment of VPLS.

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 ([1]).

1.3  Changes from version 04 to 05

   [NOTE to RFC Editor: this section is to be removed before
   publication.]

   Updated IANA section to reflect agreement with authors of [8] that
   the two docs should use the same AFI for L2VPN information.

   Addressed comments received from Alex Zinin.  No technical changes,
   but a more complete description to cover the issues that Alex raised:

   1.  encoding of BGP NEXT_HOP for the new AFI/SAFI is not described

   2.  VE ID, Block offset, Block size, Label base are not described
       anywhere

   3.  no information on how the receiving PE choose the PW label

   4.  section 3.2.2 talks about PE capabilities all of a sudden and
       introduces a L2 Info Community, whose fields and use are not
       described

   Changes to address these:

   1.  Broke up section 3.2.1 into "Concepts" and "PW Setup".

   2.  Expanded section on "Signaling PE Capabilities".

   3.  Added a new section 3.3 "BGP VPLS Operation".

   4.  Minor tweaking, e.g. to fix section number references.







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1.4  Changes from version 03 to 04

   [NOTE to RFC Editor: this section is to be removed before
   publication.]

   Incorporated IDR review comments from Eric Ji, Chaitanya Kodeboyina,
   and Mike Loomis.  Most changes are clarifications and rewording for
   better readability.  The substantive changes are to remove several
   flags from the control field.










































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2.  Functional Model

   This will be described with reference to the following figure.

                                                       -----
                                                      /  A1 \
        ----                                     ____CE1     |
       /    \          --------       --------  /    |       |
      |  A2 CE2-      /        \     /        PE1     \     /
       \    /   \    /          \___/          | \     -----
        ----     ---PE2                        |  \
                    |                          |   \   -----
                    | Service Provider Network |    \ /     \
                    |                          |     CE5  A5 |
                    |            ___           |   /  \     /
             |----|  \          /   \         PE4_/    -----
             |u-PE|--PE3       /     \       /
             |----|    --------       -------
      ----  /   |    ----
     /    \/    \   /    \               CE = Customer Edge Device
    |  A3 CE3    --CE4 A4 |              PE = Provider Edge Router
     \    /         \    /               u-PE = Layer 2 Aggregation
      ----           ----                A<n> = Customer site n

                        Figure 1: Example of a VPLS


2.1  Terminology

   Terminology similar to that in [9] is used, with the addition of
   "u-PE", a Layer 2 PE device used for Layer 2 aggregation.  The notion
   of u-PE is described further in Section 5.  PE and u-PE devices are
   "VPLS-aware", which means that they know that a VPLS service is being
   offered.  We will call these VPLS edge devices, which could be either
   a PE or an u-PE, a VE.

   In contrast, the CE device (which may be owned and operated by either
   the SP or the customer) is VPLS-unaware; as far as the CE is
   concerned, it is connected to the other CEs in the VPLS via a Layer 2
   switched network.  This means that there should be no changes to a CE
   device, either to the hardware or the software, in order to offer
   VPLS.

   A CE device may be connected to a PE or a u-PE via Layer 2 switches
   that are VPLS-unaware.  From a VPLS point of view, such Layer 2
   switches are invisible, and hence will not be discussed further.
   Furthermore, a u-PE may be connected to a PE via Layer 2 and Layer 3
   devices; this will be discussed further in a later section.



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   The term "demultiplexor" refers to an identifier in a data packet
   that identifies both the VPLS to which the packet belongs as well as
   the ingress PE.  In this document, the demultiplexor is an MPLS
   label.

   The term "VPLS" will refer to the service as well as a particular
   instantiation of the service (i.e., an emulated LAN); it should be
   clear from the context which usage is intended.

2.2  Assumptions

   The Service Provider Network is a packet switched network.  The PEs
   are assumed to be (logically) full-meshed with tunnels over which
   packets that belong to a service (such as VPLS) are encapsulated and
   forwarded.  These tunnels can be IP tunnels, such as GRE, or MPLS
   tunnels, established by RSVP-TE or LDP.  These tunnels are
   established independently of the services offered over them; the
   signaling and establishment of these tunnels are not discussed in
   this document.

   "Flooding" and MAC address "learning" (see Section 4) are an integral
   part of VPLS.  However, these activities are private to an SP device,
   i.e., in the VPLS described below, no SP device requests another SP
   device to flood packets or learn MAC addresses on its behalf.

   All the PEs participating in a VPLS are assumed to be fully meshed,
   i.e., every (ingress) PE can send a VPLS packet to the egress PE(s)
   directly, without the need for an intermediate PE (see
   Section 4.2.3.)

2.3  Interactions

   VPLS is a "LAN Service" in that CE devices that belong to VPLS V can
   interact through the SP network as if they were connected by a LAN.
   VPLS is "private" in that CE devices that belong to different VPLSs
   cannot interact.  VPLS is "virtual" in that multiple VPLSs can be
   offered over a common packet switched network.

   PE devices interact to "discover" all the other PEs participating in
   the same VPLS, and to exchange demultiplexors.  These interactions
   are control-driven, not data-driven.

   u-PEs interact with PEs to establish connections with remote PEs or
   u-PEs in the same VPLS.  This interaction is control-driven.

   PE devices can participate simultaneously in both VPLS and IP VPNs
   ([9]).  These are independent services, and the information exchanged
   for each type of service is kept separate as the Network Layer



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   Reachability Information (NLRI) used for this exchange have different
   Address Family Identifiers (AFI) and Subsequent Address Family
   Identifiers (SAFI).  Consequently, an implementation MUST maintain a
   separate routing storage for each service.  However, multiple
   services can use the same underlying tunnels; the VPLS or VPN label
   is used to demultiplex the packets belonging to different services.













































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3.  Control Plane

   There are two primary functions of the VPLS control plane:
   autodiscovery, and setup and teardown of the pseudowires that
   constitute the VPLS, often called signaling.  Section 3.1 and
   Section 3.2 describe these functions.  Both of these functions are
   accomplished with a single BGP Update advertisement; Section 3.3
   describes how this is done by detailing BGP protocol operation for
   VPLS.  Section 3.4 describes the setting up of pseudowires that span
   Autonomous Systems.  Section 3.5 describes how multi-homing is
   handled.

3.1  Autodiscovery

   Discovery refers to the process of finding all the PEs that
   participate in a given VPLS.  A PE can either be configured with the
   identities of all the other PEs in a given VPLS, or the PE can use
   some protocol to discover the other PEs.  The latter is called
   autodiscovery.

   The former approach is fairly configuration-intensive, especially
   since it is required that the PEs participating in a given VPLS are
   fully meshed (i.e., that every PE in a given VPLS establish
   pseudowires to every other PE in that VPLS).  Furthermore, when the
   topology of a VPLS changes (i.e., a PE is added to, or removed from
   the VPLS), the VPLS configuration on all PEs in that VPLS must be
   changed.

   In the autodiscovery approach, each PE "discovers" which other PEs
   are part of a given VPLS by means of some protocol, in this case BGP.
   This allows each PE's configuration to consist only of the identity
   of the VPLS instance established on this PE, not the identity of
   every other PE in that VPLS instance -- that is auto-discovered.
   Moreover, when the topology of a VPLS changes, only the affected PE's
   configuration changes; other PEs automatically find out about the
   change and adapt.

3.1.1  Functions

   A PE that participates in a given VPLS V must be able to tell all
   other PEs in VPLS V that it is also a member of V. A PE must also
   have a means of declaring that it no longer participates in a VPLS.
   To do both of these, the PE must have a means of identifying a VPLS
   and a means by which to communicate to all other PEs.

   U-PE devices also need to know what constitutes a given VPLS;
   however, they don't need the same level of detail.  The PE (or PEs)
   to which a u-PE is connected gives the u-PE an abstraction of the



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   VPLS; this is described in section 5.

3.1.2  Protocol Specification

   The specific mechanism for autodiscovery described here is based on
   [11] and [9]; it uses BGP extended communities [4] to identify
   members of a VPLS.  A more generic autodiscovery mechanism is
   described in [8].  The specific extended community used is the Route
   Target, whose format is described in [4].  The semantics of the use
   of Route Targets is described in [9]; their use in VPLS is identical.

   As it has been assumed that VPLSs are fully meshed, a single Route
   Target RT suffices for a given VPLS V, and in effect that RT is the
   identifier for VPLS V.

   A PE announces (typically via I-BGP) that it belongs to VPLS V by
   annotating its NLRIs for V (see next subsection) with Route Target
   RT, and acts on this by accepting NLRIs from other PEs that have
   Route Target RT.  A PE announces that it no longer participates in V
   by withdrawing all NLRIs that it had advertised with Route Target RT.

3.2  Signaling

   Once discovery is done, each pair of PEs in a VPLS must be able to
   establish (and tear down) pseudowires to each other, i.e., exchange
   (and withdraw) demultiplexors.  This process is known as signaling.
   Signaling is also used to transmit certain characteristics of the
   pseudowires that a PE sets up for a given VPLS.

   Recall that a demultiplexor is used to distinguish among several
   different streams of traffic carried over a tunnel, each stream
   possibly representing a different service.  In the case of VPLS, the
   demultiplexor not only says to which specific VPLS a packet belongs,
   but also identifies the ingress PE.  The former information is used
   for forwarding the packet; the latter information is used for
   learning MAC addresses.  The demultiplexor described here is an MPLS
   label.  However, note that the PE-to-PE tunnels need not be MPLS
   tunnels.

3.2.1  Concepts

   The VPLS BGP NLRI described below, with a new AFI and SAFI (see [3])
   is used to exchange VPLS membership and demultiplexors.

   A VPLS BGP NLRI has the following information elements: a VE ID, a VE
   Block Offset, a VE Block Size and a label base.  The exact format is
   given below.




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   A PE participating in a VPLS must have at least one VE ID.  If the PE
   is the VE, it typically has one VE ID.  If the PE is connected to
   several u-PEs, it has a distinct VE ID for each u-PE.  It may
   additionally have a VE ID for itself, if it itself acts as a VE for
   that VPLS.  In what follows, we will call the PE announcing the VPLS
   NLRI PE-a, and we will assume that PE-a owns VE ID V (either
   belonging to PE-a itself, or to a u-PE connected to PE-a).

   VE IDs are typically assigned by the network administrator.  Their
   scope is local to a VPLS.  A given VE ID should belong to only one
   PE, unless a CE is multi-homed (see Section 3.5).

   A label block is a set of demultiplexor labels used to reach a given
   VE ID.  A VPLS BGP NLRI with VE ID V, VE Block Offset VBO, VE Block
   Size VBS and label base LB implicitly announces

       label block for V: labels from LB to (LB + VBS - 1), and

       remote VE set for V: from VBO to (VBO + VBS - 1).

   There is a one-to-one correspondance between the remote VE set and
   the label block: VE ID (VBO + n) corresponds to label (LB + n).

3.2.2  PW Setup and Teardown

   Suppose PE-a is part of VPLS foo, and makes an announcement with VE
   ID V, VE Block Offset VBO, VE Block Size VBS and label base LB.  If
   PE-b is also part of VPLS foo, and has VE ID W, PE-b does the
   following:

   1.  is W part of PE-a's 'remote VE set': if VBO <= W < VBO + VBS,
       then W is part of PE-a's remote VE set.  If not, PE-b ignores
       this message, and skips the rest of this procedure.

   2.  set up a PW to PE-a: the demultiplexor label to send traffic from
       PE-b to PE-a is computed as (LB + W - VBO).

   3.  is V part of any 'remote VE set' that PE-b announced: PE-b checks
       if V belongs to some remote VE set that PE-b announced, say with
       VE Block Offset VBO', VE Block Size VBS' and label base LB'.  If
       not, PE-b MUST make a new announcement as described in
       Section 3.3.

   4.  set up a PW from PE-a: the demultiplexor label over which PE-b
       should expect traffic from PE-a is computed as: (LB' + V - VBO').

   If Y withdraws an NLRI for V that X was using, then X MUST tear down
   its ends of the pseudowire between X and Y.



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   The format of the VPLS NLRI is given below.  The AFI is the L2VPN AFI
   (to be assigned by IANA), and the SAFI is the VPLS SAFI (65).

      +------------------------------------+
      |  Length (2 octets)                 |
      +------------------------------------+
      |  Route Distinguisher  (8 octets)   |
      +------------------------------------+
      |  VE ID (2 octets)                  |
      +------------------------------------+
      |  VE Block Offset (2 octets)        |
      +------------------------------------+
      |  VE Block Size (2 octets)          |
      +------------------------------------+
      |  Label Base (3 octets)             |
      +------------------------------------+

                  Figure 2: BGP NLRI for VPLS Information


3.2.3  Signaling PE Capabilities

   The following extended attribute, the "Layer2 Info Extended
   Community", is used to signal control information about the
   pseudowires to be setup for a given VPLS.  This information includes
   the Encaps Type (type of encapsulation on the pseudowires), Control
   Flags (control information regarding the pseudowires) and the Maximum
   Transmission Unit (MTU) to be used on the pseudowires.

   The Encaps Type for VPLS is 19.

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

                 Figure 3: Layer2 Info Extended Community







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       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
      |   MBZ     |C|S|      (MBZ = MUST Be Zero)
      +-+-+-+-+-+-+-+-+

                    Figure 4: Control Flags Bit Vector

   With reference to Figure 4, the following bits in the Control Flags
   are defined; the remaining bits, designated MBZ, MUST be set to zero
   when sending and MUST be ignored when receiving this community.

        Name   Meaning
           C   If set to 1 (0), Control word MUST (NOT) be present when
               sending VPLS packets to this PE [10].
           S   If set to 1 (0), Sequenced delivery of frames is (not)
               required when sending VPLS packets to this PE.


3.3  BGP VPLS Operation

   To create a new VPLS, say VPLS foo, a network administrator must pick
   a RT for VPLS foo, say RT-foo.  This will be used by all PEs that
   serve VPLS foo.  To configure a given PE, say PE-a, to be part of
   VPLS foo, the network administrator only has to choose a VE ID V for
   PE-a.  (If PE-a is connected to u-PEs, PE-a may be configured with
   more than one VE ID; in that case, the following is done for each VE
   ID).  The PE may also be configured with a Route Distinguisher (RD);
   if not, it generates a unique RD for VPLS foo.  Say the RD is
   RD-foo-a.  PE-a then generates an initial label block and a remote VE
   set for V, defined by VE Block Offset VBO, VE Block Size VBS and
   label base LB.  These may be empty.

   PE-a then creates a VPLS BGP NLRI with RD RD-foo-a, VE ID V, VE Block
   Offset VBO, VE Block Size VBS and label base LB.  To this, it
   attaches a Layer2 Info Extended Community and a RT, RT-foo.  It sets
   the BGP Next Hop for this NLRI as itself, and announces this NLRI to
   its peers.  The Network Layer protocol associated with the Network
   Address of the Next Hop for the combination <AFI=L2VPN AFI, SAFI=VPLS
   SAFI> is IP; this association is required by [3], Section 5.  If the
   value of the Length of the Next Hop field is 4, then the Next Hop
   contains an IPv4 address.  If this value is 16, then the Next Hop
   contains an IPv6 address.

   If PE-a hears from another PE, say PE-b, a VPLS BGP announcement with
   RT-foo and VE ID W, then PE-a knows that PE-b is a member of the same
   VPLS (auto-discovery).  PE-a then has to set up its part of a VPLS
   pseudowire between PE-a and PE-b, using the mechanisms in
   Section 3.2.  Similarly, PE-b will have discovered that PE-a is in



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   the same VPLS, and PE-b must set up its part of the VPLS pseudowire.
   Thus, signaling and pseudowire setup is also achieved with the same
   Update message.

   If W is not in any remote VE set that PE-a announced for VE ID V in
   VPLS foo, PE-b will not be able to set up its part of the pseudowire
   to PE-a.  To address this, PE-a can choose to withdraw the old
   announcement(s) it made for VPLS foo, and announce a new Update with
   a larger remote VE set and corresponding label block that covers all
   VE IDs that are in VPLS foo.  This however, may cause some service
   disruption.  An alternative for PE-a is to create a new remote VE set
   and corresponding label block, and announce them in a new Update,
   without withdrawing previous announcements.

   If PE-a's configuration is changed to remove VE ID V from VPLS foo,
   then PE-a MUST withdraw all its announcements for VPLS foo that
   contain VE ID V. If all of PE-a's links to its CEs in VPLS foo go
   down, then PE-a SHOULD either withdraw all its NLRIs for VPLS foo, or
   let other PEs in the VPLS foo know in some way that PE-a is no longer
   connected to its CEs.

3.4  Multi-AS VPLS

   As in [11] and [9], the above autodiscovery and signaling functions
   are typically announced via I-BGP.  This assumes that all the sites
   in a VPLS are connected to PEs in a single Autonomous System (AS).

   However, sites in a VPLS may connect to PEs in different ASes.  This
   leads to two issues: 1) there would not be an I-BGP connection
   between those PEs, so some means of signaling across ASes may be
   needed; and 2) there may not be PE-to-PE tunnels between the ASes.

   A similar problem is solved in [9], Section 10.  Three methods are
   suggested to address issue (1); all these methods have analogs in
   multi-AS VPLS.
















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   Here is a diagram for reference:

        __________       ____________       ____________       __________
       /          \     /            \     /            \     /          \
                   \___/        AS 1  \   /  AS 2        \___/
                                       \ /
         +-----+           +-------+    |    +-------+           +-----+
         | PE1 | ---...--- | ASBR1 | ======= | ASBR2 | ---...--- | PE2 |
         +-----+           +-------+    |    +-------+           +-----+
                    ___                / \                ___
                   /   \              /   \              /   \
       \__________/     \____________/     \____________/     \__________/

                          Figure 6: Inter-AS VPLS


3.4.1  a) VPLS-to-VPLS connections at the AS border routers.

   In this method, an AS Border Router (ASBR1) acts as a PE for all
   VPLSs that span AS1 and an AS to which ASBR1 is connected, such as
   AS2 here.  The ASBR on the neighboring AS (ASBR2) is viewed by ASBR1
   as a CE for the VPLSs that span AS1 and AS2; similarly, ASBR2 acts as
   a PE for this VPLS from AS2's point of view, and views ASBR1 as a CE.

   This method does not require MPLS on the ASBR1-ASBR2 link, but does
   require that this link carry Ethernet traffic, and that there be a
   separate VLAN sub-interface for each VPLS traversing this link.  It
   further requires that ASBR1 does the PE operations (discovery,
   signaling, MAC address learning, flooding, encapsulation, etc.) for
   all VPLSs that traverse ASBR1.  This imposes a significant burden on
   ASBR1, both on the control plane and the data plane, which limits the
   number of multi-AS VPLSs.

   Note that in general, there will be multiple connections between a
   pair of ASes, for redundancy.  In this case, the Spanning Tree
   Protocol (STP), or some other means of loop detection and prevention,
   must be run on each VPLS that spans these ASes, so that a loop-free
   topology can be constructed in each VPLS.  This imposes a further
   burden on the ASBRs and PEs participating in those VPLSs, as these
   devices would need to run a loop detection algorithm for each such
   VPLS.  How this may be achieved is outside the scope of this
   document.

3.4.2  b) EBGP redistribution of VPLS information between ASBRs.

   This method requires I-BGP peerings between the PEs in AS1 and ASBR1
   in AS1 (perhaps via route reflectors), an E-BGP peering between ASBR1
   and ASBR2 in AS2, and I-BGP peerings between ASBR2 and the PEs in



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   AS2.  In the above example, PE1 sends a VPLS NLRI to ASBR1 with a
   label block and itself as the BGP nexthop; ASBR1 sends the NLRI to
   ASBR2 with new labels and itself as the BGP nexthop; and ASBR2 sends
   the NLRI to PE2 with new labels and itself as the nexthop.

   The VPLS NLRI that ASBR1 sends to ASBR2 (and the NLRI that ASBR2
   sends to PE2) is identical to the VPLS NLRI that PE1 sends to ASBR1,
   except for the label block.  To be precise, the Length, the Route
   Distinguisher, the VE ID, the VE Block Offset, and the VE Block Size
   MUST be the same; the Label Base may be different.  Furthermore,
   ASBR1 must also update its forwarding path as follows: if the Label
   Base sent by PE1 is L1, the Label-block Size is N, the Label Base
   sent by ASBR1 is L2, and the tunnel label from ASBR1 to PE1 is T,
   then ASBR1 must install the following in the forwarding path:

      swap L2 with L1 and push T,

      swap L2+1 with L1+1 and push T, ...

      swap L2+N-1 with L1+N-1 and push T.

   ASBR2 must act similarly, except that it may not need a tunnel label
   if it is directly connected with ASBR1.

   When PE2 wants to send a VPLS packet to PE1, PE2 uses its VE ID to
   get the right VPLS label from ASBR2's label block for PE1, and uses a
   tunnel label to reach ASBR2.  ASBR2 swaps the VPLS label with the
   label from ASBR1; ASBR1 then swaps the VPLS label with the label from
   PE1, and pushes a tunnel label to reach PE1.

   In this method, one needs MPLS on the ASBR1-ASBR2 interface, but
   there is no requirement that the link layer be Ethernet.
   Furthermore, the ASBRs take part in distributing VPLS information.
   However, the data plane requirements of the ASBRs is much simpler
   than in method (a), being limited to label operations.  Finally, the
   construction of loop-free VPLS topologies is done by routing
   decisions, viz.  BGP path and nexthop selection, so there is no need
   to run the Spanning Tree Protocol on a per-VPLS basis.  Thus, this
   method is considerably more scalable than method (a).

3.4.3  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 preferably, a Route Reflector) in AS1 and the PEs (or Route
   Reflector) in AS2.  PE1 sends a VPLS NLRI with labels and nexthop
   self to PE2; if this is via route reflectors, the BGP nexthop is not
   changed.  This requires that there be a tunnel LSP from PE1 to PE2.



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   This tunnel LSP can be created exactly as in [9], section 10 (c), for
   example using E-BGP to exchange labeled IPv4 routes for the PE
   loopbacks.

   When PE1 wants to send a VPLS packet to PE2, it pushes the VPLS label
   corresponding to its own VE ID onto the packet.  It then pushes the
   tunnel label(s) to reach PE2.

   This method requires no VPLS information (in either the control or
   the data plane) on the ASBRs.  The ASBRs only need to set up PE-to-PE
   tunnel LSPs in the control plane, and do label operations in the data
   plane.  Again, as in the case of method (b), the construction of
   loop-free VPLS topologies is done by routing decisions, i.e., BGP
   path and nexthop selection, so there is no need to run the Spanning
   Tree Protocol on a per-VPLS basis.  This option is likely to be the
   most scalable of the three methods presented here.

3.4.4  Allocation of VE IDs Across Multiple ASes

   In order to ease the allocation of VE IDs for a VPLS that spans
   multiple ASes, one can allocate ranges for each AS.  For example, AS1
   uses VE IDs in the range 1 to 100, AS2 from 101 to 200, etc.  If
   there are 10 sites attached to AS1 and 20 to AS2, the allocated VE
   IDs could be 1-10 and 101 to 120.  This minimizes the number of VPLS
   NLRIs that are exchanged while ensuring that VE IDs are kept unique.

   In the above example, if AS1 needed more than 100 sites, then another
   range can be allocated to AS1.  The only caveat is that there be no
   overlap between VE ID ranges among ASes.  The exception to this rule
   is multi-homing, which is dealt with below.

3.5  Multi-homing and Path Selection

   It is often desired to multi-home a VPLS site, i.e., to connect it to
   multiple PEs, perhaps even in different ASes.  In such a case, the
   PEs connected to the same site can either be configured with the same
   VE ID or with different VE IDs.  In the latter case, it is mandatory
   to run STP on the CE device, and possibly on the PEs, to construct a
   loop-free VPLS topology.  How this can be accomplished is outside the
   scope of this document; however, the rest of this section will
   describe in some detail the former case.

   In the case where the PEs connected to the same site are assigned the
   same VE ID, a loop-free topology is constructed by routing
   mechanisms, in particular, by BGP path selection.  When a BGP speaker
   receives two equivalent NLRIs (see below for the definition), it
   applies standard path selection criteria such as Local Preference and
   AS Path Length to determine which NLRI to choose; it MUST pick only



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   one.  If the chosen NLRI is subsequently withdrawn, the BGP speaker
   applies path selection to the remaining equivalent VPLS NLRIs to pick
   another; if none remain, the forwarding information associated with
   that NLRI is removed.

   Two VPLS NLRIs are considered equivalent from a path selection point
   of view if the Route Distinguisher, the VE ID and the VE Block Offset
   are the same.  If two PEs are assigned the same VE ID in a given
   VPLS, they MUST use the same Route Distinguisher, and they SHOULD
   announce the same VE Block Size for a given VE Offset.









































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4.  Data Plane

   This section discusses two aspects of the data plane for PEs and
   u-PEs implementing VPLS: encapsulation and forwarding.

4.1  Encapsulation

   Ethernet frames received from CE devices are encapsulated for
   transmission over the packet switched network connecting the PEs.
   The encapsulation is as in [10], with one change: a PE that sets the
   P bit in the Control Flags strips the outermost VLAN from an Ethernet
   frame received from a CE before encapsulating it, and pushes a VLAN
   onto a decapsulated frame before sending it to a CE.

4.2  Forwarding

   VPLS packets are classified as belonging to a given service instance
   and associated forwarding table based on the interface over which the
   packet is received.  Packets are forwarded in the context of the
   service instance based on the destination MAC address.  The former
   mapping is determined by configuration.  The latter is the focus of
   this section.

4.2.1  MAC address learning

   As was mentioned earlier, the key distinguishing feature of VPLS is
   that it is a multipoint service.  This means that the entire Service
   Provider network should appear as a single logical learning bridge
   for each VPLS that the SP network supports.  The logical ports for
   the SP "bridge" are the customer ports on all of the VE on a given
   service.  Just as a learning bridge learns MAC addresses on its
   ports, the SP bridge must learn MAC addresses at its VEs.

   Learning consists of associating source MAC addresses of packets with
   the (logical) ports on which they arrive; this association is the
   Forwarding Information Base (FIB).  The FIB is used for forwarding
   packets.  For example, suppose the bridge receives a packet with
   source MAC address S on (logical) port P. If subsequently, the bridge
   receives a packet with destination MAC address S, it knows that it
   should send the packet out on port P.

4.2.2  Flooding

   When a bridge receives a packet to a destination that is not in its
   FIB, it floods the packet on all the other ports.  Similarly, a VE
   will flood packets to an unknown destination to all other VEs in the
   VPLS.




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   In Figure 1 above, if CE2 sent an Ethernet frame to PE2, and the
   destination MAC address on the frame was not in PE2's FIB (for that
   VPLS), then PE2 would be responsible for flooding that frame to every
   other PE in the same VPLS.  On receiving that frame, PE1 would be
   responsible for further flooding the frame to CE1 and CE5 (unless PE1
   knew which CE "owned" that MAC address).

   On the other hand, if PE3 received the frame, it could delegate
   further flooding of the frame to its u-PE.  If PE3 was connected to 2
   u-PEs, it would announce that it has two u-PEs.  PE3 could either
   announce that it is incapable of flooding, in which case it would
   receive two frames, one for each u-PE, or it could announce that it
   is capable of flooding, in which case it would receive one copy of
   the frame, which it would then send to both u-PEs.

4.2.3  "Split Horizon" Forwarding

   When a PE capable of flooding receives a broadcast Ethernet frame, or
   one with an unknown destination MAC address, it must flood the frame.
   If the frame arrived from an attached CE, the PE must send a copy of
   the frame to every other attached CE, as well as to all PEs
   participating in the VPLS.  If the frame arrived from another PE,
   however, the PE must only send a copy of the packet to attached CEs.
   The PE MUST NOT send the frame to other PEs.  This notion has been
   termed "split horizon" forwarding, and is a consequence of the PEs
   being logically full-meshed -- if a broadcast frame is received from
   PEx, then PEx would have sent a copy to all other PEs.

   Split horizon forwarding rules also apply to multicast frames (i.e.,
   those with a multicast destination MAC address).  In this case, when
   a PE receives a multicast frame from another PE, the frame is
   replicated and sent to the relevant subset of attached CEs; however,
   it MUST NOT be sent to other PEs.


















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5.  Deployment Options

   In deploying a network that supports VPLS, the SP must decide what
   functions the VPLS-aware device closest to the customer (the VE)
   supports.  The default case described in this document is that the VE
   is a PE.  However, there are a number of reasons that the VE might be
   a device that does all the Layer 2 functions (such as MAC address
   learning and flooding), and a limited set of Layer 3 functions (such
   as communicating to its PE), but, for example, doesn't do full-
   fledged discovery and PE-to-PE signaling.  Such a device is called a
   "u-PE".

   As both of these cases have benefits, one would like to be able to
   "mix and match" these scenarios.  The signaling mechanism presented
   here allows this.  For example, in a given provider network, one PE
   may be directly connected to CE devices; another may be connected to
   u-PEs that are connected to CEs; and a third may be connected
   directly to a customer over some interfaces and to u-PEs over others.
   All these PEs perform discovery and signaling in the same manner.
   How they do learning and forwarding depends on whether or not there
   is a u-PE; however, this is a local matter, and is not signaled.
   However, the details of the operation of a u-PE and its interactions
   with PEs and other u-PEs is beyond the scope of this document.




























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6.  Security Considerations

   The focus in Virtual Private LAN Service is the privacy of data,
   i.e., that data in a VPLS is only distributed to other nodes in that
   VPLS and not to any external agent or other VPLS.  Note that VPLS
   does not offer security or authentication: VPLS packets are sent in
   the clear in the packet-switched network, and a man-in-the-middle can
   eavesdrop, and may be able to inject packets into the data stream.
   If security is desired, the PE-to-PE tunnels can be IPsec tunnels.
   For more security, the end systems in the VPLS sites can use
   appropriate means of encryption to secure their data even before it
   enters the Service Provider network.

   There are two aspects to achieving data privacy in a VPLS: securing
   the control plane, and protecting the forwarding path.  Compromise of
   the control plane could result in a PE sending data belonging to some
   VPLS to another VPLS, or blackholing VPLS data, or even sending it to
   an eavesdropper, none of which are acceptable from a data privacy
   point of view.  Since all control plane exchanges are via BGP,
   techniques such as in [2] help authenticate BGP messages, making it
   harder to spoof updates (which can be used to divert VPLS traffic to
   the wrong VPLS), or withdraws (denial of service attacks).  In the
   multi-AS options (b) and (c), this also means protecting the inter-AS
   BGP sessions, between the ASBRs, the PEs or the Route Reflectors.
   Note that [2] will not help in keeping VPLS labels private -- knowing
   the labels, one can eavesdrop on VPLS traffic.  However, this
   requires access to the data path within a Service Provider network.

   Protecting the data plane requires ensuring that PE-to-PE tunnels are
   well-behaved (this is outside the scope of this document), and that
   VPLS labels are accepted only from valid interfaces.  For a PE, valid
   interfaces comprise links from P routers.  For an ASBR, a valid
   interface is a link from an ASBR in an AS that is part of a given
   VPLS.  It is especially important in the case of multi-AS VPLSs that
   one accept VPLS packets only from valid interfaces.
















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7.  IANA Considerations

   IANA is asked to allocate an AFI for L2VPN information (suggested
   value: 25).  This should be the same as the AFI requested by [8].















































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8.  References

8.1  Normative References

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

   [2]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
        Signature Option", RFC 2385, August 1998.

   [3]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter, "Multiprotocol
        Extensions for BGP-4", draft-ietf-idr-rfc2858bis-06 (work in
        progress), May 2004.

   [4]  Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
        Communities Attribute", draft-ietf-idr-bgp-ext-communities-08
        (work in progress), February 2005.

   [5]  Martini, L., "Encapsulation Methods for Transport of Ethernet
        Frames Over MPLS Networks", draft-ietf-pwe3-ethernet-encap-09
        (work in progress), February 2005.

8.2  Informative References

   [6]   Andersson, L. and E. Rosen, "Framework for Layer 2 Virtual
         Private Networks (L2VPNs)", draft-ietf-l2vpn-l2-framework-05
         (work in progress), June 2004.

   [7]   Lasserre, M. and V. Kompella, "Virtual Private LAN Services
         over MPLS", draft-ietf-l2vpn-vpls-ldp-06 (work in progress),
         February 2005.

   [8]   Ould-Brahim, H., Rosen, E., and Y. Rekhter, "Using BGP as an
         Auto-Discovery Mechanism for Layer-3 and Layer-2 VPNs",
         draft-ietf-l3vpn-bgpvpn-auto-05 (work in progress),
         February 2005.

   [9]   Rosen, E., "BGP/MPLS IP VPNs", draft-ietf-l3vpn-rfc2547bis-03
         (work in progress), October 2004.

   [10]  Martini, L., "Pseudowire Setup and Maintenance using LDP",
         draft-ietf-pwe3-control-protocol-16 (work in progress),
         March 2005.

   [11]  Kompella, K., "Layer 2 VPNs Over Tunnels",
         draft-kompella-l2vpn-l2vpn-00 (work in progress), January 2004.





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Authors' Addresses

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

   Email: kireeti@juniper.net


   Yakov Rekhter (editor)
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   US

   Email: kireeti@juniper.net

































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Appendix A.  Contributors

   The following contributed to this document:

           Javier Achirica, Telefonica
           Loa Andersson, Acreo
           Chaitanya Kodeboyina, Juniper
           Giles Heron, Alcatel
           Sunil Khandekar, Alcatel
           Vach Kompella, Alcatel
           Marc Lasserre, Riverstone
           Pierre Lin
           Pascal Menezes
           Ashwin Moranganti, Appian
           Hamid Ould-Brahim, Nortel
           Seo Yeong-il, Korea Tel



































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Appendix B.  Acknowledgements

   Thanks to Joe Regan and Alfred Nothaft for their contributions.  Many
   thanks too to Eric Ji, Chaitanya Kodeboyina, and Mike Loomis for
   their detailed reviews.














































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