Internet Draft Document                              Marc Lasserre
   Provider Provisioned VPN Working Group               Vach Kompella
   draft-ietf-l2vpn-vpls-ldp-03.txt                         (Editors)
   Expires: October 2004                                   April 2004
                  Virtual Private LAN Services over MPLS
   1.        Status of this Memo
   This document is an Internet-Draft and is in full conformance
   with all provisions of Section 10 of RFC2026.
   Internet-Drafts are working documents of the Internet Engineering
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   reference material or to cite them other than as "work in progress."
   The list of current Internet-Drafts can be accessed at
   The list of Internet-Draft Shadow Directories can be accessed at
   2.        Abstract
   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.  It delivers 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
   demultiplexor labels, extending [PWE3-CTRL].  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 [PWE3-
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   3.        Conventions
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119
   Table of Contents
   4.        Overview
   Ethernet has become the predominant technology for Local Area
   Networks (LANs) 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 MAN/WAN network(s), 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 LSP basis, packet
   replication across LSPs for multicast/broadcast traffic and for
   flooding of unknown unicast destination traffic.
   [PWE3-ETHERNET] defines how to carry L2 PDUs over point-to-point
   MPLS LSPs, called pseudowires (PW). Such PWs can be carried over
   MPLS or GRE tunnels. 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].
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   The following discussion applies to devices that are VPLS capable
   and have a means of tunneling labeled packets amongst each other.
   While MPLS LSPs may be used to tunnel these labeled packets, other
   technologies may be used as well, e.g., GRE [MPLS-GRE].  The
   resulting set of interconnected devices forms a private MPLS VPN.
   5.        Topological Model for VPLS
   An interface participating in a VPLS must be able to flood, forward,
   and filter Ethernet frames.
   +----+                                              +----+
   + C1 +---+      ...........................     +---| C1 |
   +----+   |      .                         .     |   +----+
   Site A   |   +----+                    +----+   |   Site B
            +---| PE |------ Cloud -------| PE |---+
                +----+         |          +----+
                   .           |             .
                   .         +----+          .
                   ..........| PE |...........
                             +----+         ^
                               |            |
                               |            +-- Emulated LAN
                             | C1 |
                             Site C
   The set of PE devices interconnected via pseudowires appears as a
   single emulated LAN to customer C1. Each PE device will learn remote
   MAC address to pseudowire associations and will also learn directly
   attached MAC addresses on customer facing ports.
   We note here again that while this document shows specific examples
   using MPLS transport tunnels, other tunnels that can be used by
   pseudo-wires, 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].
   The PE device is typically an edge router capable of running the LDP
   signaling protocol and/or routing protocols to set up pseudowires.
   In addition, it is capable of setting up transport tunnels to other
   PEs and deliver traffic over a pseudowire.
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   5.1.          Flooding and Forwarding
   One of attributes of an Ethernet service is that packets to
   broadcast packets and to unknown destination MAC addresses are
   flooded to all ports. To achieve flooding within the service
   provider network, all address unknown unicast, broadcast and
   multicast frames are flooded over the corresponding pseudowires to
   all relevant PE nodes participating in the VPLS.
   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 can be used. The use of
   IGMP snooping and PIM snooping techniques should be used to improve
   multicast efficiency.
   To forward a frame, a PE MUST be able to associate a destination MAC
   address with a pseudowire. It is unreasonable and perhaps impossible
   to require PEs to statically configure an association of every
   possible destination MAC address with a pseudowire. Therefore, VPLS-
   capable PEs SHOULD have the capability to dynamically learn MAC
   addresses on both physical ports and virtual circuits and to forward
   and replicate packets across both physical ports and pseudowires.
   5.2.          Address Learning
   Unlike BGP VPNs [BGP-VPN], reachability information does not need to
   be advertised and distributed via a control plane.  Reachability is
   obtained by standard learning bridge functions in the data plane.
   A pseudowire consists of a pair of uni-directional VC LSPs.  The
   state of this pseudowire is considered operationally up when both
   incoming and outgoing VC LSPs are established.  Similarly, it is
   considered operationally down when one of these two VC LSPs is torn
   down.  When a previously unknown MAC address is learned on an
   inbound VC LSP, it needs to be associated with the its counterpart
   outbound VC LSP in that pseudowire.
   Standard learning, filtering and forwarding actions, as defined in
   [802.1D-ORIG], [802.1D-REV] and [802.1Q], are required when a
   logical link state changes.
   5.3.          Tunnel Topology
   PE routers are assumed to have the capability to establish transport
   tunnels.  Tunnels are set up between PEs to aggregate traffic.
   Pseudowires are signaled to demultiplex the L2 encapsulated packets
   that traverse the tunnels.
   In an Ethernet L2VPN, it becomes the responsibility of the service
   provider to create the loop free topology.  For the sake of
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   simplicity, we define that the topology of a VPLS is a full mesh of
   tunnels and pseudowires.
   5.4.          Loop free L2 VPN
   For simplicity, a full mesh of pseudowires is established between
   PEs.  Ethernet bridges, unlike Frame Relay or ATM where the
   termination point becomes the CE node, have to examine the layer 2
   fields of the packets to make a switching decision.  If the frame is
   directed to an unknown destination, or is a broadcast or multicast
   frame, the frame must be flooded.
   Therefore, if the topology isn't a full mesh, the PE devices may
   need to forward these frames to other PEs. However, this would
   require the use of spanning tree protocol to form a loop free
   topology that may have characteristics that are undesirable to the
   provider. The use of a full mesh and split-horizon forwarding
   obviates the need for a spanning tree protocol.
   Each PE MUST create a rooted tree to every other PE router that
   serves the same VPLS.  Each PE MUST support a "split-horizon" scheme
   in order to prevent loops, that is, a PE MUST NOT forward traffic
   from one pseudowire to another in the same VPLS mesh (since each PE
   has direct connectivity to all other PEs in the same VPLS).
   Note that customers are allowed to run STP 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 BPDUs are simply
   tunneled through the provider cloud.
   6.        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
   7.        Control Plane
   This document describes the control plane functions of Demultiplexor
   Exchange (signaling of VC 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.
   7.1.          LDP Based Signaling of Demultiplexors
   In order to establish a full mesh of pseudowires, all PEs in a VPLS
   must have a full mesh of LDP sessions.
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   Once an LDP session has been formed between two PEs, all pseudowires
   are signaled over this session.
   In [PWE3-CTRL], two types of FECs are described, the FEC type 128
   PWid FEC Element and the FEC type 129 Generalized PWid FEC Element.
   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 through the Generalized PWid FEC
   7.1.1.            Using the Generalized PWid FEC Element
   [PWE3-CTRL] describes a generalized FEC structure that is be used
   for VPLS signaling in the following manner.  The following describes
   the assignment of the Generalized PWid FEC Element fields in the
   context of VPLS signaling.
   Control bit (C): Depending on whether, on that particular
   pseudowire, the control word is desired or not, the control bit may
   be specified.
   PW type: The allowed PW types in this version are Ethernet and
   Ethernet VLAN.
   VC info length: Same as in [PWE3-CTRL].
   AGI, Length, Value: The unique name of this VPLS.  The AGI
   identifies a type of name, the length denotes the length of Value,
   which is the name of the VPLS.  We will 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.
   Interface Parameters: The relevant interface parameters are:
        MTU: the MTU 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 VLAN, this
             parameter may be used to signal the insertion of the
             appropriate VLAN ID.
   7.1.2.            Address Withdraw Message Containing MAC TLV
   When MAC addresses are being removed or relearned explicitly, e.g.,
   the primary link of a dual-homed MTU-s has failed, an Address
   Withdraw Message with the list of MAC addresses to be relearned can
   be sent to all other PEs over the corresponding directed LDP
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   The processing for MAC TLVs received in an Address Withdraw Message
     For each MAC address in the TLV:
     - Relearn the association between the MAC address and the
        interface/pseudowire over which this message is received
     For an 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 this link (over the pseudowire associated with the
        signaling link over which the message is received)
   The scope of a MAC TLV is the VPLS specified in the FEC TLV in the
   Address Withdraw Message.  The number of MAC addresses can be
   deduced from the length field in the TLV.
   7.2.          MAC Address Withdrawal
   It MAY be desirable to remove or relearn MAC addresses that have
   been dynamically learned for faster convergence.
   We introduce an optional MAC TLV that is used to specify a list of
   MAC addresses that can be removed or relearned using the Address
   Withdraw Message.
   The Address Withdraw message with MAC 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 MTU-s).
   If a notification message is sent on the backup link (blocked link),
   which has transitioned into an active state (e.g., similar to
   Topology Change Notification message of 802.1w RSTP), with a list of
   MAC entries to be relearned, the PE will update the MAC entries in
   its FIB for that VPLS instance and send the message to other PEs
   over the corresponding directed LDP sessions.
   If the notification message contains an empty list, this tells the
   receiving PE to remove all the MAC addresses learned for the
   specified VPLS instance except the ones it learned from the sending
   PE (MAC address removal is required for all VPLS instances that are
   affected).  Note that the definition of such a notification message
   is outside the scope of the document, unless it happens to come from
   an MTU connected to the PE as a spoke.  In such a scenario, the
   message will be just an Address Withdraw message as noted above.
   7.2.1.            MAC TLV
   MAC addresses to be relearned can be signaled using an LDP Address
   Withdraw Message that contains a new TLV, the MAC TLV.  Its format
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   is described below.  The encoding of a MAC TLV address is the 6-byte
   MAC address specified by IEEE 802 documents [g-ORIG] [802.1D-REV].
     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 #n                           |
   U bit
        Unknown bit.  This bit MUST be set to 0.  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 field.  This field MUST be set to 0x0404 (subject to IANA
   approval).  This identifies the TLV type as MAC TLV.
        Length field.  This field specifies the total length of the MAC
   addresses in the TLV.
   MAC Address
        The MAC address(es) being removed.
   The LDP Address Withdraw Message contains a FEC TLV (to identify the
   VPLS in consideration), a MAC Address TLV and optional parameters.
   No optional parameters have been defined for the MAC Address
   Withdraw signaling.
   8.        Data Forwarding on an Ethernet VC Pseudowire
   This section describes the dataplane behavior on an Ethernet
   pseudowire used in a VPLS.  While the encapsulation is similar to
   that described in [PWE3-ETHERNET], the NSP functions of stripping
   the service-delimiting tag and using a "normalized" Ethernet packet
   are described.
   8.1.          VPLS Encapsulation actions
   In a VPLS, a customer Ethernet packet without preamble is
   encapsulated with a header as defined in [PWE3-ETHERNET].  A
   customer Ethernet packet is defined as follows:
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      - If the packet, 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 is stripped before the packet
        is sent into the VPLS.  As the packet exits the VPLS, the
        packet may have a service-delimiting encapsulation inserted.
      - If the packet, as it arrives at the PE, has an encapsulation
        that is not service delimiting, then it is a customer packet
        whose encapsulation should not be modified by the VPLS.  This
        covers, for example, a packet that carries customer-specific
        VLAN-Ids that the service provider neither knows about nor
        wants to modify.
   As an application of these rules, a customer packet 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 packet may be tagged again, if a
   service-delimiting tag is used, or it may be untagged if none is
   Likewise, if a customer packet arrives at a customer-facing port
   over an ATM VC that identifies the customer's VPLS instance, then
   the ATM encapsulation is removed before the packet is passed into
   the VPLS.
   Contrariwise, if a customer packet 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 packet that traverses a
   VPLS is always a customer Ethernet packet.  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 pseudowire given by [PWE3-
   ETHERNET] can serve as the access side connection into a PE.  An
   RFC1483 PVC encapsulation could be another 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 VPLS deployments, as described below.
   8.1.1.            VPLS Learning actions
   Learning is done based on the customer Ethernet packet, as defined
   above.  The Forwarding Information Base (FIB) keeps track of the
   mapping of customer Ethernet packet addressing and the appropriate
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   pseudowire to use.  We define two modes of learning: qualified and
   unqualified learning.
   In unqualified learning, all the customer VLANs 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 UNI interface 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
   For STP to work in qualified 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
   Martini) can be added by MTU-s nodes such that PEs can unambiguously
   identify all customer traffic, including STP/MSTP 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/MSTP
   traffic. The STP/MSTP 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
   9.        Data Forwarding on an Ethernet VLAN Pseudowire
   This section describes the dataplane behavior on an Ethernet VLAN
   pseudowire in a VPLS.  While the encapsulation is similar to that
   described in [PWE3-ETHERNET], the NSP functions of imposing tags,
   and using a "normalized" Ethernet packet are described.  The
   learning behavior is the same as for Ethernet pseudowires.
   9.1.          VPLS Encapsulation actions
   In a VPLS, a customer Ethernet packet without preamble is
   encapsulated with a header as defined in [PWE3-ETHERNET].  A
   customer Ethernet packet is defined as follows:
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      - If the packet, 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 packet is sent into the VPLS,
        unless the Requested VLAN ID optional parameter was signaled.
        In that case, the VLAN tag is overwritten before the packet is
        sent out on the pseudowire.
      - If the packet, 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 packet 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 pseudowire is a simple way to preserve customer
   802.1p bits.
   A VPLS MAY have both Ethernet and Ethernet VLAN pseudowires.
   However, if a PE is not able to support both pseudowires
   simultaneously, it can send a Label Release on the pseudowire
   messages that it cannot support with a status code "Unknown FEC" as
   given in [RFC3036].
   10.         Operation of a VPLS
   We show here 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.
   Initially, the VPLS is set up so that PE1, PE2 and PE3 have a full-
   mesh of Ethernet pseudowires.  The VPLS instance is assigned a
   unique VCID.
   For the above example, say PE1 signals VC Label 102 to PE2 and 103
   to PE3, and PE2 signals VC Label 201 to PE1 and 203 to PE3.
   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 multicast the packet to PE2 and
   PE3.  When PE2 receives the packet, it will have an inner 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 VC Label 102.
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                                                        /  A1 \
           ----                                    ----CE1    |
          /    \          --------       -------  /     |     |
          | A2 CE2-      /        \     /       PE1     \     /
          \    /   \    /          \---/         \       -----
           ----     ---PE2                        |
                       | Service Provider Network |
                        \          /   \         /
                 -----  PE3       /     \       /
                 |Agg|_/  --------       -------
                -|   |
         ----  / -----  ----
        /    \/    \   /    \                 CE = Customer Edge Router
        | A3 CE3    --C4 A4 |                 PE = Provider Edge Router
        \    /         \    /                 Agg = Layer 2 Aggregation
         ----           ----
   10.1.           MAC Address Aging
   PEs that learn remote MAC addresses need to have an aging mechanism
   to remove unused entries associated with a VC 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.
   As packets arrive, MAC addresses are remembered.  The aging timer
   for MAC address M SHOULD be reset when a packet is received with
   source MAC address M.
   11.         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 pseudowires 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 VCs 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 device 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 PE VPLS access points.
   It is often beneficial to extend the VPLS service tunneling
   techniques into the MTU (multi-tenant unit) domain.  This can be
   accomplished by treating the MTU device as a PE device and
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   provisioning pseudowires between it and every other edge, as an
   basic VPLS.  An alternative is to utilize [PWE3-ETHERNET]
   pseudowires or Q-in-Q logical interfaces between the MTU 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
   pseudowires (Hub) are augmented with access pseudowires (Spoke) to
   form a two-tier hierarchical VPLS (H-VPLS).
   Spoke pseudowires 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 pseudowire from a Layer-2 switch that connects to it, through
   the service core network, to a bridging VPLS PE router supporting
   Hub pseudowires.  We also describe how VPLS-challenged nodes and
   low-end CEs without MPLS capabilities may participate in a
   hierarchical VPLS.
   11.1.           Hierarchical connectivity
   This section describes the hub and spoke connectivity model and
   describes the requirements of the bridging capable and non-bridging
   MTU devices for supporting the spoke connections.
   For rest of this discussion we will refer to a bridging capable MTU
   device as MTU-s and a non-bridging capable PE device as PE-r.  A
   routing and bridging capable device will be referred to as PE-rs.
   11.1.1.             Spoke connectivity for bridging-capable devices
   As shown in the figure below, consider the case where an MTU-s
   device has a single connection to the PE-rs device placed in the CO.
   The PE-rs devices are connected in a basic VPLS full mesh.  For each
   VPLS service, a single spoke pseudowire is set up between the MTU-s
   and the PE-rs based on [PWE3-CTRL]. Unlike traditional pseudowires
   that terminate on a physical (or a VLAN-tagged logical) port at each
   end, the spoke pseudowire terminates on a virtual bridge instance on
   the MTU-s and the PE-rs devices.
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                                                         /      \
                                                        |   --   |
                                                        |  /  \  |
    CE-1                                                |  \B /  |
     \                                                   \  --  /
      \                                                  /------
       \   MTU-s                          PE1-rs        /   |
        \ ------                          ------       /    |
         /      \                        /      \     /     |
        | \ --   |      VC-1            |   --   |---/      |
        |  /  \--|- - - - - - - - - - - |--/  \  |          |
        |  \B /  |                      |  \B /  |          |
         \ /--  /                        \  --  / ---\      |
          /-----                          ------      \     |
         /                                             \    |
       ----                                             \ ------
      |Agg |                                             /      \
       ----                                             |  --    |
      /    \                                            | /  \   |
     CE-2  CE-3                                         | \B /   |
                                                         \ --   /
    MTU-s = Bridging capable MTU                          ------
    PE-rs = VPLS capable PE                               PE3-rs
   /  \
   \B / = Virtual VPLS(Bridge)Instance
    Agg = Layer-2 Aggregation
   The MTU-s device and the PE-rs device treat each spoke connection
   like an access port of the VPLS service. On access ports, the
   combination of the physical port and/or the VLAN tag is used to
   associate the traffic to a VPLS instance while the pseudowire tag
   (e.g., VC label) is used to associate the traffic from the virtual
   spoke port with a VPLS instance, followed by a standard L2 lookup to
   identify which customer port the frame needs to be sent to.               MTU-s Operation
   MTU-s device 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 virtual spoke port.
   Packets to unknown destination are replicated to all ports in the
   service including the virtual spoke port.  Once the MAC address is
   learned, traffic between CE1 and CE2 will be switched locally by the
   MTU-s device saving the link capacity of the connection to the PE-
   rs.  Similarly traffic between CE1 or CE2 and any remote destination
   is switched directly on to the spoke connection and sent to the PE-
   rs over the point-to-point pseudowire.
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   Since the MTU-s is bridging capable, only a single pseudowire 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 connection
   pseudowire.               PE-rs Operation
   The PE-rs device 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 pseudowire.  Thus, the spoke pseudowire from
   the PE-r is treated as a virtual port and the PE-rs device will
   switch traffic between the spoke pseudowire, hub pseudowires, and
   access ports once it has learned the MAC addresses.
   11.1.2.             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
     pseudowires per service between all devices participating in the
     VPLS service.
  - Minimizes signaling overhead since fewer pseudowires 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 device and all of it’s
     locally connected MTU-s and PE-r.
  - Addition of other sites requires configuration of the new MTU-s
     device 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.
   11.1.3.             Spoke connectivity for non-bridging devices
   In some cases, a bridging PE-rs device may not be deployed in a CO
   or a multi-tenant building while a PE-r might already be deployed.
   If there is a need to provide VPLS service from the CO where the PE-
   rs device is not available, the service provider may prefer to use
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   the PE-r device in the interim.  In this section, we explain how a
   PE-r device that does not support any of the VPLS bridging
   functionality can participate in the VPLS service.
   As shown in this figure, the PE-r device creates a point-to-point
   tunnel LSP to a PE-rs device.  Then for every access port that needs
                                                         /      \
                                                        |   --   |
                                                        |  /  \  |
    CE-1                                                |  \B /  |
     \                                                   \  --  /
      \                                                  /------
       \   PE-r                           PE1-rs        /   |
        \ ------                          ------       /    |
         /      \                        /      \     /     |
        | \      |      VC-1            |   --   |---/      |
        |  ------|- - - - - - - - - - - |--/  \  |          |
        |   -----|- - - - - - - - - - - |--\B /  |          |
         \ /    /                        \  --  / ---\      |
          ------                          ------      \     |
         /                                             \    |
       ----                                             \------
      | Agg|                                            /      \
       ----                                            |  --    |
      /    \                                           | /  \   |
     CE-2  CE-3                                        | \B /   |
                                                        \ --   /
   to participate in a VPLS service, the PE-r device creates a point-
   to-point [PWE3-ETHERNET] pseudowire that terminates on the physical
   port at the PE-r and terminates on the virtual bridge instance of
   the VPLS service at the PE-rs.               PE-r Operation
   The PE-r device is defined as a device that supports routing but
   does not support any bridging functions.  However, it is capable of
   setting up [PWE3-ETHERNET] pseudowires between itself and the PE-rs.
   For every port that is supported in the VPLS service, a [PWE3-
   ETHERNET] pseudowire is setup from the PE-r to the PE-rs.  Once the
   pseudowires are setup, there is no learning or replication function
   required on part of the PE-r.  All traffic received on any of the
   access ports is transmitted on the pseudowire.  Similarly all
   traffic received on a pseudowire is transmitted to the access port
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   where the pseudowire terminates.  Thus traffic from CE1 destined for
   CE2 is switched at PE-rs and not at PE-r.
   This approach adds more overhead than the bridging capable (MTU-s)
   spoke approach since a pseudowire is required for every access port
   that participates in the service versus a single pseudowire required
   per service (regardless of access ports) when a MTU-s type device 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 device.
   11.2.           Redundant Spoke Connections
   An obvious weakness of the hub and spoke approach described thus far
   is that the MTU device has a single connection to the PE-rs device.
   In case of failure of the connection or the PE-rs device, the MTU
   device 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 device.
   The mechanism described is identical for both, MTU-s and PE-r type
   of devices
   11.2.1.             Dual-homed MTU device
   To protect from connection failure of the pseudowire or the failure
   of the PE-rs device, the MTU-s device or the PE-r is dual-homed into
   two PE-rs devices, as shown in figure-3.  The PE-rs devices must be
   part of the same VPLS service instance.
   An MTU-s device will setup two [PWE3-ETHERNET] pseudowires (one each
   to PE-rs1 and PE-rs2) for each VPLS instance. One of the two
   pseudowires is designated as primary and is the one that is actively
   used under normal conditions, while the second pseudowire is
   designated as secondary and is held in a standby state.  The MTU
   device negotiates the pseudowire labels for both the primary and
   secondary pseudowires, but does not use the secondary pseudowire
   unless the primary pseudowire fails.  Since only one link is active
   at a given time, a loop does not exist and hence 802.1D spanning
   tree is not required.
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                                                         /      \
                                                        |   --   |
                                                        |  /  \  |
    CE-1                                                |  \B /  |
      \                                                  \  --  /
       \                                                 /------
        \  MTU-s                          PE1-rs        /   |
         \------                          ------       /    |
         /      \                        /      \     /     |
        |   --   |   Primary PW         |   --   |---/      |
        |  /  \--|- - - - - - - - - - - |--/  \  |          |
        |  \B /  |                      |  \B /  |          |
         \  -- \/                        \  --  / ---\      |
          ------\                         ------      \     |
          /      \                                     \    |
         /        \                                     \ ------
        /          \                                     /      \
       CE-2         \                                   |  --    |
                     \     Secondary PW                 | /  \   |
                      - - - - - - - - - - - - - - - - - |-\B /   |
                                                         \ --   /
   11.2.2.             Failure detection and recovery
   The MTU-s device controls the usage of the pseudowires to the PE-rs
   nodes.  Since LDP signaling is used to negotiate the pseudowire
   labels, the hello messages used for the LDP session can be used to
   detect failure of the primary pseudowire.
   Upon failure of the primary pseudowire, MTU-s device immediately
   switches to the secondary pseudowire.  At this point the PE3-rs
   device that terminates the secondary pseudowire starts learning MAC
   addresses on the spoke pseudowire.  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 relearning 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 device where the
   secondary pseudowire got activated may send out a flush message,
   using the MAC 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.
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   11.3.           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 pseudowire 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 5.
   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.
   12.         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 are 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 device needs to have the capability of
   adding the additional P-VLAN tag for non-multiplexed customer UNI
   port where customer VLANs are not used as service delimiter. If
   customer VLANs need to be treated as service delimiter (e.g.,
   customer UNI port 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 in order to resolve overlapping VLAN-ids used by
   different customers. Therefore, the MTU-s device in this model can be
   considered as a typical bridge with this additional UNI capability.
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   The PE-rs device needs to be able to perform bridging functionality
   over the standard Ethernet ports toward the access network as well as
   over the pseudowires toward the network core. The set of pseudowires
   that corresponds to a VPLS instance would look just like a P-VLAN to
   the bridge portion of the PE-rs and that is why sometimes it is
   referred to as Emulated VLAN. In this model the PE-rs may need to run
   STP protocol in addition to split-horizon. Split horizon is run over
   MPLS-core; whereas, STP is run over the access network to accommodate
   any arbitrary access topology. In this model, the PE-rs needs to map
   a P-VLAN to a VPLS-instance and its associated pseudowires and vise
   The details regarding bridge operation for MTU-s and PE-rs (e.g.,
   encapsulation format for QinQ 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 pseudowires in
   the PE-rs device.
   12.1.           Scalability
   Given that each P-VLAN corresponds to a VPLS instance, one may think
   that the total number of VPLS instances supported is limited to 4K.
   However, the 4K limit applies only to each Ethernet access network
   (Ethernet island) and not to the entire network. The SP network, in
   this model, 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). Each island may consist of
   many MTU-s devices, several aggregators, and one or more PE-rs
   devices. The PE-rs devices enable a P-VLAN to be extended from one
   island to others using a set of pseudowires (associated with that
   VPLS instance) and providing a loop free mechanism across the core
   network through split-horizon.  Since a P-VLAN serves as a service
   delimiter within the provider's network, it does not get carried over
   the pseudowires and furthermore the mapping between P-VLAN and the
   pseudowires is a local matter. This means a VPLS instance can be
   represented by different P-VLAN in different Ethernet islands and
   furthermore each island can support 4K VPLS instances independent
   from one another.
   12.2.           Dual Homing and Failure Recovery
   In this model, an MTU-s can be dual or triple homed to different
   devices (aggregators and/or PE-rs devices). The failure protection
   for access network nodes and links can be provided through running
   MSTP in each island. The MSTP 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 pseudowires is
   used among these PE-rs devices for carrying the SP BPDU packets for
   that island. On a per P-VLAN basis, the MSTP will designate a single
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   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.
   13.         Significant Modifications
   Between rev 02 and this one, these are the changes:
       o Introduction of the Generalized PWid FEC in the signaling of
          a VPLS
       o Description of the use of Ethernet VLAN pseudowires
   14.         Contributors
   Loa Andersson, TLA
   Ron Haberman, Masergy
   Juha Heinanen, Independent
   Giles Heron, Tellabs
   Sunil Khandekar, Alcatel
   Luca Martini, Cisco
   Pascal Menezes, Terabeam
   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
   15.         Acknowledgments
   We wish to thank Joe Regan, Kireeti Kompella, Anoop Ghanwani, Joel
   Halpern, Rick Wilder, Jim Guichard, Steve Phillips, Norm Finn, Matt
   Squire, Muneyoshi Suzuki, Waldemar Augustyn, Eric Rosen, Yakov
   Rekhter, and Sasha Vainshtein for their valuable feedback.  In
   addition, we would like to thank Rajiv Papneja (ISOCORE), Winston
   Liu (ISOCORE), and Charlie Hundall (Extreme) for identifying issues
   with the draft in the course of the interoperability tests.
   16.         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
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          o Traffic isolation between VPLS domains is guaranteed by
             the use of per VPLS L2 FIB table and the use of per VPLS
          o 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
          o 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
          o LDP security (authentication) methods as described in
             [RFC-3036] SHOULD be applied.  This would prevent
             unauthorized participation by a PE in a VPLS.
     . Denial of service attacks
          o Some means to limit the number of MAC addresses (per site
             per VPLS) that a PE can learn SHOULD be implemented.
   17.         Intellectual Property Considerations
   This document is being submitted for use in IETF standards
   18.         Full Copyright Statement
      Copyright (C) The Internet Society (2001).  All Rights Reserved.
   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.
   This document and the information contained herein is provided on an
   19.         References
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   [PWE3-ETHERNET] "Encapsulation Methods for Transport of Ethernet
   Frames Over IP/MPLS Networks", draft-ietf-pwe3-ethernet-encap-
   06.txt, Work in progress, April 2004.
   [PWE3-CTRL] "Transport of Layer 2 Frames over MPLS", draft-ietf-
   pwe3-control-protocol-06.txt, Work in progress, March 2004.
   [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.
   [BGP-VPN] "BGP/MPLS VPNs". draft-ietf-l3vpn-rfc2547bis-01.txt, Work
   in Progress, September 2003.
   [RFC3036] "LDP Specification", L. Andersson, et al.  RFC 3036.
   January 2001.
   [RADIUS-DISC] "Using Radius for PE-Based VPN Discovery", draft-ietf-
   l2vpn-radius-pe-discovery-00.txt, Work in Progress, February 2004.
   [BGP-DISC] "Using BGP as an Auto-Discovery Mechanism for Network-
   based VPNs", draft-ietf-l3vpn-bgpvpn-auto-02.txt, Work in Progress,
   April 2004.
   [LDP-DISC] "Discovering Nodes and Services in a VPLS Network",
   draft-stokes-ppvpn-vpls-discover-00.txt, Work in Progress, June
   [L2FRAME] "Framework for Layer 2 Virtual Private Networks (L2VPNs)",
   draft-ietf-l2vpn-l2-framework-04, Work in Progress, March 2004.
   [L2VPN-REQ] "Service Requirements for Layer-2 Provider Provisioned
   Virtual Private  Networks", draft-ietf-l2vpn-requirements-01.txt,
   Work in Progress, February 2004.
   [802.1ad] "IEEE standard for Provider Bridges", Work in Progress,
   December 2002.
   [VPN-SEC] "Security Framework for Provider Provisioned Virtual
   Private Networks", draft-ietf-l3vpn-security-framework-01.txt, Work in
   Progress, February 2004.
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   Appendix 1.  Signaling a VPLS 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 VC
   VC, C, VC 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
    |    VC tlv     |C|         VC Type             |VC info Length |
    |                      Group ID                                 |
    |                        VCID                                   |
    |                       Interface parameters                    |
    ~                                                               ~
    |                                                               |
   We use the Ethernet pseudowire type to identify pseudowires that
   carry Ethernet traffic for multipoint connectivity.
   In a VPLS, we use a VCID (which has been substituted with a more
   general identifier, 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.
   20.         Authors' Addresses
   Marc Lasserre
   Riverstone Networks
   Vach Kompella
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