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RFC 4762 - Virtual Private LAN Service (VPLS) Using Label Distri

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Network Working Group                                   M. Lasserre, Ed.
Request for Comments: 4762                              V. Kompella, Ed.
Category: Standards Track                                 Alcatel-Lucent
                                                            January 2007

               Virtual Private LAN Service (VPLS) Using
              Label Distribution Protocol (LDP) Signaling

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).


   The L2VPN Working Group produced two separate documents, RFC 4761 and
   this document, that perform similar functions using different
   signaling protocols.  Be aware that each method is commonly referred
   to as "VPLS" even though they are distinct and incompatible with one


   This document describes a Virtual Private LAN Service (VPLS) solution
   using pseudowires, a service previously implemented over other
   tunneling technologies and known as Transparent LAN Services (TLS).
   A VPLS creates an emulated LAN segment for a given set of users;
   i.e., it creates a Layer 2 broadcast domain that is fully capable of
   learning and forwarding on Ethernet MAC addresses and that is closed
   to a given set of users.  Multiple VPLS services can be supported
   from a single Provider Edge (PE) node.

   This document describes the control plane functions of signaling
   pseudowire labels using Label Distribution Protocol (LDP), extending
   RFC 4447.  It is agnostic to discovery protocols.  The data plane
   functions of forwarding are also described, focusing in particular on
   the learning of MAC addresses.  The encapsulation of VPLS packets is
   described by RFC 4448.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................3
      2.1. Conventions ................................................4
   3. Acronyms ........................................................4
   4. Topological Model for VPLS ......................................5
      4.1. Flooding and Forwarding ....................................6
      4.2. Address Learning ...........................................6
      4.3. Tunnel Topology ............................................7
      4.4. Loop free VPLS .............................................7
   5. Discovery .......................................................7
   6. Control Plane ...................................................7
      6.1. LDP-Based Signaling of Demultiplexers ......................8
           6.1.1. Using the Generalized PWid FEC Element ..............8
      6.2. MAC Address Withdrawal .....................................9
           6.2.1. MAC List TLV ........................................9
           6.2.2. Address Withdraw Message Containing MAC List TLV ...11
   7. Data Forwarding on an Ethernet PW ..............................11
      7.1. VPLS Encapsulation Actions ................................11
      7.2. VPLS Learning Actions .....................................12
   8. Data Forwarding on an Ethernet VLAN PW .........................13
      8.1. VPLS Encapsulation Actions ................................13
   9. Operation of a VPLS ............................................14
      9.1. MAC Address Aging .........................................15
   10. A Hierarchical VPLS Model .....................................16
      10.1. Hierarchical Connectivity ................................16
           10.1.1. Spoke Connectivity for Bridging-Capable Devices ...17
           10.1.2. Advantages of Spoke Connectivity ..................18
           10.1.3. Spoke Connectivity for Non-Bridging Devices .......19
      10.2. Redundant Spoke Connections ..............................21
           10.2.1. Dual-Homed MTU-s ..................................21
           10.2.2. Failure Detection and Recovery ....................22
      10.3. Multi-domain VPLS Service ................................23
   11. Hierarchical VPLS Model Using Ethernet Access Network .........23
      11.1. Scalability ..............................................24
      11.2. Dual Homing and Failure Recovery .........................24
   12. Contributors ..................................................25
   13. Acknowledgements ..............................................25
   14. Security Considerations .......................................26
   15. IANA Considerations ...........................................26
   16. References ....................................................27
      16.1. Normative References .....................................27
      16.2. Informative References ...................................27
   Appendix A. VPLS Signaling using the PWid FEC Element .............29

1.  Introduction

   Ethernet has become the predominant technology for Local Area Network
   (LAN) connectivity and is gaining acceptance as an access technology,
   specifically in Metropolitan and Wide Area Networks (MAN and WAN,
   respectively).  The primary motivation behind Virtual Private LAN
   Services (VPLS) is to provide connectivity between geographically
   dispersed customer sites across MANs and WANs, as if they were
   connected using a LAN.  The intended application for the end-user can
   be divided into the following two categories:

   -  Connectivity between customer routers: LAN routing application

   -  Connectivity between customer Ethernet switches: LAN switching

   Broadcast and multicast services are available over traditional LANs.
   Sites that belong to the same broadcast domain and that are connected
   via an MPLS network expect broadcast, multicast, and unicast traffic
   to be forwarded to the proper location(s).  This requires MAC address
   learning/aging on a per-pseudowire basis, and packet replication
   across pseudowires for multicast/broadcast traffic and for flooding
   of unknown unicast destination traffic.

   [RFC4448] defines how to carry Layer 2 (L2) frames over point-to-
   point pseudowires (PW).  This document describes extensions to
   [RFC4447] for transporting Ethernet/802.3 and VLAN [802.1Q] traffic
   across multiple sites that belong to the same L2 broadcast domain or
   VPLS.  Note that the same model can be applied to other 802.1
   technologies.  It describes a simple and scalable way to offer
   Virtual LAN services, including the appropriate flooding of
   broadcast, multicast, and unknown unicast destination traffic over
   MPLS, without the need for address resolution servers or other
   external servers, as discussed in [L2VPN-REQ].

   The following discussion applies to devices that are VPLS capable and
   have a means of tunneling labeled packets amongst each other.  The
   resulting set of interconnected devices forms a private MPLS VPN.

2.  Terminology

   Q-in-Q               802.1ad Provider Bridge extensions also known
                        as stackable VLANs or Q-in-Q.

   Qualified learning   Learning mode in which each customer VLAN is
                        mapped to its own VPLS instance.

   Service delimiter    Information used to identify a specific customer
                        service instance.  This is typically encoded in
                        the encapsulation header of customer frames
                        (e.g., VLAN Id).

   Tagged frame         Frame with an 802.1Q VLAN identifier.

   Unqualified learning Learning mode where all the VLANs of a single
                        customer are mapped to a single VPLS.

   Untagged frame       Frame without an 802.1Q VLAN identifier.

2.1.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Acronyms

   AC            Attachment Circuit

   BPDU          Bridge Protocol Data Unit

   CE            Customer Edge device

   FEC           Forwarding Equivalence Class

   FIB           Forwarding Information Base

   GRE           Generic Routing Encapsulation

   IPsec         IP security

   L2TP          Layer Two Tunneling Protocol

   LAN           Local Area Network

   LDP           Label Distribution Protocol

   MTU-s         Multi-Tenant Unit switch

   PE            Provider Edge device

   PW            Pseudowire

   STP           Spanning Tree Protocol

   VLAN          Virtual LAN

   VLAN tag      VLAN Identifier

4.  Topological Model for VPLS

   An interface participating in a VPLS must be able to flood, forward,
   and filter Ethernet frames.  Figure 1, below, shows the topological
   model of a VPLS.  The set of PE devices interconnected via PWs
   appears as a single emulated LAN to customer X.  Each PE will form
   remote MAC address to PW associations and associate directly attached
   MAC addresses to local customer facing ports.  This is modeled on
   standard IEEE 802.1 MAC address learning.

    +-----+                                              +-----+
    | CE1 +---+      ...........................     +---| CE2 |
    +-----+   |      .                         .     |   +-----+
     Site 1   |   +----+                    +----+   |   Site 2
              +---| PE |       Cloud        | PE |---+
                  +----+                    +----+
                     .                         .
                     .         +----+          .
                     ..........| PE |...........
                               +----+         ^
                                 |            |
                                 |            +-- Emulated LAN
                               | CE3 |
                               Site 3

               Figure 1: Topological Model of a VPLS for
                      Customer X with three sites

   We note here again that while this document shows specific examples
   using MPLS transport tunnels, other tunnels that can be used by PWs
   (as mentioned in [RFC4447]) -- e.g., GRE, L2TP, IPsec -- can also be
   used, as long as the originating PE can be identified, since this is
   used in the MAC learning process.

   The scope of the VPLS lies within the PEs in the service provider
   network, highlighting the fact that apart from customer service
   delineation, the form of access to a customer site is not relevant to
   the VPLS [L2VPN-REQ].  In other words, the attachment circuit (AC)
   connected to the customer could be a physical Ethernet port, a
   logical (tagged) Ethernet port, an ATM PVC carrying Ethernet frames,
   etc., or even an Ethernet PW.

   The PE is typically an edge router capable of running the LDP
   signaling protocol and/or routing protocols to set up PWs.  In
   addition, it is capable of setting up transport tunnels to other PEs
   and delivering traffic over PWs.

4.1.  Flooding and Forwarding

   One of attributes of an Ethernet service is that frames sent to
   broadcast addresses and to unknown destination MAC addresses are
   flooded to all ports.  To achieve flooding within the service
   provider network, all unknown unicast, broadcast and multicast frames
   are flooded over the corresponding PWs to all PE nodes participating
   in the VPLS, as well as to all ACs.

   Note that multicast frames are a special case and do not necessarily
   have to be sent to all VPN members.  For simplicity, the default
   approach of broadcasting multicast frames is used.

   To forward a frame, a PE MUST be able to associate a destination MAC
   address with a PW.  It is unreasonable and perhaps impossible to
   require that PEs statically configure an association of every
   possible destination MAC address with a PW.  Therefore, VPLS-capable
   PEs SHOULD have the capability to dynamically learn MAC addresses on
   both ACs and PWs and to forward and replicate packets across both ACs
   and PWs.

4.2.  Address Learning

   Unlike BGP VPNs [RFC4364], reachability information is not advertised
   and distributed via a control plane.  Reachability is obtained by
   standard learning bridge functions in the data plane.

   When a packet arrives on a PW, if the source MAC address is unknown,
   it needs to be associated with the PW, so that outbound packets to
   that MAC address can be delivered over the associated PW.  Likewise,
   when a packet arrives on an AC, if the source MAC address is unknown,
   it needs to be associated with the AC, so that outbound packets to
   that MAC address can be delivered over the associated AC.

   Standard learning, filtering, and forwarding actions, as defined in
   [802.1D-ORIG], [802.1D-REV], and [802.1Q], are required when a PW or
   AC state changes.

4.3.  Tunnel Topology

   PE routers are assumed to have the capability to establish transport
   tunnels.  Tunnels are set up between PEs to aggregate traffic.  PWs
   are signaled to demultiplex encapsulated Ethernet frames from
   multiple VPLS instances that traverse the transport tunnels.

   In an Ethernet L2VPN, it becomes the responsibility of the service
   provider to create the loop-free topology.  For the sake of
   simplicity, we define that the topology of a VPLS is a full mesh of

4.4.  Loop free VPLS

   If the topology of the VPLS is not restricted to a full mesh, then it
   may be that for two PEs not directly connected via PWs, they would
   have to use an intermediary PE to relay packets.  This topology would
   require the use of some loop-breaking protocol, like a spanning tree

   Instead, a full mesh of PWs is established between PEs.  Since every
   PE is now directly connected to every other PE in the VPLS via a PW,
   there is no longer any need to relay packets, and we can instantiate
   a simpler loop-breaking rule: the "split horizon" rule, whereby a PE
   MUST NOT forward traffic from one PW to another in the same VPLS

   Note that customers are allowed to run a Spanning Tree Protocol (STP)
   (e.g., as defined in [802.1D-REV]), such as when a customer has "back
   door" links used to provide redundancy in the case of a failure
   within the VPLS.  In such a case, STP Bridge PDUs (BPDUs) are simply
   tunneled through the provider cloud.

5.  Discovery

   The capability to manually configure the addresses of the remote PEs
   is REQUIRED.  However, the use of manual configuration is not
   necessary if an auto-discovery procedure is used.  A number of auto-
   discovery procedures are compatible with this document

6.  Control Plane

   This document describes the control plane functions of signaling of
   PW labels.  Some foundational work in the area of support for multi-
   homing is laid.  The extensions to provide multi-homing support
   should work independently of the basic VPLS operation, and they are
   not described here.

6.1.  LDP-Based Signaling of Demultiplexers

   A full mesh of LDP sessions is used to establish the mesh of PWs.
   The requirement for a full mesh of PWs may result in a large number
   of targeted LDP sessions.  Section 10 discusses the option of setting
   up hierarchical topologies in order to minimize the size of the VPLS
   full mesh.

   Once an LDP session has been formed between two PEs, all PWs between
   these two PEs are signaled over this session.

   In [RFC4447], two types of FECs are described: the PWid FEC Element
   (FEC type 128) and the Generalized PWid FEC Element (FEC type 129).
   The original FEC element used for VPLS was compatible with the PWid
   FEC Element.  The text for signaling using the PWid FEC Element has
   been moved to Appendix A.  What we describe below replaces that with
   a more generalized L2VPN descriptor, the Generalized PWid FEC

6.1.1.  Using the Generalized PWid FEC Element

   [RFC4447] describes a generalized FEC structure that is be used for
   VPLS signaling in the following manner.  We describe the assignment
   of the Generalized PWid FEC Element fields in the context of VPLS

   Control bit (C): This bit is used to signal the use of the control
   word as specified in [RFC4447].

   PW type: The allowed PW types are Ethernet (0x0005) and Ethernet
   tagged mode (0x004), as specified in [RFC4446].

   PW info length: As specified in [RFC4447].

   Attachment Group Identifier (AGI), Length, Value: The unique name of
   this VPLS.  The AGI identifies a type of name, and Length denotes the
   length of Value, which is the name of the VPLS.  We use the term AGI
   interchangeably with VPLS identifier.

   Target Attachment Individual Identifier (TAII), Source Attachment
   Individual Identifier (SAII): These are null because the mesh of PWs
   in a VPLS terminates on MAC learning tables, rather than on
   individual attachment circuits.  The use of non-null TAII and SAII is
   reserved for future enhancements.

   Interface Parameters: The relevant interface parameters are:

   -  MTU: The MTU (Maximum Transmission Unit) of the VPLS MUST be the
      same across all the PWs in the mesh.

   -  Optional Description String: Same as [RFC4447].

   -  Requested VLAN ID: If the PW type is Ethernet tagged mode, this
      parameter may be used to signal the insertion of the appropriate
      VLAN ID, as defined in [RFC4448].

6.2.  MAC Address Withdrawal

   It MAY be desirable to remove or unlearn MAC addresses that have been
   dynamically learned for faster convergence.  This is accomplished by
   sending an LDP Address Withdraw Message with the list of MAC
   addresses to be removed to all other PEs over the corresponding LDP

   We introduce an optional MAC List TLV in LDP to specify a list of MAC
   addresses that can be removed or unlearned using the LDP Address
   Withdraw Message.

   The Address Withdraw message with MAC List TLVs MAY be supported in
   order to expedite removal of MAC addresses as the result of a
   topology change (e.g., failure of the primary link for a dual-homed
   VPLS-capable switch).

   In order to minimize the impact on LDP convergence time, when the MAC
   list TLV contains a large number of MAC addresses, it may be
   preferable to send a MAC address withdrawal message with an empty

6.2.1.  MAC List TLV

   MAC addresses to be unlearned can be signaled using an LDP Address
   Withdraw Message that contains a new TLV, the MAC List TLV.  Its
   format is described below.  The encoding of a MAC List TLV address is
   the 6-octet MAC address specified by IEEE 802 documents [802.1D-ORIG]

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |U|F|       Type                |            Length             |
   |                      MAC address #1                           |
   |        MAC address #1         |      MAC Address #2           |
   |                      MAC address #2                           |
   ~                              ...                              ~
   |                      MAC address #n                           |
   |        MAC address #n         |

   U bit: Unknown bit.  This bit MUST be set to 1.  If the MAC address
   format is not understood, then the TLV is not understood and MUST be

   F bit: Forward bit.  This bit MUST be set to 0.  Since the LDP
   mechanism used here is targeted, the TLV MUST NOT be forwarded.

   Type: Type field.  This field MUST be set to 0x0404.  This identifies
   the TLV type as MAC List TLV.

   Length: Length field.  This field specifies the total length in
   octets of the MAC addresses in the TLV.  The length MUST be a
   multiple of 6.

   MAC Address: The MAC address(es) being removed.

   The MAC Address Withdraw Message contains a FEC TLV (to identify the
   VPLS affected), a MAC Address TLV, and optional parameters.  No
   optional parameters have been defined for the MAC Address Withdraw
   signaling.  Note that if a PE receives a MAC Address Withdraw Message
   and does not understand it, it MUST ignore the message.  In this
   case, instead of flushing its MAC address table, it will continue to
   use stale information, unless:

   -  it receives a packet with a known MAC address association, but
      from a different PW, in which case it replaces the old
      association; or

   -  it ages out the old association.

   The MAC Address Withdraw message only helps speed up convergence, so
   PEs that do not understand the message can continue to participate in
   the VPLS.

6.2.2.  Address Withdraw Message Containing MAC List TLV

   The processing for MAC List TLV received in an Address Withdraw
   Message is:

   For each MAC address in the TLV:

   -  Remove the association between the MAC address and the AC or PW
      over which this message is received.

   For a MAC Address Withdraw message with empty list:

   -  Remove all the MAC addresses associated with the VPLS instance
      (specified by the FEC TLV) except the MAC addresses learned over
      the PW associated with this signaling session over which the
      message was received.

   The scope of a MAC List TLV is the VPLS specified in the FEC TLV in
   the MAC Address Withdraw Message.  The number of MAC addresses can be
   deduced from the length field in the TLV.

7.  Data Forwarding on an Ethernet PW

   This section describes the data plane behavior on an Ethernet PW used
   in a VPLS.  While the encapsulation is similar to that described in
   [RFC4448], the functions of stripping the service-delimiting tag and
   using a "normalized" Ethernet frame are described.

7.1.  VPLS Encapsulation Actions

   In a VPLS, a customer Ethernet frame without preamble is encapsulated
   with a header as defined in [RFC4448].  A customer Ethernet frame is
   defined as follows:

   -  If the frame, as it arrives at the PE, has an encapsulation that
      is used by the local PE as a service delimiter, i.e., to identify
      the customer and/or the particular service of that customer, then
      that encapsulation may be stripped before the frame is sent into
      the VPLS.  As the frame exits the VPLS, the frame may have a
      service-delimiting encapsulation inserted.

   -  If the frame, as it arrives at the PE, has an encapsulation that
      is not service delimiting, then it is a customer frame whose
      encapsulation should not be modified by the VPLS.  This covers,
      for example, a frame that carries customer-specific VLAN tags that
      the service provider neither knows about nor wants to modify.

   As an application of these rules, a customer frame may arrive at a
   customer-facing port with a VLAN tag that identifies the customer's
   VPLS instance.  That tag would be stripped before it is encapsulated
   in the VPLS.  At egress, the frame may be tagged again, if a
   service-delimiting tag is used, or it may be untagged if none is

   Likewise, if a customer frame arrives at a customer-facing port over
   an ATM or Frame Relay VC that identifies the customer's VPLS
   instance, then the ATM or FR encapsulation is removed before the
   frame is passed into the VPLS.

   Contrariwise, if a customer frame arrives at a customer-facing port
   with a VLAN tag that identifies a VLAN domain in the customer L2
   network, then the tag is not modified or stripped, as it belongs with
   the rest of the customer frame.

   By following the above rules, the Ethernet frame that traverses a
   VPLS is always a customer Ethernet frame.  Note that the two actions,
   at ingress and egress, of dealing with service delimiters are local
   actions that neither PE has to signal to the other.  They allow, for
   example, a mix-and-match of VLAN tagged and untagged services at
   either end, and they do not carry across a VPLS a VLAN tag that has
   local significance only.  The service delimiter may be an MPLS label
   also, whereby an Ethernet PW given by [RFC4448] can serve as the
   access side connection into a PE.  An RFC1483 Bridged PVC
   encapsulation could also serve as a service delimiter.  By limiting
   the scope of locally significant encapsulations to the edge,
   hierarchical VPLS models can be developed that provide the capability
   to network-engineer scalable VPLS deployments, as described below.

7.2.  VPLS Learning Actions

   Learning is done based on the customer Ethernet frame as defined
   above.  The Forwarding Information Base (FIB) keeps track of the
   mapping of customer Ethernet frame addressing and the appropriate PW
   to use.  We define two modes of learning: qualified and unqualified
   learning.  Qualified learning is the default mode and MUST be
   supported.  Support of unqualified learning is OPTIONAL.

   In unqualified learning, all the VLANs of a single customer are
   handled by a single VPLS, which means they all share a single
   broadcast domain and a single MAC address space.  This means that MAC
   addresses need to be unique and non-overlapping among customer VLANs,
   or else they cannot be differentiated within the VPLS instance, and
   this can result in loss of customer frames.  An application of
   unqualified learning is port-based VPLS service for a given customer
   (e.g., customer with non-multiplexed AC where all the traffic on a
   physical port, which may include multiple customer VLANs, is mapped
   to a single VPLS instance).

   In qualified learning, each customer VLAN is assigned to its own VPLS
   instance, which means each customer VLAN has its own broadcast domain
   and MAC address space.  Therefore, in qualified learning, MAC
   addresses among customer VLANs may overlap with each other, but they
   will be handled correctly since each customer VLAN has its own FIB;
   i.e., each customer VLAN has its own MAC address space.  Since VPLS
   broadcasts multicast frames by default, qualified learning offers the
   advantage of limiting the broadcast scope to a given customer VLAN.
   Qualified learning can result in large FIB table sizes, because the
   logical MAC address is now a VLAN tag + MAC address.

   For STP to work in qualified learning mode, a VPLS PE must be able to
   forward STP BPDUs over the proper VPLS instance.  In a hierarchical
   VPLS case (see details in Section 10), service delimiting tags
   (Q-in-Q or [RFC4448]) can be added such that PEs can unambiguously
   identify all customer traffic, including STP BPDUs.  In a basic VPLS
   case, upstream switches must insert such service delimiting tags.
   When an access port is shared among multiple customers, a reserved
   VLAN per customer domain must be used to carry STP traffic.  The STP
   frames are encapsulated with a unique provider tag per customer (as
   the regular customer traffic), and a PEs looks up the provider tag to
   send such frames across the proper VPLS instance.

8.  Data Forwarding on an Ethernet VLAN PW

   This section describes the data plane behavior on an Ethernet VLAN PW
   in a VPLS.  While the encapsulation is similar to that described in
   [RFC4448], the functions of imposing tags and using a "normalized"
   Ethernet frame are described.  The learning behavior is the same as
   for Ethernet PWs.

8.1.  VPLS Encapsulation Actions

   In a VPLS, a customer Ethernet frame without preamble is encapsulated
   with a header as defined in [RFC4448].  A customer Ethernet frame is
   defined as follows:

   -  If the frame, as it arrives at the PE, has an encapsulation that
      is part of the customer frame and is also used by the local PE as
      a service delimiter, i.e., to identify the customer and/or the
      particular service of that customer, then that encapsulation is
      preserved as the frame is sent into the VPLS, unless the Requested
      VLAN ID optional parameter was signaled.  In that case, the VLAN
      tag is overwritten before the frame is sent out on the PW.

   -  If the frame, as it arrives at the PE, has an encapsulation that
      does not have the required VLAN tag, a null tag is imposed if the
      Requested VLAN ID optional parameter was not signaled.

   As an application of these rules, a customer frame may arrive at a
   customer-facing port with a VLAN tag that identifies the customer's
   VPLS instance and also identifies a customer VLAN.  That tag would be
   preserved as it is encapsulated in the VPLS.

   The Ethernet VLAN PW provides a simple way to preserve customer
   802.1p bits.

   A VPLS MAY have both Ethernet and Ethernet VLAN PWs.  However, if a
   PE is not able to support both PWs simultaneously, it SHOULD send a
   Label Release on the PW messages that it cannot support with a status
   code "Unknown FEC" as given in [RFC3036].

9.  Operation of a VPLS

   We show here, in Figure 2, below, an example of how a VPLS works.
   The following discussion uses the figure below, where a VPLS has been
   set up between PE1, PE2, and PE3.  The VPLS connects a customer with
   4 sites labeled A1, A2, A3, and A4 through CE1, CE2, CE3, and CE4,

   Initially, the VPLS is set up so that PE1, PE2, and PE3 have a full
   mesh of Ethernet PWs.  The VPLS instance is assigned an identifier
   (AGI).  For the above example, say PE1 signals PW label 102 to PE2
   and 103 to PE3, and PE2 signals PW label 201 to PE1 and 203 to PE3.

                                                     /  A1 \
        ----                                    ----CE1    |
       /    \          --------       -------  /     |     |
       | A2 CE2-      /        \     /       PE1     \     /
       \    /   \    /          \---/         \       -----
        ----     ---PE2                        |
                    | Service Provider Network |
                     \          /   \         /
              -----  PE3       /     \       /
              |Agg|_/  --------       -------
             -|   |
      ----  / -----  ----
     /    \/    \   /    \             CE = Customer Edge Router
     | A3 CE3    -CE4 A4 |             PE = Provider Edge Router
     \    /         \    /             Agg = Layer 2 Aggregation
      ----           ----

                      Figure 2: Example of a VPLS

   Assume a packet from A1 is bound for A2.  When it leaves CE1, say it
   has a source MAC address of M1 and a destination MAC of M2.  If PE1
   does not know where M2 is, it will flood the packet; i.e., send it to
   PE2 and PE3.  When PE2 receives the packet, it will have a PW label
   of 201.  PE2 can conclude that the source MAC address M1 is behind
   PE1, since it distributed the label 201 to PE1.  It can therefore
   associate MAC address M1 with PW label 102.

9.1.  MAC Address Aging

   PEs that learn remote MAC addresses SHOULD have an aging mechanism to
   remove unused entries associated with a PW label.  This is important
   both for conservation of memory and for administrative purposes.  For
   example, if a customer site A, is shut down, eventually the other PEs
   should unlearn A's MAC address.

   The aging timer for MAC address M SHOULD be reset when a packet with
   source MAC address M is received.

10.  A Hierarchical VPLS Model

   The solution described above requires a full mesh of tunnel LSPs
   between all the PE routers that participate in the VPLS service.  For
   each VPLS service, n*(n-1)/2 PWs must be set up between the PE
   routers.  While this creates signaling overhead, the real detriment
   to large scale deployment is the packet replication requirements for
   each provisioned PWs on a PE router.  Hierarchical connectivity,
   described in this document, reduces signaling and replication
   overhead to allow large-scale deployment.

   In many cases, service providers place smaller edge devices in
   multi-tenant buildings and aggregate them into a PE in a large
   Central Office (CO) facility.  In some instances, standard IEEE
   802.1q (Dot 1Q) tagging techniques may be used to facilitate mapping
   CE interfaces to VPLS access circuits at a PE.

   It is often beneficial to extend the VPLS service tunneling
   techniques into the access switch domain.  This can be accomplished
   by treating the access device as a PE and provisioning PWs between it
   and every other edge, as a basic VPLS.  An alternative is to utilize
   [RFC4448] PWs or Q-in-Q logical interfaces between the access device
   and selected VPLS enabled PE routers.  Q-in-Q encapsulation is
   another form of L2 tunneling technique, which can be used in
   conjunction with MPLS signaling, as will be described later.  The
   following two sections focus on this alternative approach.  The VPLS
   core PWs (hub) are augmented with access PWs (spoke) to form a two-
   tier hierarchical VPLS (H-VPLS).

   Spoke PWs may be implemented using any L2 tunneling mechanism, and by
   expanding the scope of the first tier to include non-bridging VPLS PE
   routers.  The non-bridging PE router would extend a spoke PW from a
   Layer-2 switch that connects to it, through the service core network,
   to a bridging VPLS PE router supporting hub PWs.  We also describe
   how VPLS-challenged nodes and low-end CEs without MPLS capabilities
   may participate in a hierarchical VPLS.

   For rest of this discussion we refer to a bridging capable access
   device as MTU-s and a non-bridging capable PE as PE-r.  We refer to a
   routing and bridging capable device as PE-rs.

10.1.  Hierarchical Connectivity

   This section describes the hub and spoke connectivity model and
   describes the requirements of the bridging capable and non-bridging
   MTU-s devices for supporting the spoke connections.

10.1.1.  Spoke Connectivity for Bridging-Capable Devices

   In Figure 3, below, three customer sites are connected to an MTU-s
   through CE-1, CE-2, and CE-3.  The MTU-s has a single connection
   (PW-1) to PE1-rs.  The PE-rs devices are connected in a basic VPLS
   full mesh.  For each VPLS service, a single spoke PW is set up
   between the MTU-s and the PE-rs based on [RFC4447].  Unlike
   traditional PWs that terminate on a physical (or a VLAN-tagged
   logical) port, a spoke PW terminates on a virtual switch instance
   (VSI; see [L2FRAME]) on the MTU-s and the PE-rs devices.

                                                        |        |
                                                        |   --   |
                                                        |  /  \  |
    CE-1                                                |  \S /  |
     \                                                  |   --   |
      \                                                 +--------+
       \   MTU-s                          PE1-rs        /   |
        +--------+                      +--------+     /    |
        |        |                      |        |    /     |
        |   --   |      PW-1            |   --   |---/      |
        |  /  \--|- - - - - - - - - - - |  /  \  |          |
        |  \S /  |                      |  \S /  |          |
        |   --   |                      |   --   |---\      |
        +--------+                      +--------+    \     |
         /                                             \    |
       ----                                             +--------+
      |Agg |                                            |        |
       ----                                             |  --    |
      /    \                                            | /  \   |
     CE-2  CE-3                                         | \S /   |
                                                        |  --    |
    Agg = Layer-2 Aggregation
   /  \
   \S / = Virtual Switch Instance

           Figure 3: An example of a hierarchical VPLS model

   The MTU-s and the PE-rs treat each spoke connection like an AC of the
   VPLS service.  The PW label is used to associate the traffic from the
   spoke to a VPLS instance.  MTU-s Operation

   An MTU-s is defined as a device that supports layer-2 switching
   functionality and does all the normal bridging functions of learning
   and replication on all its ports, including the spoke, which is
   treated as a virtual port.  Packets to unknown destinations are
   replicated to all ports in the service including the spoke.  Once the
   MAC address is learned, traffic between CE1 and CE2 will be switched
   locally by the MTU-s, saving the capacity of the spoke to the PE-rs.
   Similarly traffic between CE1 or CE2 and any remote destination is
   switched directly onto the spoke and sent to the PE-rs over the
   point-to-point PW.

   Since the MTU-s is bridging capable, only a single PW is required per
   VPLS instance for any number of access connections in the same VPLS
   service.  This further reduces the signaling overhead between the
   MTU-s and PE-rs.

   If the MTU-s is directly connected to the PE-rs, other encapsulation
   techniques, such as Q-in-Q, can be used for the spoke.  PE-rs Operation

   A PE-rs is a device that supports all the bridging functions for VPLS
   service and supports the routing and MPLS encapsulation; i.e., it
   supports all the functions described for a basic VPLS, as described

   The operation of PE-rs is independent of the type of device at the
   other end of the spoke.  Thus, the spoke from the MTU-s is treated as
   a virtual port, and the PE-rs will switch traffic between the spoke
   PW, hub PWs, and ACs once it has learned the MAC addresses.

10.1.2.  Advantages of Spoke Connectivity

   Spoke connectivity offers several scaling and operational advantages
   for creating large-scale VPLS implementations, while retaining the
   ability to offer all the functionality of the VPLS service.

   -  Eliminates the need for a full mesh of tunnels and full mesh of
      PWs per service between all devices participating in the VPLS

   -  Minimizes signaling overhead, since fewer PWs are required for the
      VPLS service.

   -  Segments VPLS nodal discovery.  MTU-s needs to be aware of only
      the PE-rs node, although it is participating in the VPLS service
      that spans multiple devices.  On the other hand, every VPLS PE-rs
      must be aware of every other VPLS PE-rs and all of its locally
      connected MTU-s and PE-r devices.

   -  Addition of other sites requires configuration of the new MTU-s
      but does not require any provisioning of the existing MTU-s
      devices on that service.

   -  Hierarchical connections can be used to create VPLS service that
      spans multiple service provider domains.  This is explained in a
      later section.

   Note that as more devices participate in the VPLS, there are more
   devices that require the capability for learning and replication.

10.1.3.  Spoke Connectivity for Non-Bridging Devices

   In some cases, a bridging PE-rs may not be deployed, or a PE-r might
   already have been deployed.  In this section, we explain how a PE-r
   that does not support any of the VPLS bridging functionality can
   participate in the VPLS service.

   In Figure 4, three customer sites are connected through CE-1, CE-2,
   and CE-3 to the VPLS through PE-r.  For every attachment circuit that
   participates in the VPLS service, PE-r creates a point-to-point PW
   that terminates on the VSI of PE1-rs.

                                                       |        |
                                                       |   --   |
                                                       |  /  \  |
   CE-1                                                |  \S /  |
    \                                                  |   --   |
     \                                                 +--------+
      \   PE-r                           PE1-rs        /   |
       +--------+                      +--------+     /    |
       |\       |                      |        |    /     |
       | \      |      PW-1            |   --   |---/      |
       |  ------|- - - - - - - - - - - |  /  \  |          |
       |   -----|- - - - - - - - - - - |  \S /  |          |
       |  /     |                      |   --   |---\      |
       +--------+                      +--------+    \     |
        /                                             \    |
      ----                                            +--------+
     | Agg|                                           |        |
      ----                                            |  --    |
     /    \                                           | /  \   |
    CE-2  CE-3                                        | \S /   |
                                                      |  --    |

              Figure 4: An example of a hierarchical VPLS
                       with non-bridging spokes

   The PE-r is defined as a device that supports routing but does not
   support any bridging functions.  However, it is capable of setting up
   PWs between itself and the PE-rs.  For every port that is supported
   in the VPLS service, a PW is set up from the PE-r to the PE-rs.  Once
   the PWs are set up, there is no learning or replication function
   required on the part of the PE-r.  All traffic received on any of the
   ACs is transmitted on the PW.  Similarly, all traffic received on a
   PW is transmitted to the AC where the PW terminates.  Thus, traffic
   from CE1 destined for CE2 is switched at PE1-rs and not at PE-r.

   Note that in the case where PE-r devices use Provider VLANs (P-VLAN)
   as demultiplexers instead of PWs, PE1-rs can treat them as such and
   map these "circuits" into a VPLS domain to provide bridging support
   between them.

   This approach adds more overhead than the bridging-capable (MTU-s)
   spoke approach, since a PW is required for every AC that participates
   in the service versus a single PW required per service (regardless of
   ACs) when an MTU-s is used.  However, this approach offers the
   advantage of offering a VPLS service in conjunction with a routed
   internet service without requiring the addition of new MTU-s.

10.2.  Redundant Spoke Connections

   An obvious weakness of the hub and spoke approach described thus far
   is that the MTU-s has a single connection to the PE-rs.  In case of
   failure of the connection or the PE-rs, the MTU-s suffers total loss
   of connectivity.

   In this section, we describe how the redundant connections can be
   provided to avoid total loss of connectivity from the MTU-s.  The
   mechanism described is identical for both, MTU-s and PE-r devices.

10.2.1.  Dual-Homed MTU-s

   To protect from connection failure of the PW or the failure of the
   PE-rs, the MTU-s or the PE-r is dual-homed into two PE-rs devices.
   The PE-rs devices must be part of the same VPLS service instance.

   In Figure 5, two customer sites are connected through CE-1 and CE-2
   to an MTU-s.  The MTU-s sets up two PWs (one each to PE1-rs and
   PE3-rs) for each VPLS instance.  One of the two PWs is designated as
   primary and is the one that is actively used under normal conditions,
   whereas the second PW is designated as secondary and is held in a
   standby state.  The MTU-s negotiates the PW labels for both the
   primary and secondary PWs, but does not use the secondary PW unless
   the primary PW fails.  How a spoke is designated primary or secondary
   is outside the scope of this document.  For example, a spanning tree
   instance running between only the MTU-s and the two PE-rs nodes is
   one possible method.  Another method could be configuration.

                                                       |        |
                                                       |   --   |
                                                       |  /  \  |
   CE-1                                                |  \S /  |
     \                                                 |   --   |
      \                                                +--------+
       \  MTU-s                          PE1-rs        /   |
       +--------+                      +--------+     /    |
       |        |                      |        |    /     |
       |   --   |   Primary PW         |   --   |---/      |
       |  /  \  |- - - - - - - - - - - |  /  \  |          |
       |  \S /  |                      |  \S /  |          |
       |   --   |                      |   --   |---\      |
       +--------+                      +--------+    \     |
         /      \                                     \    |
        /        \                                     +--------+
       /          \                                    |        |
      CE-2         \                                   |  --    |
                    \     Secondary PW                 | /  \   |
                     - - - - - - - - - - - - - - - - - | \S /   |
                                                       |  --    |
              Figure 5: An example of a dual-homed MTU-s

10.2.2.  Failure Detection and Recovery

   The MTU-s should control the usage of the spokes to the PE-rs
   devices.  If the spokes are PWs, then LDP signaling is used to
   negotiate the PW labels, and the hello messages used for the LDP
   session could be used to detect failure of the primary PW.  The use
   of other mechanisms that could provide faster detection failures is
   outside the scope of this document.

   Upon failure of the primary PW, MTU-s immediately switches to the
   secondary PW.  At this point, the PE3-rs that terminates the
   secondary PW starts learning MAC addresses on the spoke PW.  All
   other PE-rs nodes in the network think that CE-1 and CE-2 are behind
   PE1-rs and may continue to send traffic to PE1-rs until they learn
   that the devices are now behind PE3-rs.  The unlearning process can
   take a long time and may adversely affect the connectivity of
   higher-level protocols from CE1 and CE2.  To enable faster
   convergence, the PE3-rs where the secondary PW got activated may send
   out a flush message (as explained in Section 6.2), using the MAC List

   TLV, as defined in Section 6, to all PE-rs nodes.  Upon receiving the
   message, PE-rs nodes flush the MAC addresses associated with that
   VPLS instance.

10.3.  Multi-domain VPLS Service

   Hierarchy can also be used to create a large-scale VPLS service
   within a single domain or a service that spans multiple domains
   without requiring full mesh connectivity between all VPLS-capable
   devices.  Two fully meshed VPLS networks are connected together using
   a single LSP tunnel between the VPLS "border" devices.  A single
   spoke PW per VPLS service is set up to connect the two domains

   When more than two domains need to be connected, a full mesh of
   inter-domain spokes is created between border PEs.  Forwarding rules
   over this mesh are identical to the rules defined in Section 4.

   This creates a three-tier hierarchical model that consists of a hub-
   and-spoke topology between MTU-s and PE-rs devices, a full-mesh
   topology between PE-rs, and a full mesh of inter-domain spokes
   between border PE-rs devices.

   This document does not specify how redundant border PEs per domain
   per VPLS instance can be supported.

11.  Hierarchical VPLS Model Using Ethernet Access Network

   In this section, the hierarchical model is expanded to include an
   Ethernet access network.  This model retains the hierarchical
   architecture discussed previously in that it leverages the full-mesh
   topology among PE-rs devices; however, no restriction is imposed on
   the topology of the Ethernet access network (e.g., the topology
   between MTU-s and PE-rs devices is not restricted to hub and spoke).

   The motivation for an Ethernet access network is that Ethernet-based
   networks are currently deployed by some service providers to offer
   VPLS services to their customers.  Therefore, it is important to
   provide a mechanism that allows these networks to integrate with an
   IP or MPLS core to provide scalable VPLS services.

   One approach of tunneling a customer's Ethernet traffic via an
   Ethernet access network is to add an additional VLAN tag to the
   customer's data (which may be either tagged or untagged).  The
   additional tag is referred to as Provider's VLAN (P-VLAN).  Inside
   the provider's network each P-VLAN designates a customer or more
   specifically a VPLS instance for that customer.  Therefore, there is
   a one-to-one correspondence between a P-VLAN and a VPLS instance.  In

   this model, the MTU-s needs to have the capability of adding the
   additional P-VLAN tag to non-multiplexed ACs where customer VLANs are
   not used as service delimiters.  This functionality is described in

   If customer VLANs need to be treated as service delimiters (e.g., the
   AC is a multiplexed port), then the MTU-s needs to have the
   additional capability of translating a customer VLAN (C-VLAN) to a
   P-VLAN, or to push an additional P-VLAN tag, in order to resolve
   overlapping VLAN tags used by different customers.  Therefore, the
   MTU-s in this model can be considered a typical bridge with this
   additional capability.  This functionality is described in [802.1ad].

   The PE-rs needs to be able to perform bridging functionality over the
   standard Ethernet ports toward the access network, as well as over
   the PWs toward the network core.  In this model, the PE-rs may need
   to run STP towards the access network, in addition to split-horizon
   over the MPLS core.  The PE-rs needs to map a P-VLAN to a VPLS-
   instance and its associated PWs, and vice versa.

   The details regarding bridge operation for MTU-s and PE-rs (e.g.,
   encapsulation format for Q-in-Q messages, customer's Ethernet control
   protocol handling, etc.) are outside the scope of this document and
   are covered in [802.1ad].  However, the relevant part is the
   interaction between the bridge module and the MPLS/IP PWs in the
   PE-rs, which behaves just as in a regular VPLS.

11.1.  Scalability

   Since each P-VLAN corresponds to a VPLS instance, the total number of
   VPLS instances supported is limited to 4K.  The P-VLAN serves as a
   local service delimiter within the provider's network that is
   stripped as it gets mapped to a PW in a VPLS instance.  Therefore,
   the 4K limit applies only within an Ethernet access network (Ethernet
   island) and not to the entire network.  The SP network consists of a
   core MPLS/IP network that connects many Ethernet islands.  Therefore,
   the number of VPLS instances can scale accordingly with the number of
   Ethernet islands (a metro region can be represented by one or more

11.2.  Dual Homing and Failure Recovery

   In this model, an MTU-s can be dual homed to different devices
   (aggregators and/or PE-rs devices).  The failure protection for
   access network nodes and links can be provided through running STP in
   each island.  The STP of each island is independent of other islands
   and do not interact with others.  If an island has more than one
   PE-rs, then a dedicated full-mesh of PWs is used among these PE-rs

   devices for carrying the SP BPDU packets for that island.  On a
   per-P-VLAN basis, STP will designate a single PE-rs to be used for
   carrying the traffic across the core.  The loop-free protection
   through the core is performed using split-horizon, and the failure
   protection in the core is performed through standard IP/MPLS re-

12.  Contributors

   Loa Andersson, TLA
   Ron Haberman, Alcatel-Lucent
   Juha Heinanen, Independent
   Giles Heron, Tellabs
   Sunil Khandekar, Alcatel-Lucent
   Luca Martini, Cisco
   Pascal Menezes, Independent
   Rob Nath, Alcatel-Lucent
   Eric Puetz, AT&T
   Vasile Radoaca, Independent
   Ali Sajassi, Cisco
   Yetik Serbest, AT&T
   Nick Slabakov, Juniper
   Andrew Smith, Consultant
   Tom Soon, AT&T
   Nick Tingle, Alcatel-Lucent

13.  Acknowledgments

   We wish to thank Joe Regan, Kireeti Kompella, Anoop Ghanwani, Joel
   Halpern, Bill Hong, Rick Wilder, Jim Guichard, Steve Phillips, Norm
   Finn, Matt Squire, Muneyoshi Suzuki, Waldemar Augustyn, Eric Rosen,
   Yakov Rekhter, Sasha Vainshtein, and Du Wenhua for their valuable

   We would also like to thank Rajiv Papneja (ISOCORE), Winston Liu
   (Ixia), and Charlie Hundall for identifying issues with the draft in
   the course of the interoperability tests.

   We would also like to thank Ina Minei, Bob Thomas, Eric Gray and
   Dimitri Papadimitriou for their thorough technical review of the

14.  Security Considerations

   A more comprehensive description of the security issues involved in
   L2VPNs is covered in [RFC4111].  An unguarded VPLS service is
   vulnerable to some security issues that pose risks to the customer
   and provider networks.  Most of the security issues can be avoided
   through implementation of appropriate guards.  A couple of them can
   be prevented through existing protocols.

   -  Data plane aspects

        -  Traffic isolation between VPLS domains is guaranteed by the
           use of per VPLS L2 FIB table and the use of per VPLS PWs.

        -  The customer traffic, which consists of Ethernet frames, is
           carried unchanged over VPLS.  If security is required, the
           customer traffic SHOULD be encrypted and/or authenticated
           before entering the service provider network.

        -  Preventing broadcast storms can be achieved by using routers
           as CPE devices or by rate policing the amount of broadcast
           traffic that customers can send.

   -  Control plane aspects

        -  LDP security (authentication) methods as described in
           [RFC3036] SHOULD be applied.  This would prevent
           unauthenticated messages from disrupting a PE in a VPLS.

   -  Denial of service attacks

        -  Some means to limit the number of MAC addresses (per site per
           VPLS) that a PE can learn SHOULD be implemented.

15.  IANA Considerations

   The type field in the MAC List TLV is defined as 0x404 in Section

16.  References

16.1.  Normative References

   [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

   [RFC4448]      Martini, L., Rosen, E., El-Aawar, N., and G. Heron,
                  "Encapsulation Methods for Transport of Ethernet over
                  MPLS Networks", RFC 4448, April 2006.

   [802.1D-ORIG]  Original 802.1D - ISO/IEC 10038, ANSI/IEEE Std
                  802.1D-1993 "MAC Bridges".

   [802.1D-REV]   802.1D - "Information technology - Telecommunications
                  and information exchange between systems - Local and
                  metropolitan area networks - Common specifications -
                  Part 3: Media Access Control (MAC) Bridges: Revision.
                  This is a revision of ISO/IEC 10038: 1993, 802.1j-1992
                  and 802.6k-1992.  It incorporates P802.11c, P802.1p
                  and P802.12e." ISO/IEC 15802-3: 1998.

   [802.1Q]       802.1Q - ANSI/IEEE Draft Standard P802.1Q/D11, "IEEE
                  Standards for Local and Metropolitan Area Networks:
                  Virtual Bridged Local Area Networks", July 1998.

   [RFC3036]      Andersson, L., Doolan, P., Feldman, N., Fredette, A.,
                  and B. Thomas, "LDP Specification", RFC 3036, January

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

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

16.2.  Informative References

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

   [RADIUS-DISC]  Heinanen, J., Weber, G., Ed., Townsley, W., Booth, S.,
                  and W. Luo, "Using Radius for PE-Based VPN Discovery",
                  Work in Progress, October 2005.

   [BGP-DISC]     Ould-Brahim, H., Ed., Rosen, E., Ed., and Y. Rekhter,
                  Ed., "Using BGP as an Auto-Discovery Mechanism for
                  Network-based VPNs", Work in Progress, September 2006.

   [L2FRAME]      Andersson, L. and E. Rosen, "Framework for Layer 2
                  Virtual Private Networks (L2VPNs)", RFC 4664,
                  September 2006.

   [L2VPN-REQ]    Augustyn, W. and Y. Serbest, "Service Requirements for
                  Layer 2 Provider-Provisioned Virtual Private
                  Networks", RFC 4665, September 2006.

   [RFC4111]      Fang, L., "Security Framework for Provider-Provisioned
                  Virtual Private Networks (PPVPNs)", RFC 4111, July

   [802.1ad]      "IEEE standard for Provider Bridges", Work in
                  Progress, December 2002.

Appendix A.  VPLS Signaling using the PWid FEC Element

   This section is being retained because live deployments use this
   version of the signaling for VPLS.

   The VPLS signaling information is carried in a Label Mapping message
   sent in downstream unsolicited mode, which contains the following
   PWid FEC TLV.

   PW, C, PW Info Length, Group ID, and Interface parameters are as
   defined in [RFC4447].

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  |    PW TLV     |C|         PW Type             |PW info Length |
  |                      Group ID                                 |
  |                        PWID                                   |
  |                       Interface parameters                    |
  ~                                                               ~
  |                                                               |

   We use the Ethernet PW type to identify PWs that carry Ethernet
   traffic for multipoint connectivity.

   In a VPLS, we use a VCID (which, when using the PWid FEC, has been
   substituted with a more general identifier (AGI), to address
   extending the scope of a VPLS) to identify an emulated LAN segment.
   Note that the VCID as specified in [RFC4447] is a service identifier,
   identifying a service emulating a point-to-point virtual circuit.  In
   a VPLS, the VCID is a single service identifier, so it has global
   significance across all PEs involved in the VPLS instance.

Authors' Addresses

   Marc Lasserre
   EMail: mlasserre@alcatel-lucent.com

   Vach Kompella
   EMail: vach.kompella@alcatel-lucent.com

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