Internet Draft Document                            Marc Lasserre
  L2VPN Working Group                                Vach Kompella
  draft-ietf-l2vpn-vpls-ldp-08.txt                       (Editors)
  Expires: May 2006                                  November 2005

                Virtual Private LAN Services over MPLS

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  By submitting this Internet-Draft, each author represents that any
  applicable patent or other IPR claims of which he or she is aware
  have been or will be disclosed, and any of which he or she becomes
  aware will be disclosed, in accordance with Section 6 of BCP 79.


  This document describes a Virtual Private LAN Service (VPLS)
  solution using pseudo-wires, 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 that
  is closed to a given set of users.  Multiple VPLS services can be
  supported from a single PE node.

  This document describes the control plane functions of signaling
  pseudo-wire labels using LDP [RFC3036], extending [PWE3-CTRL].  It
  is agnostic to discovery protocols.  The data plane functions of
  forwarding are also described, focusing, in particular, on the

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  learning of MAC addresses.  The encapsulation of VPLS packets is
  described by [PWE3-ETHERNET].

1. Conventions

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

2. Table of Contents

   1. Conventions.....................................................2
   2. Table of Contents...............................................2
   3. Introduction....................................................3
   3.1. Terminology...................................................3
   3.2. Acronyms......................................................4
   4. Topological Model for VPLS......................................4
   4.1. Flooding and Forwarding.......................................5
   4.2. Address Learning..............................................6
   4.3. Tunnel Topology...............................................6
   4.4. Loop free VPLS................................................6
   5. Discovery.......................................................7
   6. Control Plane...................................................7
   6.1. LDP Based Signaling of Demultiplexers.........................7
   6.1.1. Using the Generalized PWid FEC Element......................7
   6.2. MAC Address Withdrawal........................................8
   6.2.1. MAC List TLV................................................9
   6.2.2. Address Withdraw Message Containing MAC List TLV...........10
   7. Data Forwarding on an Ethernet PW..............................10
   7.1. VPLS Encapsulation actions...................................10
   7.2. VPLS Learning actions........................................11
   8. Data Forwarding on an Ethernet VLAN PW.........................12
   8.1. VPLS Encapsulation actions...................................12
   9. Operation of a VPLS............................................13
   9.1. MAC Address Aging............................................14
   10. A Hierarchical VPLS Model.....................................14
   10.1. Hierarchical connectivity...................................15
   10.1.1. Spoke connectivity for bridging-capable devices...........15
   10.1.2. Advantages of spoke connectivity..........................17
   10.1.3. Spoke connectivity for non-bridging devices...............17
   10.2. Redundant Spoke Connections.................................19
   10.2.1. Dual-homed MTU-s..........................................19
   10.2.2. Failure detection and recovery............................20
   10.3. Multi-domain VPLS service...................................20
   11. Hierarchical VPLS model using Ethernet Access Network.........21
   11.1. Scalability.................................................22
   11.2. Dual Homing and Failure Recovery............................22
   12. Contributors..................................................22

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   13. Acknowledgments...............................................22
   14. Security Considerations.......................................23
   15. IANA Considerations...........................................23
   16. References....................................................24
   16.1. Normative References........................................24
   16.2. Informative References......................................24
   17. Appendix: VPLS Signaling using the PWid FEC Element...........25
   18. Authors' Addresses............................................25

3. 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 pseudo-wire basis,
  packet replication across pseudo-wires for multicast/broadcast
  traffic and for flooding of unknown unicast destination traffic.

  [PWE3-ETHERNET] defines how to carry Layer 2 (L2) frames over
  point-to-point pseudo-wires (PW).  This document describes
  extensions to [PWE3-CTRL] 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

3.1. Terminology

  Q-in-Q                802.1ad Provider Bridge extensions also known

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                        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

3.2. Acronyms

  AC            Attachment Circuit

  BPDU          Bridge Protocol Data Unit

  CE            Customer Edge device

  FEC           Forwarding Equivalence Class

  FIB           Forwarding Information Base

  LAN           Local Area Network

  LDP           Label Distribution Protocol

  MTU-s         Multi-Tenant Unit switch

  PE            Provider Edge device

  PW            Pseudo-wire

  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

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  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 [PWE-CTRL]), e.g., GRE, L2TP, IPSEC, etc., 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,

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  the default approach of broadcasting multicast frames can be used.
  The use of IGMP snooping and PIM snooping techniques should be used
  to improve multicast efficiency.  A description of these techniques
  is beyond the scope of this document.

  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 PEs to 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 [BGP-VPN], 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

  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 protocol.

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  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: a PE MUST NOT forward traffic from one PW to another in the
  same VPLS mesh.

  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
  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 8 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 [PWE3-CTRL], 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 PWid FEC
  Element has been moved to Appendix 1.  What we describe below
  replaces that with a more generalized L2VPN descriptor, the
  Generalized PWid FEC Element.

6.1.1. Using the Generalized PWid FEC Element

  [PWE3-CTRL] describes a generalized FEC structure that is be used
  for VPLS signaling in the following manner.  We describe the

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  assignment of the Generalized PWid FEC Element fields in the
  context of VPLS signaling.

  Control bit (C): This bit is used to signal the use of the control
  word as specified in [PWE3-CTRL].

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

  PW info length: As specified in [PWE3-CTRL].

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

  TAII, SAII: These are null because the mesh of PWs in a VPLS
  terminate 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 [PWE3-CTRL].

     - 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 [PWE3-ETH].

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 sessions.

  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

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  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 [g-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 ignored.

  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 (subject to
  IANA approval).  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:

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     - 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 to 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 [PWE3-ETHERNET], the functions of stripping the
  service-delimiting tag and using a "normalized" Ethernet frame are

7.1. VPLS Encapsulation actions

  In a VPLS, a customer Ethernet frame without preamble is
  encapsulated with a header as defined in [PWE3-ETHERNET].  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

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     - 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 used.

  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 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 [PWE3-ETHERNET]
  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.

  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

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  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

  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 [PWE3-ETHERNET]) 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 [PWE3-ETHERNET], 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 [PWE3-ETHERNET].  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

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        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

  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, respectively.

  Initially, the VPLS is set up so that PE1, PE2 and PE3 have a full
  mesh of Ethernet PWs.  The VPLS instance is assigned a 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
           ----           ----

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               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 as well as 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 setup 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 [PWE3-ETHERNET] 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,
  expanding the scope of the first tier to include non-bridging VPLS
  PE routers.  The non-bridging PE router would extend a spoke PW

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  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 [PWE3-CTRL].  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.

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                                                          |        |
                                                          |   --   |
                                                          |  /  \  |
      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 on to 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

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  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 above.

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

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  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 setup from the PE-r to the
  PE-rs.  Once the PWs are setup, 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

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  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

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.

                                                          |        |
                                                          |   --   |
                                                          |  /  \  |
      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

  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

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  primary and is the one that is actively used under normal
  conditions, while 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 of 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.

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 which 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 4.2), 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 together.

  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

  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.

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  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 [802.1ad].

  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 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 as a typical bridge with this
  additional capability.  This functionality is described in

  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 of the scope of this
  document and they 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.

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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 islands).

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 from other
  islands and do not interact with each other.  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-routing.

12. Contributors

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

13. Acknowledgments

  We wish to thank Joe Regan, Kireeti Kompella, Anoop Ghanwani, Joel
  Halpern, Rick Wilder, Jim Guichard, Steve Phillips, Norm Finn, Matt

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  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 [VPN-SEC].  An unguarded VPLS service is
  vulnerable to some security issues which 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
          - 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
            [RFC-3036] 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
  6.2.1 and is subject to IANA approval.

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16. References

16.1. Normative References

  [PWE3-ETHERNET] "Encapsulation Methods for Transport of Ethernet
  Frames Over IP/MPLS Networks", draft-ietf-pwe3-ethernet-encap-
  10.txt, Work in progress, June 2005.

  [PWE3-CTRL] "Transport of Layer 2 Frames over MPLS", draft-ietf-
  pwe3-control-protocol-17.txt, Work in progress, June 2005.

  [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] "LDP Specification", L. Andersson, et al., RFC 3036,
  January 2001.

  [IANA] "IANA Allocations for pseudo Wire Edge to Edge Emulation
  (PWE3)" Martini,Townsley, draft-ietf-pwe3-iana-allocation-08.txt,
  Work in progress, February 2005.

16.2. Informative References

  [BGP-VPN] "BGP/MPLS VPNs", draft-ietf-l3vpn-rfc2547bis-03.txt, Work
  in Progress, October 2004.

  [RADIUS-DISC] "Using Radius for PE-Based VPN Discovery", draft-
  ietf-l2vpn-radius-pe-discovery-01.txt, Work in Progress, February

  [BGP-DISC] "Using BGP as an Auto-Discovery Mechanism for Network-
  based VPNs", draft-ietf-l3vpn-bgpvpn-auto-06.txt, Work in Progress,
  June 2005.

  [L2FRAME] "Framework for Layer 2 Virtual Private Networks
  (L2VPNs)", draft-ietf-l2vpn-l2-framework-05, Work in Progress, June

  [L2VPN-REQ] "Service Requirements for Layer-2 Provider Provisioned
  Virtual Private  Networks", draft-ietf-l2vpn-requirements-04.txt,
  Work in Progress, October 2005.

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  [VPN-SEC] "Security Framework for Provider Provisioned Virtual
  Private Networks", draft-ietf-l3vpn-security-framework-03.txt, Work
  in Progress, November 2004.

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

17. Appendix: 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, Interface parameters are as
  defined in [PWE3-CTRL].

   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 [PWE3-CTRL] 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.

18. Authors' Addresses

  Marc Lasserre
  Riverstone Networks

  Vach Kompella

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  Internet Draft      Virtual Private LAN Service        November 2005

IPR Disclosure Acknowledgement

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  Lasserre, et al.                                           [Page 26]