Internet Draft Document                              Marc Lasserre
   Provider Provisioned VPN Working Group         Riverstone Networks
   draft-ietf-l2vpn-vpls-ldp-00.txt                     Vach Kompella
                                                           Nick Tingle
                                                       Sunil Khandekar
                                                     Timetra Networks
                                                          Ali Sajassi
                                                         Cisco Systems
   Pascal Menezes                                       Loa Andersson
   Terabeam Networks                                       Consultant
   Andrew Smith                                            Pierre Lin
   Consultant                                     Yipes Communication
   Juha Heinanen                                          Giles Heron
   Song Networks                                  PacketExchange Ltd.
   Ron Haberman                                         Tom S.C. Soon
   Masergy, Inc.                                        Yetik Serbest
                                                           Eric Puetz
   Nick Slabakov                                   SBC Communications
   Rob Nath
   Riverstone Networks
                                                         Luca Martini
   Vasile Radaoca                                             Level 3
   Nortel Networks                                     Communications
   Expires: December 2003                                   June 2003
                  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
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   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
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   The list of Internet-Draft Shadow Directories can be accessed at
   2.  Abstract
   This document describes a  virtual private LAN service (VPLS)
   solution over MPLS, also 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.  Many VPLS services can be supported from a
   single PE node.
   This document describes the control plane functions of signaling
   demultiplexor labels, extending [PWE3-CTRL] and rudimentary support
   for availability (multi-homing).  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-
   3.  Conventions
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119
   Placement of this Memo in Sub-IP Area
   The charter of the PPVPN WG includes L2 VPN services and this draft
   specifies a model for Ethernet L2 VPN services over MPLS.
   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........................................................4
   5. Topological Model for VPLS......................................5
   5.1. Flooding and Forwarding.......................................5
   5.2. Address Learning..............................................6
   5.3. LSP Topology..................................................6
   5.4. Loop free L2 VPN..............................................7
   6. Discovery.......................................................7
   7. Control Plane...................................................7
   7.1. LDP Based Signaling of Demultiplexors.........................7
   7.2. MAC Address Withdrawal........................................9
   7.2.1. MAC TLV.....................................................9
   7.2.2. Address Withdraw Message Containing MAC TLV................10
   8. Data Forwarding on an Ethernet VC Type.........................11
   8.1. VPLS Encapsulation actions...................................11
   8.1.1. VPLS Learning actions......................................12
   9. Operation of a VPLS............................................12
   9.1. MAC Address Aging............................................13
   10. A Hierarchical VPLS Model.....................................13
   10.1. Hierarchical connectivity...................................14
   10.1.1. Spoke connectivity for bridging-capable devices...........14
   10.1.2. Advantages of spoke connectivity..........................16
   10.1.3. Spoke connectivity for non-bridging devices...............17
   10.2. Redundant Spoke Connections.................................18
   10.2.1. Dual-homed MTU device.....................................18
   10.2.2. Failure detection and recovery............................19
   10.3. Multi-domain VPLS service...................................20
   11. Hierarchical VPLS model using Ethernet Access Network.........20
   11.1. Scalability.................................................21
   11.2. Dual Homing and Failure Recovery............................21
   12. Significant Modifications.....................................22
   13. Acknowledgments...............................................22
   14. Security Considerations.......................................22
   15. Intellectual Property Considerations..........................22
   16. Full Copyright Statement......................................22
   17. References....................................................23
   18. Authors' Addresses............................................24
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   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). An Ethernet port is used to connect a
   customer to the Provider Edge (PE) router acting as an LER. Customer
   traffic is subsequently mapped to a specific MPLS L2 VPN by
   configuring L2 FECs based upon the input port ID and/or VLAN tag
   depending upon the VPLS service.
   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.
   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
   [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].
   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.
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   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 a
   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.
   5.1.  Flooding and Forwarding
   One of attributes of an Ethernet service is that all broadcast and
   destination unknown 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
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   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. Extensions
   explaining how to interact with 802.1 GMRP protocol, IGMP snooping
   and static MAC multicast filters will be discussed in a future
   revision if needed.
   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 must 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.
   As discussed previously, a pseudowire consists of a pair of uni-
   directional VC LSPs.  When a new MAC address is learned on an
   inbound VC LSP, it needs to be associated with the outbound VC LSP
   that is part of the same pair. 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.
   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 typically run an IGP between them, and are assumed to
   have the capability to establish transport tunnels.  Tunnel 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
   simplicity, we define that the topology of a VPLS is a full mesh of
   tunnels and pseudowires.
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   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
   serve 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 (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
   Currently, no discovery mechanism has been prescribed for VPLS.
   There are three potential candidates, [BGP-DISC], [RADIUS-DISC],
   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, although that work is
   described in a different document [VPLS-BRIDGING].
   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.
   Once an LDP session has been formed between two PEs, all pseudowires
   are signaled over this session.
   In [PWE3-CTRL], the L2 VPN information is carried in a Label Mapping
   message sent in downstream unsolicited mode, which contains the
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   following VC FEC TLV.  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                    |
    ~                                                               ~
    |                                                               |
   This document uses the VC type value for Ethernet as defined in
   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
   VC types 0x0004 and 0x0005 identify pseudowire types that carry VLAN
   tagged and untagged Ethernet traffic respectively, for point-to-
   point connectivity.
   We use the VC type Ethernet with codepoint 0x0005 to identify
   pseudowires that carry Ethernet traffic for multipoint connectivity.
   The Ethernet VC Type described below, conforms to the Ethernet VC
   Type defined in [PWE3-CTRL].
   In a VPLS, we use a VCID (to be substituted with a VPNID TLV later,
   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
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   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
   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.
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   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.
   7.2.2.  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 can be sent with the list of MAC addresses to be
   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
     - Send the same message to all other PEs over the corresponding
        directed LDP sessions.
     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)
     - Send the same message to all other PEs over the corresponding
        directed LDP sessions.
   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.
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   Further descriptions of how to deal with failures expeditiously with
   different configurations will be described in other documents, such
   8.  Data Forwarding on an Ethernet VC Type
   This section describes the dataplane behavior on an Ethernet VPLS
   pseudowire.  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
   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:
      - 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 packets 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
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   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
   pseudowire to use.  We define two modes of learning: qualified and
   unqualified learning.  However, the model followed in this VPLS
   document is the unqualified learning model.
   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, qualified learning offers the advantage
   of limiting the broadcast scope to a given customer VLAN.
   9.  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.
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   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.
                                                        /  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
         ----           ----
   9.1.  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.
   10.  A Hierarchical VPLS Model
   The solution described above requires a full mesh of tunnel LSPs
   between all the PE routers that participate in the VPLS service.
   For each VPLS service, n*(n-1)/2 pseudowires must be setup between
   the PE routers.  While this creates signaling overhead, the real
   detriment to large scale deployment is the packet replication
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   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
   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.
   10.1.  Hierarchical connectivity
   This section describes the hub and spoke connectivity model and
   describes the requirements of the bridging capable and non-bridging
   MTU 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.
   10.1.1.  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
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   end, the spoke pseudowire terminates on a virtual bridge instance on
   the MTU-s and the PE-rs devices.
                                                         /      \
                                                        |   --   |
                                                        |  /  \  |
    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-
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   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.
   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. However, to maintain a uniform end-to-end control plane
   based on MPLS signaling, [PWE3-CTRL] can be used for distribution of
   pseudowire tags (e.g., Q-in-Q tags or pseudowire labels) between
   MTU-s and PE-rs.  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
   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.
   10.1.2.  Advantages of spoke connectivity
   Spoke connectivity offers several scaling and operational advantages
   for creating large scale VPLS implementations, while retaining the
   ability to offer all the functionality of the VPLS service.
  - Eliminates the need for a full mesh of tunnels and full mesh of
     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.
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   10.1.3.  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
   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
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   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
   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.
   10.2.  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
   10.2.1.  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 /   |
                                                         \ --   /
   10.2.2.  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 PE1-rs, who relays it
   to all other PE-rs devices participating in the VPLS service.  Upon
   receiving the message, all PE-rs nodes flush the MAC addresses
   associated with that VPLS instance.
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   10.3.  Multi-domain VPLS service
   Hierarchy can also be used to create a large scale VPLS service
   within a single domain or a service that spans multiple domains
   without requiring full mesh connectivity between all VPLS capable
   devices.   Two fully meshed VPLS networks are connected together
   using a single LSP tunnel between the VPLS gateway devices.  A
   single spoke pseudowire 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.  The
   requirements and functionality required from a VPLS gateway device
   will be explained in a future version of this document.
   11.  Hierarchical VPLS model using Ethernet Access Network
   In the previous section, a two-tier hierarchical model that consists
   of hub-and-spoke topology between MTU-s devices and PE-rs devices and
   a full-mesh topology among PE-rs devices was discussed. In this
   section the two-tier 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.
   11.1.  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.
   11.2.  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
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   that island. On a per P-VLAN basis, the MSTP will designate a single
   PE-rs to be used for carrying the traffic across the core. The loop-
   free protection through the core is performed using split-horizon and
   the failure protection in the core is performed through standard
   IP/MPLS re-routing.
   12.  Significant Modifications
   Between rev 04 and this one, these are the changes:
       o minor revisions of text
       o cleanup of use of MPLS LSPs for tunnels
       o clearly states qualified learning is out of scope for current
       o corrected MAC TLV description
   13.  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, and Eric Rosen 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.
   14.  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.  In addition, the
   L2FRAME] document describes security issues in greater depth.
   15.  Intellectual Property Considerations
   This document is being submitted for use in IETF standards
   16.  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
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   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
   17.  References
   [PWE3-ETHERNET] "Encapsulation Methods for Transport of Ethernet
   Frames Over IP/MPLS Networks", draft-ietf-pwe3-ethernet-encap-
   02.txt, Work in progress, February 2003.
   [PWE3-CTRL] "Transport of Layer 2 Frames Over MPLS", draft-ietf-
   pwe3-control-protocol-02.txt, Work in progress, February 2003.
   [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". draft-ietf-ppvpn-
   rfc2547bis-04.txt, Work in Progress, May 2003.
   [RFC3036] "LDP Specification", L. Andersson, et al.  RFC 3036.
   January 2001.
   [RADIUS-DISC] " Using Radius for PE-Based VPN Discovery", Juha
   Heinanen, draft-heinanen-radius-pe-discovery-04.txt, Work in
   Progress, June 2003.
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   [BGP-DISC] "Using BGP as an Auto-Discovery Mechanism for Network-
   based VPNs", Ould-Brahim, et. al., draft-ietf-ppvpn-bgpvpn-auto-
   05.txt, Work in Progress, May 2003.
   [LDP-DISC] "Discovering Nodes and Services in a VPLS Network", O.
   Stokes et al, draft-stokes-ppvpn-vpls-discover-00.txt, Work in
   Progress, June 2002.
   [VPLS-BRIDGING] "Bridging and VPLS", draft-finn-ppvpn-bridging-vpls-
   00.txt, Work in Progress, June 2002.
   [L2VPN-SIG] "LDP-based Signaling for L2VPNs", draft-rosen-ppvpn-l2-
   signaling-03.txt, Work in Progress, May 2003.
   [L2FRAME] "L2VPN Framework", draft-ietf-ppvpn-l2-framework-03, Work
   in Progress, February 2003.
   [L2VPN-REQ] "Service Requirements for Layer 2 Provider Provisioned
   Virtual Private Networks", draft-ietf-ppvpn-l2vpn-requirements-
   00.txt, Work in Progress, May 2003.
   [802.1ad] "IEEE standard for Provider Bridges", Work in Progress,
   December 2002.
   18.  Authors' Addresses
   Marc Lasserre
   Riverstone Networks
   Vach Kompella
   TiMetra Networks
   274 Ferguson Dr.
   Mountain View, CA 94043
   Sunil Khandekar
   TiMetra Networks
   274 Ferguson Dr.
   Mountain View, CA 94043
   Nick Tingle
   TiMetra Networks
   274 Ferguson Dr.
   Mountain View, CA 94043
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   Ali Sajassi
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134
   Loa Andersson
   Pascal Menezes
   Andrew Smith
   Giles Heron
   PacketExchange Ltd.
   Juha Heinanen
   Tom S. C. Soon
   SBC Technology Resources Inc.
   Yetik Serbest
   SBC Communications
   Eric Puetz
   SBC Communications
   Ron Haberman
   Masergy Inc.
   Luca Martini
   Level 3 Communications, LLC.
   Rob Nath
   Riverstone Networks
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