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   Internet Draft Document                              Marc Lasserre
   draft-lasserre-vkompella-ppvpn-vpls-01.txt     Riverstone Networks
                                                        Vach Kompella
                                                          Nick Tingle
                                                      Sunil Khandekar
                                                     Timetra Networks
  
   Pascal Menezes                                       Loa Andersson
   Terabeam Networks                                           Utfors
  
   Andrew Smith                                            Pierre Lin
   Consultant                                     Yipes Communication
  
   Juha Heinanen                                          Giles Heron
   Song Networks                                  PacketExchange Ltd.
  
   Ron Haberman                                         Tom S.C. Soon
   Masergy, Inc.                                   SBC Communications
  
   Nick Slabakov                                         Luca Martini
   Rob Nath                                                   Level 3
   Riverstone Networks                                 Communications
  
   Expires: September 2002                                 March 2002
  
  
  
                  Virtual Private LAN Services over MPLS
                draft-lasserre-vkompella-ppvpn-vpls-01.txt
  
  
   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
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.
  
   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time.  It is inappropriate to use Internet-Drafts as
   reference material or to cite them other than as "work in progress."
  
   The list of current Internet-Drafts can be accessed at
        http://www.ietf.org/ietf/1id-abstracts.txt
   The list of Internet-Draft Shadow Directories can be accessed at
        http://www.ietf.org/shadow.html.
  
  
  
  
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   2.  Abstract
  
   This document describes a  virtual private LAN service (VPLS)
   solution over MPLS, also known as Transparent LAN Services (TLS).
   VPLS simulates an Ethernet virtual 802.1D bridge [802.1D-ORIG]
   [802.1D-REV] 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.  Many
   VLS services can be supported from a single PE node.
  
   3.  Conventions
  
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119
  
   Placement of this Memo in Sub-IP Area
  
   RELATED DOCUMENTS
  
   http:// search.ietf.org/internet-drafts/draft-martini-l2circuit-
   trans-mpls-06.txt
  
   http://search.ietf.org/internet-drafts/draft-martini-l2circuit-
   encap-mpls-02.txt
  
   http://search.ietf.org/internet-drafts/draft-augustyn-ppvpn-vpls-
   reqmts-00.txt
  
   WHERE DOES THIS FIT IN THE PICTURE OF THE SUB-IP WORK
  
   PPVPN
  
   WHY IS IT TARGETTED AT THIS WG
  
   The charter of the PPVPN WG includes L2 VPN services and this draft
   specifies a model for Ethernet L2 VPN services over MPLS.
  
   JUSTIFICATION
  
   Existing Internet drafts specify how to provide point-to-point
   Ethernet L2 VPN services over MPLS. This draft defines how
   multipoint Ethernet services can be provided.
  
  
  
  
  
  
  
  
  
  
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   Table of Contents
  
   1. Status of this Memo.............................................1
   2. Abstract........................................................2
   3. Conventions.....................................................2
   4. Overview........................................................3
   5. Bridging Model for MPLS.........................................4
   5.1. Flooding and Forwarding.......................................5
   5.2. Address Learning..............................................5
   5.3. LSP Topology..................................................6
   5.4. Loop free L2 VPN..............................................6
   5.5. LDP Based Signaling...........................................6
   5.6. Ethernet VPLS VC Type.........................................8
   5.6.1. VPLS Encapsulation actions..................................8
   5.6.2. VPLS Learning actions.......................................8
   5.6.3. VPLS Forwarding actions.....................................9
   6. MAC Address Withdrawal..........................................9
   6.1. MAC TLV......................................................10
   6.2. Address Withdraw Message Containing MAC TLV..................11
   7. Operation of a VPLS............................................11
   7.1. MAC Address Aging............................................12
   8. A Hierarchical VPLS Model......................................12
   8.1. Hierarchical connectivity....................................13
   8.1.1. Spoke connectivity for bridging-capable devices............13
   8.1.2. Advantages of spoke connectivity...........................15
   8.1.3. Spoke connectivity for non-bridging devices................16
   8.2. Redundant Spoke Connections..................................17
   8.2.1. Dual-homed MTU device......................................17
   8.2.2. Failure detection and recovery.............................18
   8.3. Multi-domain VPLS service....................................19
   9. Acknowledgments................................................19
   10. Security Considerations.......................................19
   11. Intellectual Property Considerations..........................19
   12. Full Copyright Statement......................................19
   13. References....................................................20
   14. Authors' Addresses............................................21
  
   4.  Overview
  
   Ethernet has become a predominant technology initially for Local
   Area Networks (LANs) and now as an access technology, specifically
   in metropolitan networks. Ethernet ports or IEEE VLANs are dedicated
   to customers on Provider Edge (PE) routers acting as LERs. Customer
   traffic gets mapped to a specific MPLS L2 VPN by configuring L2 FECs
   based upon the input port and/or VLAN.
  
  
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   Broadcast and multicast services are available over traditional
   LANs. MPLS does not support such services currently. 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.
  
   [MARTINI-ENCAP] defines how to carry L2 PDUs over point-to-point
   MPLS LSPs, called VC LSPs. Such VC LSPs can be carried across MPLS
   or GRE tunnels. This document describes extensions to [MARTINI-
   ENCAP] for transporting Ethernet/802.3 and VLAN [802.1Q] traffic
   across multiple sites that belong to the same L2 broadcast domain.
   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 [VPLS-REQ].
  
   The following discussion applies to devices that serve as Label Edge
   Routers (LERs) on an MPLS network that is VPLS capable. It will not
   discuss the behavior of transit Label Switch Routers (LSRs) that are
   considered a part of MPLS network. The MPLS network provides a
   number of Label Switch Paths (LSPs) that form the basis for
   connections between LERs attached to the same MPLS network. The
   resulting set of interconnected LERs forms a private MPLS VPN where
   each LSP is uniquely identified at each MPLS interface by a label.
  
   5.  Bridging Model for MPLS
  
   An MPLS interface acting as a bridge must be able to flood, forward,
   and filter bridged frames.
  
   +----+                                              +----+
   + C1 +---+      ...........................     +---| C1 |
   +----+   |      .                         .     |   +----+
   Site A   |   +----+                    +----+   |   Site B
            +---| PE |---- MPLS Cloud ----| PE |---+
                +----+         |          +----+
                   .           |             .
                   .         +----+          .
                   ..........| PE |...........
                             +----+         ^
                               |            |
                               |            +-- Logical bridge
                             +----+
                             | C1 |
                             +----+
                             Site C
  
  
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   The set of PE devices interconnected via transport tunnels appears
   as a single 802.1D bridge/switch to customer C1. Each PE device will
   learn remote MAC addresses on LSPs (and keeps learning directly
   attached MAC addresses on customer facing ports).  We note here 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 sender PE can be
   identified, since this is used in the learning algorithm.
  
   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 [VPLS-REQ].
  
   The PE device is typically an edge router capable of running a
   signaling protocol and/or routing protocols to exchange VC label
   information.  In addition, it is capable of setting up transport
   tunnels to other PEs to deliver VC LSP traffic.
  
  
   5.1.  Flooding and Forwarding
  
   Flooding within the service provider network is performed by sending
   unknown unicast and multicast frames to all relevant PE nodes
   participating in the VPLS. In the MPLS environment this means
   sending the PDU through each relevant VC LSP.
  
   Note that multicast frames do not necessarily have to be sent to all
   VPN members. For simplicity, the default approach of broadcasting
   multicast frames can be used. Extensions explaining how to interact
   with 802.1 GMRP protocol, IGMP snooping and static MAC multicast
   filters will be discussed in a future revision.
  
   To forward a frame, a bridge must be able to associate a destination
   MAC address with a VC LSP. It is unreasonable and perhaps impossible
   to require bridges to statically configure an association of every
   possible destination MAC address with a VC LSP. Therefore, VPLS
   bridges must provide enough information to allow an MPLS interface
   to dynamically learn about foreign destinations beyond the set of
   LSRs. To accomplish dynamic learning, a bridged PDU MUST conform to
   the encapsulation described within [MARTINI-ENCAP].
  
  
   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.
  
   Since VC LSPs are uni-directional, two LSPs of opposite directions
   are required to form a logical bi-directional link. When a new MAC
  
  
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   address is learned on an inbound LSP, it needs to be associated with
   the outbound LSP that is part of the same pair. The state of this
   logical link can be considered as up as soon as both incoming and
   outgoing LSPs are established. Similarly, it can be considered as
   down as soon as one of these two LSPs is torn down.
   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.  LSP Topology
  
   PE routers typically run an IGP between them, and are assumed to
   have the capability to establish MPLS tunnels.  Tunnel LSPs are set
   up between PEs to aggregate traffic.  VC LSPs are signaled to
   demultiplex the L2 encapsulated packets that traverse the tunnel
   LSPs.
  
   In this Ethernet L2VPN, it becomes the responsibility of the service
   provider to create the loop free topology, since the PEs have to
   examine the Layer 2 fields of the packets, unlike Frame Relay or
   ATM, where the termination point becomes the CE node.  Therefore,
   for the sake of simplicity, we assume that the topology of a VPLS is
   a full mesh of tunnel and VC LSPs.
  
  
   5.4.  Loop free L2 VPN
  
   For simplicity, a full mesh of LSPs is established between PEs.
  
   Each PE MUST create a rooted tree to every other PE router that
   serve the same L2 VPN. Each PE MUST support a "split-horizon" scheme
   in order to prevent loops, that is, a PE MUST NOT forward traffic
   from one VC LSP to another in the same VPN (since each PE has direct
   connectivity to all other PEs in the same VPN).
  
   Note that customers are allowed to run STP such as when a customer
   has a back door link used for backup. In such a case STP BPDUs are
   simply tunneled through the MPLS cloud.
  
   5.5.  LDP Based Signaling
  
   In order to establish a full mesh of VC LSPs, all PEs in a VPLS must
   have a full mesh of LDP sessions.
  
   Once an LDP session has been formed between two PEs, all VC LSPs are
   signaled over this session.
  
   In [MARTINI-SIG], the L2 VPN information is carried in a Label
   Mapping message sent in downstream unsolicited mode, which contains
  
  
  
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   the following VC FEC TLV.  VC, C, VC Info Length, Group ID,
   Interface parameters are as defined in [MARTINI-SIG].
  
  
    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                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        VC ID                                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Interface parameters                    |
    |                              "                                |
    |                              "                                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  
  
   This document defines a new VC type value in addition to the
   following values already defined in [MARTINI-SIG]:
  
   VC Type  Description
  
   0x0001   Frame Relay DLCI
   0x0002   ATM AAL5 VCC transport
   0x0003   ATM transparent cell transport
   0x0004   Ethernet VLAN
   0x0005   Ethernet
   0x0006   HDLC
   0x0007   PPP
   0x8008   CEM [8]
   0x0009   ATM VCC cell transport
   0x000A   ATM VPC cell transport
   0x000B   Ethernet VPLS
  
   VC types 0x0004 and 0x0005 identify VC LSPs that carry VLAN tagged
   and untagged Ethernet traffic respectively, for point-to-point
   connectivity.
  
   We define a new VC type, Ethernet VPLS, with codepoint 0x000B to
   identify VC LSPs that carry Ethernet traffic for multipoint
   connectivity.  The Ethernet VC Type is described below.
  
   For VC types 0x0001 to 0x000A, The VC ID identifies a particular VC.
   For the VPLS VC type, the VC ID is a VPN identifier globally unique
   within a service provider domain.
  
   Note that the VCID as specified in [MARTINI_SIG] is a service
   identifier, identifying a service emulating a point-to-point virtual
  
  
  
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   circuit.  In a VPLS, the VCID is a single service identifier,
   identifying an emulated LAN segment.
  
  
   5.6.  Ethernet VPLS VC Type
   5.6.1.  VPLS Encapsulation actions
  
   In a VPLS, a customer Ethernet packet without preamble is
   encapsulated with a header as defined in [MARTINI-ENCAP].  A
   customer Ethernet packet is defined as follows:
  
      - If the packet, as it arrives at the PE, has an encapsulation
        that is used by the local PE as a service delimiter, 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.
  
   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 may have only local significance.  The service delimiter may be
   a VC label also, whereby an Ethernet VC given by [MARTINI-ENCAP] 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.
  
  
   5.6.2.  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
   VC label to use.  We define two modes of learning: qualified and
   unqualified learning.
  
   In qualified learning, the learning decisions at the PE are based on
   the customer Ethernet packet's MAC address and VLAN tag, if one
   exists.  If no VLAN tag exists, the default VLAN is assumed.
  
  
  
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   Effectively, within one VPLS, there are multiple logical FIBs, one
   for each customer VLAN tag identified in a customer packet.
  
   In unqualified learning, learning is based on a customer Ethernet
   packet's MAC address only.  In other words, at any PE, there is only
   one FIB per VPLS, which maps the MAC address in a customer Ethernet
   packet to a VC label.
  
   5.6.3.  VPLS Forwarding actions
  
   The forwarding decisions taken at a PE couple with the learning
   mode.  When using unqualified learning, unknown destination packets
   are flooded to the entire VPLS.  When using qualified learning, the
   scope of the flooding domain may be reduced (to the scope of the
   customer VLAN).  How this may be achieved is outside the scope of
   this draft.
  
   It is important to ensure that the above learning and forwarding
   modes are used consistently across the VPLS.  For example, when the
   intention is to use qualified learning, duplicate MAC addresses with
   different VLAN tags should not trigger re-learn events, which will
   lead to incorrect forwarding decisions.  We propose that signaling
   an optional parameter in the VC FEC will provide an adequate guard
   against such misconfigurations.  By default, the behavior is
   unqualified learning.
  
   In order to signal the learning mode, we introduce a new interface
   parameter [MARTINI-SIG].
  
   Optional Interface Parameter
        0x06     VPLS Learning Mode
                 Length: 1 byte.
                 Value: 0 - unqualified learning
                        1 - qualified learning
  
   6.  MAC Address Withdrawal
  
   It MAY be desirable to remove 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 using the Address Withdraw
   Message.
  
   The Address Withdraw message with MAC TLVs MAY be supported in order
   to uninstall learned MAC addresses that have moved or gone away more
   quickly.  Once a MAC address is unlearned, re-learning occurs
   through flooding.
  
  
  
  
  
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   6.1.  MAC TLV
  
   MAC addresses to be unlearned can be signaled using an LDP Address
   Withdraw Message.  We define a new TLV, the MAC TLV.  Its format is
   described below.  The encoding of a MAC TLV address is a 2-byte
   802.1Q tag, followed by the 6-byte MAC address encoding specified by
   IEEE 802 documents [g-ORIG] [802.1D-REV].  The 802.1Q tag and the
   MAC address MUST appear in pairs.  If no tag is required, the value
   of the tag field MUST be zero.
  
     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             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Reserved            |       802.1Q Tag #1           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      MAC address #1                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          ...                  |       802.1Q Tag #n           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      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
        Type field.  This field MUST be set to 0x0404 (subject to IANA
   approval).  This identifies the TLV type as MAC TLV.
  
   Length
        Length field.  This field specifies the total length of the
   TLV, including the Type and Length fields.
  
   Reserved
        Reserved bits.  They MUST NOT be interpreted at the receiver,
   and MUST be set to zero by the sender.
  
   802.1Q Tag
        The 802.1Q Tag.  The value MUST be zero if the Ethernet VLAN
   encapsulation is used.  If the Ethernet encapsulation is used, and
   the Ethernet address is associated with a VLAN, it MUST be set to
   the VLAN tag.  If the Ethernet encapsulation is used, and the MAC
  
  
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   address is not associated with a VLAN, it MUST be set to zero.
   Since an 802.1Q tag is 12-bits, the high 4 bits of the field MUST be
   set to zero.
  
   MAC Address
        The MAC address 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.
  
   6.2.  Address Withdraw Message Containing MAC TLV
  
   When MAC addresses are being removed explicitly, e.g., an adjacent
   CE router has been disconnected, an Address Withdraw Message can be
   sent with the list of MAC addresses to be withdrawn.
  
   The processing for MAC TLVs received in an Address Withdraw Message
   is:
     For each (VLAN tag, MAC address) pair in the TLV:
     - Remove the association between the (VLAN tag, MAC address) pair
        and VC label.  It does not matter whether the MAC address was
        installed as a static or dynamic address.
  
   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.  The address list MAY be empty.  This tells the receiving
   LSR to delete any MAC addresses learned from the sending LSR for the
   VPLS specified by the FEC TLV.
  
   7.  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 tunnels between them for carrying tunneled traffic.  The
   VPLS service is assigned a VCID (a 32-bit quantity that is unique
   across the provider network across all VPLSs). (Allocation of
   domain-wide unique VCIDs is outside the scope of this draft.)
  
   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.
  
  
  
  
  
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   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.
                                                         -----
                                                        /  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
         ----           ----
  
  
   7.1.  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.
  
  
   8.  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) VCs 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.
  
  
  
  
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   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.  When this is done, a
   hierarchical architecture is created outside the context of VPLS; no
   service level signaling is present between the PE router and the MTU
   bridge.
  
   It is often beneficial to extend the VPLS service tunneling
   techniques into the MTU domain.  This can be accomplished by
   treating the MTU device as a PE device and provisioning VCs between
   it and every other edge, as an basic VPLS.  An alternative is to
   utilize [MARTINI-ENCAP] VCs between the MTU and selected VPLS
   enabled PE routers.  This section focuses on this alternative
   approach.  The [VPLS] mesh core tier VCs (Hub) are augmented with
   access tier VCs (Spoke) to form a two tier hierarchical VPLS (H-
   VPLS).
  
   Spoke VCs may be expanded to include 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 VC from
   a Layer-2 switch that connects to it, through the service core
   network, to a bridging VPLS PE router supporting Hub VCs.  We also
   describe how VPLS-challenged nodes and low-end CEs without MPLS
   capabilities may participate in a hierarchical VPLS.
  
  
   8.1.  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.
  
  
   8.1.1.  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.   To
   participate in the VPLS service, MTU-s device creates a single
   point-to-point tunnel LSP to the PE-rs device in the CO.  We will
   call this the spoke connection.  For each VPLS service, a single
   spoke VC is setup between the MTU-s and the PE-rs based on [MARTINI-
   SIG] and [MARTINI-ENCAP].  Unlike traditional [MARTINI-ENCAP] VCs
   that terminate on a physical (or a VLAN-tagged logical) port at each
   end, the spoke VC terminates on a virtual bridge instance on the
  
  
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   MTU-s and the PE-rs devices.  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 the VLAN
   tag is used to associate the traffic to a VPLS instance while the 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.
  
   The signaling and association of the spoke connection to the VPLS
   service may be done by introducing extensions to the LDP signaling
   as specified in [SHAH-PECE].
                                                          PE2-rs
                                                          ------
                                                         /      \
                                                        |   --   |
                                                        |  /  \  |
    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
  
  
   8.1.1.1.  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
  
  
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   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 VC LSP.
  
   Since the MTU-s is bridging capable, only a single VC 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.
  
  
   8.1.1.2.  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 in
   [VPLS].   The operation on the PE-rs node is identical to that
   described in [VPLS] with one addition.  A point-to-point VC
   associated with the VPLS is regarded as a virtual port (see
   discussion in Section 5.6.1 on service delimiting).  The operation
   on the virtual spoke port is identical to the operation on an access
   port as described in the earlier section.  As shown in the figure
   above, each PE-rs device switches traffic between aggregated
   [MARTINI-ENCAP] VCs that look like virtual ports and the network
   side VPLS VCs.
  
  
   8.1.2.  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
     VCs per service between all devices participating in the VPLS
     service.
  - Minimizes signaling overhead since fewer VC-LSPs 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 itÂ’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.
  
  
  
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   8.1.3.  Spoke connectivity for non-bridging devices
  
   In some cases, a bridging PE-rs device may not be deployed in some
   CO 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 the PE-r device in
   the interim.  In this section, we explain how a PE-r device that
   does not support any of the bridging functionality as described in
   [VPLS] can participate in the VPLS service.
  
                                                          PE2-rs
                                                          ------
                                                         /      \
                                                        |   --   |
                                                        |  /  \  |
    CE-1                                                |  \B /  |
     \                                                   \  --  /
      \                                                  /------
       \   PE-r                           PE1-rs        /   |
        \ ------                          ------       /    |
         /      \                        /      \     /     |
        | \      |      VC-1            |   --   |---/      |
        |  ------|- - - - - - - - - - - |--/  \  |          |
        |   -----|- - - - - - - - - - - |--\B /  |          |
         \ /    /                        \  --  / ---\      |
          ------                          ------      \     |
         /                                             \    |
       ----                                             \------
      | Agg|                                            /      \
       ----                                            |  --    |
      /    \                                           | /  \   |
     CE-2  CE-3                                        | \B /   |
                                                        \ --   /
    PE-r = Non-Bridging PE (router)                      ------
    PE-rs = VPLS capable PE                               PE3-rs
  
    --
   /  \
   \B / = Virtual VPLS(Bridge)Instance
    --
    Agg = Layer-2 Aggregation
  
   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
   to participate in a VPLS service, the PE-r device creates a point-
   to-point [MARTINI-ENCAP] VC 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.
  
  
  
  
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   8.1.3.1.  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 [Martini-Encap] VCs between itself and the PE-rs.  For
   every port that is supported in the VPLS service, a [MARTINI-ENCAP]
   VC is setup from the PE-r to the PE-rs.  Once the VCs 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 VC.  Similarly all traffic received on a VC is
   transmitted to the access port where the VC 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 VC is required for every access port that
   participates in the service versus a single VC 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.
  
  
   8.1.3.2.  PE-rs Operation
  
   The operation of PE-rs is independent of the type of device at the
   other end of the spoke connection.  Whether there is a bridging
   capable device (MTU-s) at the other end of the spoke connection or
   there is a non-bridging device (PE-r) at the other end of the spoke
   connection, the operation of PE-rs is exactly the same.  Thus, the
   spoke connection from the PE-r is treated as a virtual port and the
   PE-rs device switches traffic between the virtual port, access ports
   and the network side VPLS VCs once it has learned the MAC addresses.
  
  
   8.2.  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
  
   8.2.1.  Dual-homed MTU device
  
   To protect from connection failure of the VC or the failure of the
   PE-rs device, the MTU-s device or the PE-r is dual-homed into two
  
  
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   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 [MARTINI-ENCAP] VCs (one each to PE-
   rs1 and PE-rs2) for each VPLS instances. One of the two VC is
   designated as primary and is the one that is actively used under
   normal conditions, while the second VC is designated as secondary
   and is held in a standby state.  The MTU device negotiates the VC-
   labels for both the primary and secondary VC, but does not use the
   secondary VC unless the primary VC 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.
  
                                                          PE2-rs
                                                          ------
                                                         /      \
                                                        |   --   |
                                                        |  /  \  |
    CE-1                                                |  \B /  |
      \                                                  \  --  /
       \                                                 /------
        \  MTU-s                          PE1-rs        /   |
         \------                          ------       /    |
         /      \                        /      \     /     |
        |   --   |   Primary VC         |   --   |---/      |
        |  /  \--|- - - - - - - - - - - |--/  \  |          |
        |  \B /  |                      |  \B /  |          |
         \  -- \/                        \  --  / ---\      |
          ------\                         ------      \     |
          /      \                                     \    |
         /        \                                     \ ------
        /          \                                     /      \
       CE-2         \                                   |  --    |
                     \     Secondary VC                 | /  \   |
                      - - - - - - - - - - - - - - - - - |-\B /   |
                                                         \ --   /
    MTU-s = Bridging capable MTU                          ------
    PE-rs = VPLS capable PE                               PE3-rs
  
    --
   /  \
   \B / = Virtual VPLS(Bridge)Instance
    --
  
  
   8.2.2.   Failure detection and recovery
  
   The MTU-s device controls the usage of the VC links to the PE-rs
   nodes.  Since LDP signaling is used to negotiate the VC-labels, the
   hello messages used for the LDP session are used to detect failure
   of the primary VC.
  
  
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   Upon failure of the primary VC, MTU-s device immediately switches to
   the secondary VC.  At this point the PE3-rs device that terminates
   the secondary VC starts learning MAC addresses on the spoke VC.  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
   PE1-rs device where the primary VC failed sends out a flush message,
   using the MAC TLV as defined in Section 6, to all other PE-rs
   devices participating in the VPLS service.  Upon receiving the
   message, all PE-rs flush the MAC addresses learned from PE1-rs.
   8.3.  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 gateway devices.  A
   single VC is setup per VPLS service to connect the two domains
   together.  The VPLS gateway device joins two VPLS services together
   to form a single multi-domain VPLS service.
  
   9.  Acknowledgments
  
   We wish to thank Joe Regan, Kireeti Kompella, Anoop Ghanwani, Joel
   Halpern, Rick Wilder and Eric Rosen for their valuable feedback.
  
   10.  Security Considerations
  
   Security issues resulting from this draft will be discussed in
   greater depth at a later point.  It is recommended in [RFC3036] that
   LDP security (authentication) methods be applied.  This would
   prevent unauthorized participation by a PE in a VPLS.  Traffic
   separation for a VPLS is effected by using VC labels.  However, for
   additional levels of security, the customer MAY deploy end-to-end
   security, which is out of the scope of this draft.
  
   11.  Intellectual Property Considerations
  
   This document is being submitted for use in IETF standards
   discussions.
  
   12.  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
  
  
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   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
   English.
  
   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
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
  
  
   13.  References
  
   [MARTINI-ENCAP] "Encapsulation Methods for Transport of Layer 2
   Frames Over MPLS", draft-martini-l2circuit-encap-mpls-04.txt (Work
   in progress)
  
   [MARTINI-SIG] "Transport of Layer 2 Frames Over MPLS", draft-
   martini-l2circuit-trans-mpls-08.txt (Work in progress)
  
   [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] Rosen and Rekhter, "BGP/MPLS VPNs". RFC 2547, March 1999
  
   [VPLS-REQ] "Requirements for Virtual Private LAN Services (VPLS)",
   draft-augustyn-vpls-requirements-00.txt (Work in progress).
  
   [RFC3036] "LDP Specification", L. Andersson, et al.  RFC 3036.
   January 2001.
  
  
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   [SHAH-PECE] " Signaling between PE and L2PE/MTU for Decoupled VPLS
   and Hierarchical VPLS ", draft-shah-ppvpn-vpls-pe-mtu-signaling-
   00.txt, February, 2002. (Work in progress)
  
   14.  Authors' Addresses
  
   Marc Lasserre
   Riverstone Networks
   5200 Great America Pkwy
   Santa Clara, CA 95054
   Email: marc@riverstonenet.com
  
   Vach Kompella
   TiMetra Networks
   274 Ferguson Dr.
   Mountain View, CA 94043
   Email: vkompella@timetra.com
  
   Sunil Khandekar
   TiMetra Networks
   274 Ferguson Dr.
   Mountain View, CA 94043
   Email: sunil@timetra.com
  
   Nick Tingle
   TiMetra Networks
   274 Ferguson Dr.
   Mountain View, CA 94043
   Email: ntingle@timetra.com
  
   Loa Andersson
   Utfors Bredband AB
   Rasundavagen 12 169 29 Solna
   Email: loa.andersson@utfors.se
  
   Pascal Menezes
   TeraBeam Networks
   2300 Seventh Ave
   Seattle, WA 98121
   Email: Pascal.Menezes@Terabeam.com
  
   Pierre Lin
   Yipes Communication
   114 Sansome St
   San Francisco, CA 94104
   Email: pierre.lin@yipes.com
  
   Andrew Smith
   Consultant
   Email: ah_smith@pacbell.net
  
  
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   Giles Heron
   PacketExchange Ltd.
   The Truman Brewery
   91 Brick Lane
   LONDON E1 6QL
   United Kingdom
   Email: giles@packetexchange.net
  
   Juha Heinanen
   Song Networks, Inc.
   Email: jh@lohi.eng.song.fi
  
   Tom S. C. Soon
   SBC Technology Resources Inc.
   4698 Willow Road
   Pleasanton, CA 94588
   Email: sxsoon@tri.sbc.com
  
   Ron Haberman
   Masergy Inc.
   2901 Telestar Ct.
   Falls Church, VA 22042
   Email: ronh@masergy.com
  
   Luca Martini
   Level 3 Communications, LLC.
   1025 Eldorado Blvd.
   Broomfield, CO, 80021
   Email: luca@level3.net
  
   Nick Slabakov
   Riverstone Networks
   5200 Great America Pkwy
   Santa Clara, CA 95054
   Email: nslabakov@riverstonenet.com
  
   Rob Nath
   Riverstone Networks
   5200 Great America Pkwy
   Santa Clara, CA 95054
   Email: rnath@riverstonenet.com
  
  
  
  
  
  
  
  
  
  
  
  
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