[Docs] [txt|pdf] [Tracker] [Email] [Diff1] [Diff2] [Nits] [IPR]

Versions: 00 01 02

Network Working Group                                   Kireeti Kompella
Internet Draft                                          Manoj Leelanivas
Expiration Date:  May 2001                                 Quaizar Vohra
                                                        Juniper Networks

Javier Achirica                                            Ronald Bonica
Telefonica Data                                                 WorldCom

Chris Liljenstolpe                                           Eduard Metz
Cable & Wireless                                       KPN Dutch Telecom

Chandramouli Sargor
Vijay Srinivasan
CoSine Communications


                        MPLS-based Layer 2 VPNs

                    draft-kompella-mpls-l2vpn-02.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.










Kompella et al.                                                 [Page 1]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


2. Abstract

   Virtual Private Networks (VPNs) based on Frame Relay or ATM circuits
   have been around a long time.  While these VPNs work well, the costs
   of maintaining separate networks for Internet traffic and VPNs and
   the administrative burden of provisioning these VPNs have led Service
   Providers to look for alternative solutions.  In this document, we
   present a VPN solution where from the customer's point of view, the
   VPN is based on Layer 2 circuits, but the Service Provider maintains
   and manages a single MPLS-based network for IP, MPLS IP VPNs, and
   Layer 2 VPNs.


3. Introduction

   The first corporate networks were based on dedicated leased lines
   interconnecting the various offices of the corporation.  Such
   networks offered connectivity and little else: they didn't scale
   well, they were expensive for the service providers (and hence for
   their customers), and provisioning them was a slow and arduous task.

   The first Virtual Private Networks (VPNs) were based on Layer 2
   circuits: X.25, Frame Relay and ATM (see [VPN]).  Layer 2 VPNs were
   easier to provision, and virtual circuits allowed the service
   provider to share a common infrastructure for all the VPNs.  These
   features were passed on to the customers in terms of cost savings.
   However, while Layer 2 VPNs were a significant step forward from
   dedicated lines, they still had their drawbacks.  First, they tied
   the service provider VPN infrastructure to a single medium (e.g.,
   ATM).  This became even more of a burden if the Internet
   infrastructure was to share the same physical links.  Second, the
   Internet infrastructure and the VPN infrastructure, even if they
   shared the same physical network, needed separate administration and
   maintenance.  Third, while provisioning was much easier than for
   dedicated lines, it was still complex.  This was especially evident
   in the effort to add a site to an existing VPN.

   This document offers a solution that preserves the advantages of a
   Layer 2 VPN while allowing the Service Provider to maintain and
   manage a single (MPLS-based) network for IP, MPLS IP VPNs ([IPVPN])
   and Layer 2 VPNs, and reducing the provisioning problem
   significantly.  In particular, adding a site to an existing VPN in
   most cases requires configuring just the Provider Edge router
   connected to the new site.

   The rest of this section discusses the relative merits of MPLS-based
   Layer 2 and Layer 3 VPNs.  Section 4 describes the operation of an
   MPLS-based Layer 2 VPN.  Sections 5 and 6 offer two alternative means



Kompella et al.                                                 [Page 2]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


   of signalling Layer 2 VPNs, one using LDP and the other using BGP.


3.1. Terminology

   We assume that the reader is familiar with Multi-Protocol Label
   Switching (MPLS [MPLS]), the Label Distribution Protocol (LDP [LDP])
   and the Border Gateway Protocol version 4 (BGP [BGP]).

   The terminology we use follows.  A "customer" is a customer of a
   Service Provider seeking to interconnect the various "sites"
   (independently connected networks) through the Service Provider's
   network, while maintaining privacy of communication and address
   space.  The device in a customer site that connects to a Service
   Provider router is termed the CE (customer edge device); this device
   may be a router or a switch.  The Service Provider router to which a
   CE connects is termed a PE.  A router in the Service Provider's
   network which doesn't connect directly to any CE is termed P.  These
   definitions follow those given in [IPVPN].


3.2. Advantages of Layer 2 VPNs

   We define a Layer 2 VPN as one where a Service Provider provides a
   layer 2 network to the customer.  As far as the customer is
   concerned, they have (say) Frame Relay circuits connecting the
   various sites; each CE is configured with a DLCI with which to talk
   to other CEs.  Within the Service Provider's network, though, the
   layer 2 packets are transported within MPLS Label-Switched Paths
   (LSPs).

   The Service Provider does not participate in the customer's layer 3
   network, in particular, in the routing, resulting in several
   advantages to the SP as a whole and to PE routers in particular.


3.2.1. Separation of Administrative Responsibilities

   In a Layer 2 VPN, the Service Provider is responsible for Layer 2
   connectivity; the customer is responsible for Layer 3 connectivity,
   which includes routing.  If the customer says that host x in site A
   cannot reach host y in site B, the Service Provider need only
   demonstrate that site A is connected to site B.  The details of how
   routes for host y reach host x are the customer's responsibility.

   Another very important factor is that once a PE provides Layer 2
   connectivity to its connected CE, its job is done.  A misbehaving CE
   can at worst flap its interface.  On the other hand, a misbehaving CE



Kompella et al.                                                 [Page 3]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


   in a Layer 3 VPN can flap its routes, leading to instability of the
   PE router or even the entire SP network.  This means that the Service
   Provider must aggressively damp route flaps from a CE; this is common
   enough with external BGP peers, but in the case of VPNs, the scale of
   the problem is much larger; also, the CE-PE routing protocol may not
   be BGP, and thus not have BGP's flap damping control.


3.2.2. Migrating from Traditional Layer 2 VPNs

   Since "traditional" Layer 2 VPNs (i.e., real Frame Relay circuits
   connecting sites) are indistinguishable from MPLS-based VPNs from the
   customer's point-of-view, migrating from one to the other raises few
   issues.  With Layer 3 VPNs, special care has to be taken that routes
   within the traditional VPN are not preferred over the Layer 3 VPN
   routes (the so-called "backdoor routing" problem, whose solution
   requires protocol changes that are somewhat ad hoc).


3.2.3. Privacy of Routing

   In a Layer 2 VPN, the privacy of customer routing is a natural
   fallout of the fact that the Service Provider does not participate in
   routing.  The SP routers need not do anything special to keep
   customer routes separate from other customers or from the Internet;
   there is no need for per-VPN routing tables, and the additional
   complexity this imposes on PE routers.


3.2.4. Layer 3 Independence

   Since the Service Provider simply provides Layer 2 connectivity, the
   customer can run any Layer 3 protocols they choose.  If the SP were
   participating in customer routing, it would be vital that the
   customer and SP both use the same layer 3 protocol(s) and routing
   protocols.


3.2.5. Multicast Routing

   Supporting IP multicast over MPLS-based Layer 3 VPN is as yet
   undocumented.

   In the Layer 2 VPN case, the CE routers run native multicast routing
   directly.  The SP backbone just provides pipes to connect the CE
   routers; whether the CE routers run IP unicast or IP multicast or
   some other network protocol is irrelevant to the SP routers.




Kompella et al.                                                 [Page 4]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


3.2.6. PE Scaling

   In the Layer 2 VPN scheme described below, each PE transmits a single
   small chunk of information about every CE that the PE is connected to
   to every other PE.  That means that each PE need only maintain a
   single chunk of information from each CE in each VPN, and keep a
   single "route" to every site in every VPN.  This means that both the
   Forwarding Information Base and the Routing Information Base scale
   well with the number of sites and number of VPNs.  Furthermore, the
   scaling properties are independent of the customer: the only germane
   quantity is the total number of VPN sites.

   This is to be contrasted with Layer 3 VPNs, where each CE in a VPN
   may have an arbitrary number of routes that need to be carried by the
   SP.  This leads to two issues.  First, both the information stored at
   each PE and the number of routes installed by the PE for a CE in a
   VPN can be (in principle) unbounded, which means in practice that a
   PE must restrict itself to installing routes associated with the VPNs
   that it is currently a member of.  Second, a CE can send a large
   number of routes to its PE, which means that the PE must protect
   itself against such a condition.  Thus, the SP must enforce limits on
   the number of prefixes accepted from a CE; this in turn requires the
   PE router to offer such control.

   The scaling issues of Layer 3 VPNs come into sharp focus at a BGP
   route reflector (RR).  An RR cannot keep all the advertised routes in
   every VPN since the number of routes will be too large.  The
   following solutions/extensions are needed to address this issue:

      1) RRs could be partitioned so that each RR services a subset of
         VPNs so that no single RR has to carry all the routes.  This
         method has the disadvantage that a PE changing its VPN
         membership could force a change in the RR configuration, and
         would require carefully constructing RR topologies.
      2) An RR could use a preconfigured list of Route-Targets for its
         inbound route filtering.  The RR may also need to install
         Outbound Route Filters [BGP-ORF] which contain the above list
         of Route-Targets on each of its peers so that they do not send
         unnecessary VPN routes.  This method also requires significant
         extensions along with the fact that multiple RRs are needed to
         service different sets of VPNs.










Kompella et al.                                                 [Page 5]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


3.2.7. Ease of Configuration

   Configuring traditional Layer 2 VPNs was a burden primarily because
   of the O(n*n) nature of the task.  If there are n CEs in a Frame
   Relay VPN, say full-mesh connected, n*(n-1)/2 DLCI PVCs must be
   provisioned across the SP network.  At each CE, (n-1) DLCIs must be
   configured to reach each of the other CEs.  Furthermore, when a new
   CE is added, n new DLCI PVCs must be provisioned; also, each existing
   CE must be updated with a new DLCI to reach the new CE.

   In our proposal, the provisioning of "PVCs" across the SP network is
   handled by signalling protocols (LDP, RSVP-TE), reducing a large part
   of the provisioning burden.  Furthermore, we assume that DLCIs at the
   CE edge are relatively cheap; and labels in the SP network are cheap.
   This allows the SP to "over-provision" VPNs: for example, allocate 50
   CEs to a VPN when only 20 are needed.  With this over-provisioning,
   adding a new CE to a VPN requires configuring just the new CE and its
   associated PE; existing CEs and their PEs need not be re-configured.


3.3. Advantages of Layer 3 VPNs

   Layer 3 VPNs ([IPVPN] in particular) offer a good solution when the
   customer traffic is wholly IP, customer routing is reasonably simple,
   and the customer sites connect to the SP with a variety of Layer 2
   technologies.


3.3.1. Layer 2 Independence

   One major restriction in a Layer 2 VPN is that the Layer 2 medium
   with which the various sites of a single VPN connect to the SP must
   be uniform.  On the other hand, the various sites of a Layer 3 VPN
   can connect to the SP with any supported media; for example, some
   sites may connect with Frame Relay circuits, and others with
   Ethernet.

   A corollary to this is that the number of sites that can be in a
   Layer 2 VPN is determined by the number of Layer 2 circuits that the
   Layer 2 technology provides.  For example, if the Layer 2 technology
   is Frame Relay with 2-octet DLCIs, a CE can connect to at most about
   a thousand other CEs in a VPN.









Kompella et al.                                                 [Page 6]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


3.3.2. SP Routing as Added Value

   Another problem with Layer 2 VPNs is that the CE router in a VPN must
   be able to deal with having N routing peers, where N is the number of
   sites in the VPN.  This can be alleviated by manipulating the
   topology of the VPN.  For example, a hub-and-spoke VPN architecture
   means that only one CE router (the hub) needs to deal with N
   neighbors.  However, in a Layer 3 VPN, a CE router need only deal
   with one neighbor, the PE router.  Thus, the SP can offer Layer 3
   VPNs as a value-added service to its customers.

   Moreover, with layer 2 VPNs it is up to a customer to build and
   operate the whole network.  With Layer 3 VPNs, a customer is just
   responsible for building and operating routing within each site,
   which is likely to be much simpler than building and operating
   routing for the whole VPN.  That, in turn, makes Layer 3 VPNs more
   suitable for customers who don't have sufficient routing expertise,
   again allowing the SP to provide added value.


3.3.3. Class-of-Service

   Class-of-Service issues have been addressed for Layer 3 VPNs.  Since
   the PE router has visibility into the network layer (IP), the PE
   router can take on the tasks of CoS classification and routing.

   Class-of-Service issues for Layer 2 VPNs will be addressed in a
   future revision.


4. Operation of a Layer 2 VPN

   The following simple example of a customer with 4 sites connected to
   3 PE routers in a Service Provider network will hopefully illustrate
   the various aspects of the operation of a Layer 2 VPN.  For
   simplicity, we assume that a full-mesh topology is desired.

   In what follows, Frame Relay serves as the Layer 2 medium, and each
   CE has multiple DLCIs to its PE, each to connect to another CE in the
   VPN.  If the Layer 2 medium were ATM, then each CE would have
   multiple VPI/VCIs to connect to other CEs.  For PPP and Cisco HDLC,
   each CE would have multiple physical interfaces to connect to other
   CEs.








Kompella et al.                                                 [Page 7]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


4.1. Network Topology

   Consider a Service Provider network with edge routers PE0, PE1, and
   PE2.  Assume that PE0 and PE1 are IGP neighbors, and PE2 is more than
   one hop away from PE0.

   Suppose that a customer C has 4 sites S0, S1, S2 and S3 that C wants
   to connect via the Service Provider's network using Frame Relay.
   Site S0 has CE0 and CE1 both connected to PE0.  Site S1 has CE2
   connected to PE0.  Site S2 has CE3 connected to PE1 and CE4 connected
   to PE2.  Site S3 has CE5 connected to PE2.  (See the Figure 1 below.)
   Suppose further that C wants to "over-provision" each current site,
   in expectation that the number of sites will grow to at least 10 in
   the near future.  However, CE4 is only provisioned with 9 DLCIs.

   Suppose finally that CE0 and CE2 have DLCIs 100 through 109 free; CE1
   and CE3 have DLCIs 200 through 209 free; CE4 has DLCIs 107, 209, 265,
   301, 414, 555, 654, 777 and 888 free; and CE5 has DLCIs 417-426.



4.2. Configuration

   The following sub-sections detail the configuration that is needed to
   provision the above VPN.  For the purpose of exposition, we assume
   that the customer will connect to the SP with Frame Relay circuits,
   and that the customer's IGP of choice is OSPF.

   While we focus primarily on the configuration that an SP has to do,
   we touch upon the configuration requirements of CEs as well.  The
   main point of contact in CE-PE configuration is that both must agree
   on the DLCIs that will be used on the interface connecting them.

   If the PE-CE connection is Frame Relay, it is recommended to run LMI
   between the PE and CE with the PE as DCE and the CE as DTE.  For the
   case of ATM VCs, OAM cells may be used; for PPP and Cisco HDLC,
   keepalives may be used.


4.2.1. CE Configuration

   Each CE that belongs to a VPN is given a "CE ID".  CE IDs must be
   unique in the context of a VPN.  We assume that the CE ID for CE-k is
   k.  Each CE is also configured with a maximum number of CEs that it
   can connect to; this is the CE's "range".

   Each CE is configured to communicate with its corresponding PE with
   the set of DLCIs given above; for example, CE0 is configured with



Kompella et al.                                                 [Page 8]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000



Figure 1: Example Network Topology


          S0                                                   S3
    ..............                                       ..............
    .            .                                       .            .
    .    +-----+ .                                       .            .
    .    | CE0 |-----------+                             .   +-----+  .
    .    +-----+ .         |                             .   | CE5 |  .
    .            .         |                             .   +--+--+  .
    .    +-----+ .         |                             .      |     .
    .    | CE1 |-------+   |                             .......|......
    .    +-----+ .     |   |                                   /
    .            .     |   |                                  /
    ..............     |   |                                 /
                       |   |         SP Network             /
                  .....|...|.............................../.....
                  .    |   |                              /     .
                  .  +-+---+-+       +-------+           /      .
                  .  |  PE0  |-------|   P   |--        |       .
                  .  +-+---+-+       +-------+  \       |       .
                  .   /    \                     \  +---+---+   .
                  .  |      -----+                --|  PE2  |   .
                  .  |           |                  +---+---+   .
                  .  |       +---+---+                 /        .
                  .  |       |  PE1  |                /         .
                  .  |       +---+---+               /          .
                  .  |            \                 /           .
                  ...|.............|.............../.............
                     |             |              /
                     |             |             /
                     |             |            /
         S1          |             |    S2     /
    ..............   |     ........|........../......
    .            .   |     .       |         |      .
    .    +-----+ .   |     .    +--+--+   +--+--+   .
    .    | CE2 |-----+     .    | CE3 |   | CE4 |   .
    .    +-----+ .         .    +-----+   +-----+   .
    .            .         .                        .
    ..............         ..........................

   DLCIs 100 through 109.  OSPF is configured to run over each DLCI.

   Each CE also "knows" which DLCI connects it to each other CE.  A
   simple algorithm is to use the CE ID of the other CE as an index into
   the DLCI list this CE has (with zero-based indexing, i.e., 0 is the
   first index).  For example, CE0 is connected to CE3 through its



Kompella et al.                                                 [Page 9]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


   fourth DLCI, 103; CE4 is connected to CE2 by the third DLCI in its
   list, namely 265.  This is the methodology used in the examples
   below; the actual methodology used to pick the DLCI to be used is a
   local matter; the key factor is that CE-k may communicate with CE-m
   using a different DLCI from the DLCI that CE-m uses to communicate to
   CE-k, i.e., the SP network effectively acts as a giant Frame Relay
   switch.  This is very important, as it decouples the DLCIs used at
   each CE site, making for much simpler provisioning.


4.2.2. PE Configuration

   Each PE is configured with the VPNs in which it participates.  Each
   VPN has an VPN ID that is unique within the SP network.  For each
   VPN, the PE has a list of CEs that are members of that VPN.  For each
   CE, the PE knows the CE ID, which DLCIs to expect from the CE, and
   the CE's range.


4.2.3. Adding a New Site

   The first step in adding a new site to a VPN is to pick a new CE ID.
   If all current members of the VPN are over-provisioned, i.e., their
   range includes the new CE ID, adding the new site is a purely local
   task.  Otherwise, the sites whose range doesn't include the new CE ID
   and wish to communicate directly with the new CE must have their
   ranges increased to incorporate the new CE ID.

   The next step is ensuring that the new site has the required
   connectivity (see below).  This may require tweaking the connectivity
   mechanism; however, in several common cases, the only configuration
   needed is local to the PE to which the CE is attached.

   The rest of the configuration is a local matter between the new CE
   and the PE to which it is attached.

   It bears repeating that the key to making additions easy is over-
   provisioning.  However, what is being over-provisioned is the number
   of DLCIs/VCIs that connect the CE to the PE.  This is a local matter,
   and generally is not an issue.


4.3. PE Information Exchange

   When a PE is configured with all the needed information for a CE, it
   first of all chooses a contiguous set of labels with n labels, where
   n is the CE's range.  Call the smallest label in this set the label-
   base.  The PE then advertises (for this CE): its Router ID, the VPN



Kompella et al.                                                [Page 10]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


   ID, the CE ID, the CE's range, and the label-base.  This is the basic
   Layer 2 VPN advertisement.  This same advertisement is sent to all
   other PEs.  Note that PEs that may not be part of the VPN can receive
   and keep this information, in case at some future point, a CE
   connected to the PE joins the VPN.

   If the PE-CE connection goes down, or the CE configuration is
   removed, the above advertisement is withdrawn.


4.3.1. PE Advertisement Processing

   When a PE receives a Layer 2 VPN advertisement, it checks if the VPN
   ID matches any VPN that it is a member of.  If not, the PE just
   stores the advertisement for future use.

   Otherwise, suppose the advertisement is from PE A for VPN X, CE m
   with range Rm and label base Lm.  For each CE that the receiving PE B
   is connected to that is a member of VPN X, PE B does the following.

      0) Look up the configuration information associated with the CE.
         If the encapsulation type for VPN X in the advertisement does
         not match the configured encapsulation type for VPN X, stop.
      1) Say the configured CE ID is k, the range is Rk, and the DLCI
         list is Dk[].  Also, get the label base PE B allocated for
         this CE, say Lk.
      2) Check if k = m.  If so, issue an error: "CE ID k has been
         allocated to two CEs in VPN X (check CE at PE A)".  Stop.
      3) Check if k >= Rm, or m >= Rk.  If so, issue a warning: "Cannot
         communicate with CE m (PE A) of VPN X: outside range".  Stop.
      4) Look in the appropriate table to see which label will get to
         PE A.  This is the "outer" label, Z.
      5) The DLCI that CE-k will use to talk to CE-m is Dk[m].  The
         "inner" label for sending packets to CE-m is (Lm + k).  The
         "inner" label on which to expect packets from CE-m is (Lk + m).
      6) Install a "route" such that packets from CE-k with DLCI Dk[m]
         will be sent with outer label Z, inner label (Lm + k).  Also,
         install a route such that packets received with label (Lk + m)
         will be mapped to DLCI Dk[m] and be sent to CE-k.
      7) Activate DLCI Dk[m] to the CE.  This can be done using LMI.

   If an advertisement is withdrawn, the appropriate DLCI must be de-
   activated, and the corresponding routes must be removed from the
   forwarding table.







Kompella et al.                                                [Page 11]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


4.3.2. Example of PE Advertisment Processing

   Consider the example network of Figure 1.  Let the VPN connecting S0,
   S1, S2 and S3 has a VPN id of 1.  Suppose PE2 receives an
   advertisement from PE0 for VPN 1, CE ID 0 with CE range R0 = 10 and
   label base L0 = 1000.  Since PE2 is connected to CE4 which is also in
   VPN 1, PE2 does the following:

      0) Look up the configuration information associated with CE4.
         The advertised encapsulation type matches the configured
         encapsulation type (both are Frame Relay), so proceed.
      1) CE4's range R4 is 9, its DLCI list D4[] is [ 107, 209, 265,
         301, 414, 555, 654, 777, 888], and its label base L4 is 4000.
      2) CE0 and CE4 have ids 0 and 4 respectively, so step 2 of 4.3.1
         is skipped.
      3) Since CE4's id is less than R0, and CE0's id is less than R4,
         step 3 of 4.3.1 is skipped.
      4) Look in the appropriate table on PE2 to see which label will
         get to PE0.  Let the label be 10001.
      5) The DLCI that CE4 will use to talk to CE0 is D4[0], i.e., 107.
         The inner label for sending packets to CE0 is (L0 + 4), i.e
         1004.  The inner label on which to expect packets from CE0 is
         (L4 + 0), i.e., 4000.
      6) Install a "route" such that packets from CE4 with DLCI 107
         will be sent with  outer label 10001, inner label 1004.  Also,
         install a route such that packets received with label 4000 will
         be mapped to DLCI 107 and be sent to CE4.
      7) Activate DLCI 107 to CE4.

   Since CE5 is also attached to PE2, PE2 needs to do processing similar
   to the above for CE5.

   Similarly, when PE0 receives an advertisment from PE2 for VPN1, CE4,
   with range R4 = 9, and label base L4 = 4000.  PE0 processes the
   advertisment for CE0 (and CE1, which is also in VPN 1).

      0) Look up the configuration information associated with CE0.
         The advertised encapsulation type matches the configured
         encapsulation type (both are Frame Relay), so proceed.
      1) CE0's range, R0, is 9, its DLCI list D0[] is [100 - 109],
         and its label base L0 is 1000.
      2) CE0 and CE4 have ids 0 and 4 respectively, so step 2 of 4.3.1
         is skipped.
      3) Since CE4's id is less than R0, and CE0's id is less than R4,
         step 3 of 4.3.1 is skipped.
      4) Let the outer label to reach PE2 be 9999.
      5) The DLCI which CE0 will use to talk to CE4 is D0[4], i.e., 104.
         The inner label for sending packets to CE4 is (L4 + 0), i.e.



Kompella et al.                                                [Page 12]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


         4000.  The inner label on which to expect packets from CE4 is
         (L0 + 4), i.e., 1004.
      6) Install a "route" such that packets from CE0 with DLCI 104
         will be sent with  outer label 9999, inner label 4000.  Also,
         install a route that packets received with label 1004 will be
         mapped to DLCI 104 and be sent to CE0.
      7) Activate DLCI 104 to CE0.

   Note that the inner label of 4000, computed by PE0, for sending
   packets from CE0 to CE4 is the same as what PE2 computed as the
   incoming label for receiving packets originated at CE0 and destined
   to CE4.  Similarly, the inner label of 1004, computed by PE0, for
   receiving packets from CE4 to CE0 is same as what PE2 computed as the
   outgoing label for sending packets originated at CE4 and destined to
   CE0.


4.3.3. Generalizing the VPN Topology

   In the above, we assumed for simplicity that the VPN was a full mesh.
   To allow for more general VPN topologies when using LDP for
   signalling, we introduce the notion of node colors, and the "spoke"
   attribute; together, these constitute a node's "connectivity".  A
   node (CE) in a VPN can be colored with one or more colors.
   Furthermore, a node may be a hub or a spoke.  Two nodes are connected
   iff they share a color in common, and they are not both spokes.

   To incorporate connectivity into the processing of advertisements,
   add step 3' to the above:

      3') If CE k and CE m are not connected, stop.

   This notion of connectivity does not allow arbitrary topologies to be
   built; however, it is a compromise of generality and efficiency.

   A more general mechanism based on BGP extended communities can also
   be used; naturally, this mechanism can only be used when signalling
   VPNs with BGP.  See below for details.













Kompella et al.                                                [Page 13]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


5. Packet Transport

   When a packet arrives at a PE from a CE in a Layer 2 VPN, the layer 2
   address of the packet identifies to which other CE the packet is
   destined.  The procedure outlined above installs a route that maps
   the layer 2 address to an outer label (which identifies the PE to
   which the destination CE is attached) and an inner label (which
   identifies the destination CE).  If the destination PE is one hop
   away from the source PE, and Penultimate Hop Popping (PHP) is used,
   there is no outer label.  If the destination PE is the same as the
   source PE, no labels are needed.

   The packet may then be modified (depending on the layer 2
   encapsulation) and then sent to the destination PE with the
   appropriate number of labels.

   If the destination PE is the same as the source, the packet "arrives"
   with no labels.  Otherwise, the packet arrives with one label (if PHP
   is used) or two labels, in which case the outer label is discarded;
   the remaining (inner) label is used to determine which CE is the
   destination CE.  The packet is restored to a fully-formed layer 2
   packet, and then sent to the CE.

   The MTU on the Layer 2 access links MUST be chosen such that the size
   of the L2 frames plus the L2VPN header does not exceed the MTU of the
   MPLS network.  Layer 2 frames that exceed the MPLS MTU after
   encapsulation MUST be dropped.


5.1. Layer 2 Frame Format

   For each VPN encapsulation type (see section 5.1.3), we describe
   below the format of the frame as it is transported in the MPLS LSP.


Figure 2: Format of a Layer 2 Packet Carried in MPLS

   +---------------------------------------------------+
   | MPLS  | Outer | Inner | Sequence | Modified Layer |
   | Encap | Label | Label |  Number  |     2 Frame    |
   +---------------------------------------------------+


   The "Outer Label" is used to transport the packet to the PE that is
   attached to the destination CE.

   The "Inner Label" is used by the destination PE to distinguish which
   CE to send the packet to, and what layer 2 address to use (if



Kompella et al.                                                [Page 14]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


   applicable).  The Inner Label may also carry "non-address"
   information in its experimental bits.  The label itself is 20 bits;
   the stack bit is as defined in [ENCAP].  The experimental bits are
   named N (Notification), C (Control) and L (Loss) as in the following
   figure.  Note that the inner label is only used for forwarding from
   the destination PE to the destination CE, not within the MPLS
   network.


Figure 3: Format of the Inner Label

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Label (20 bits)         |N C L|S|   (Unused)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The "Sequence Number" is an optional two octet unsigned number that
   wraps back to zero that is used to ensure in-sequence delivery of L2
   frames.  The sequence number field is only included if its use is
   indicated via VPN signalling.  A Layer 2 'connection' between two
   specific CEs is characterized within the MPLS network by the PEs to
   which the CEs are attached and a specific Inner Label in each
   direction.  For each such Layer 2 connection, the sequence number
   field is set to zero for the first packet transmitted and incremented
   by 1 for each subsequent packet sent on the same Layer 2 connection.
   When an out-of-sequence packet arrives at the receiver, it MAY be
   buffered for future delivery or discarded.

   The modification to the Layer 2 frame depends on the Layer 2 type.
   The following sections describe the modification for each protocol
   type, and other per-protocol information.


5.2. Frame Relay

   A Frame Relay frame has the following format:
              <DLCI><UI><proto><layer 3 packet>

   For transport over an MPLS LSP, the <DLCI> octets are removed.  The
   rest of the frame is transported as is.

   At the destination PE, a new DLCI is added, and the fully-formed
   Frame Relay frame sent to the CE.

   A DLCI contains "non-address" bits, namely, Forward and Backward
   Explicit Congestion Notification (FECN and BECN), the



Kompella et al.                                                [Page 15]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


   Command/Response (C/R) bit and the Discard Eligible (DE) bit.  The
   ingress LSR MAY set the experimental bits as follows: copy BECN to
   the N bit; copy the C/R bit to the C bit; and copy DE to the L bit.
   Otherwise, the ingress SHOULD set the experimental bits to 0.  The
   egress LSR MAY in turn copy the N bit to BECN of the outgoing DLCI,
   the C bit to C/R and the L bit to DE.

   Note that this is orthogonal to preferential treatment of the layer 2
   frame in the MPLS network.  If there are two LSPs (L-LSPs) to the
   destination PE, one for normal traffic and another for out-of-spec
   traffic, the ingress LSR MAY choose which LSP to use (i.e., which
   outer label) based on the DE bit.  If there is one LSP (E-LSP), but
   an experimental bit is used to denote out-of-spec traffic, the
   ingress LSR MAY set this experimental bit based on the DE bit.


5.3. ATM AAL/5

   For ATM AAL/5 VPNs, the AAL/5 PDU is transported without indication
   of the VPI/VCI.  At the receiving PE, the AAL/5 PDU is fragmented, a
   cell header with the correct VPI/VCI added to each cell, and the
   cells sent to the CE.

   If any of the cells that constitute the AAL/5 PDU have the CLP bit
   set, the ingress LSR MAY set the L bit.  If the L bit is set in the
   inner label at the destination PE, this PE MAY set the CLP bit in
   each cell when fragmenting the AAL/5 PDU.

   Again, the ingress PE may give preferential treatment to the ATM PDU
   based on whether any cell had the CLP bit set or all cells had their
   CLP bits clear.


5.4. ATM Cells

   For ATM Cell VPNs, ATM cells (including the 5 octet header) are
   transported.  At the receiving PE, the cells are sent to the CE.

   The experimental bits of the inner label SHOULD be set to zero at the
   ingress and ignored by the destination PE.


5.5. PPP, Cisco HDLC, Ethernet

   For PPP, Cisco HDLC and unswitched Ethernet VLANs VPNs, the Layer 2
   frame is transported whole, without any modification.  The Layer 2
   frame does not include HLDC flags or Ethernet preamble, nor CRCs; we
   assume that bit/byte stuffing has been undone.  At the receiving PE,



Kompella et al.                                                [Page 16]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


   the frame is sent to the CE.

   The experimental bits of the inner label SHOULD be set to zero at the
   ingress and ignored by the destination PE.


6. Signalling MPLS-Based Layer 2 VPNs

   There are two alternative means of signalling the MPLS-based Layer 2
   VPNs described in this document: using LDP ([LDP]) or using BGP
   version 4 ([BGP]).

   In LDP, VPN CE information and its associated label base are carried
   in a Label Mapping message, distributed in the downstream unsolicited
   mode described in [LDP].  A new FEC element is defined below to carry
   all the information corresponding to a VPN CE, except from the label
   base.  The label base is carried in the Label TLV following the FEC
   TLV.  If a FEC element in a FEC TLV encodes Layer 2 VPN information,
   it MUST be the only FEC element in the FEC TLV.

   The Layer 2 VPN FEC element is depicted in Figure 4 below.


Figure 4: L2 VPN FEC Element

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |  Encaps. Type |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Control Flags |          Reserved (Must Be Zero)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            VPN ID                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             CE ID             |           CE Range            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        CE Connectivity                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Sub-TLVs                            |
   .                              ...                              .
   .                              ...                              .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   In BGP, the Multiprotocol Extensions [BGP-MP] are used to carry
   L2-VPN signalling information.  [BGP-MP] defines the format of two
   BGP attributes (MP_REACH_NLRI and MP_UNREACH_NLRI) that can be used
   to announce and withdraw the announcement of reachability



Kompella et al.                                                [Page 17]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


   information.  We introduce a new address family identifier (AFI) for
   L2-VPN [to be assigned by IANA], a new subsequent address family
   identifier (SAFI) [to be assigned by IANA], and also a new NLRI
   format for carrying the individual L2-VPN CE information.  This NLRI
   will be carried in the above-mentioned BGP attributes.  This NLRI
   MUST be accompanied by one or more extended communities.  The
   extended community type is "Layer 2 VPN" (to be assigned by IANA);
   and the format is <VPN-ID>:<connectivity>, where <VPN-ID> is 4 octets
   in length, and <connectivity> is two octets.  All extended
   communities accompanying one or more Layer 2 VPN NLRIs MUST have the
   same <VPN-ID>.

   PEs receiving VPN information may filter advertisements based on the
   extended communities, thus controlling CE-to-CE connectivity.

   The format of the Layer 2 VPN NLRI is as shown in Figure 5 below.


Figure 5: BGP NLRI for L2 VPN Information

   +------------------------------------+
   |  Length (2 octets)                 |
   +------------------------------------+
   |  Encaps Type (1 octet)             |
   +------------------------------------+
   |  Control Flags (1 octet)           |
   +------------------------------------+
   |  Label base (3 octets)             |
   +------------------------------------+
   |  Reserved (Must Be Zero) (1 octet) |
   +------------------------------------+
   |  CE ID (2 octets)                  |
   +------------------------------------+
   |  CE Range (2 octets)               |
   +------------------------------------+
   |  Variable TLVs (0 to n octets)     |
   |    ...                             |
   +------------------------------------+



6.1. Signalled Information









Kompella et al.                                                [Page 18]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


6.1.1. Type (LDP only)

   The Type is L2-VPN (to be decided by IETF Consensus Action).


6.1.2. Length

   In LDP, the Length is the entire length of the L2 VPN FEC element,
   including the fixed header and all the sub-TLVs.

   In BGP, the Length field indicates the length in octets of the L2-VPN
   address prefix.


6.1.3. Encapsulation Type

   Identifies the layer 2 encapsulation, e.g., ATM, Frame Relay etc.
   The following encapsulation types are defined:

      Value   Encapsulation
          0   Reserved
          1   ATM PDUs (AAL/5)
          2   ATM Cells
          3   Frame Relay
          4   PPP
          5   Cisco-HDLC
          6   Ethernet VLAN (unswitched)
          7   MPLS


6.1.4. Control Flags

   This is a bit vector, defined as in the following Figure.


Figure 6: Control Flags Bit Vector

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |  Reserved   |S|
   +-+-+-+-+-+-+-+-+


   The following bit is defined; the rest MUST be set to zero.

        Name   Bit   Meaning
           S     0   Sequenced delivery of frames is required




Kompella et al.                                                [Page 19]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


6.1.5. Label base (BGP only)

   The label-base which is to be used for determining the inner label
   for forwarding packets to the CE identified by CE ID.  (Note: LDP
   carries the label-base in the Label TLV following the FEC TLV.)


6.1.6. VPN ID (LDP only)

   A 32 bit number which uniquely identifies a VPN in a provider's
   domain.


6.1.7. CE ID

   A 16 bit number which uniquely identifies a CE in a VPN.


6.1.8. CE Range

   A 16 bit number which describes the range of CE IDs to which the
   advertised CE is willing to connect.  In particular, a PE receiving
   an L2 VPN TLV MUST NOT use a label greater than or equal to
                         <label-base> + <CE range>
   when sending traffic for this VPN to the advertising PE.


6.1.9. CE Connectivity (LDP only)

   A 32-bit number encoding connectivity.  If the leftmost bit is 1, the
   CE is a spoke.  The remaining 31 bits encode the CE colors (bit i = 1
   means the CE has color i).


6.1.10. Sub-TLVs

   New sub-TLVs can be introduced as needed.

   In LDP, the TLV encoding mechanism described in [LDP] must be used.

   In BGP, TLVs (type takes 1 octet) can be added to extend the
   information carried in the L2 VPN address prefix.

   A TLV (type = 1) will be used for carrying VLAN IDs if the
   encapsulation is VLAN.






Kompella et al.                                                [Page 20]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


6.2. BGP L2 VPN capability

   The BGP Multiprotocol capability extension [BGP-CAP] is used to
   indicate that the BGP speaker wants to negotiate L2 VPN capability
   with its peers.  The capability code is 1, the capability length is
   4, and the AFI and SAFI values will be set to the L2 VPN AFI and L2
   VPN SAFI (discussed in section 5) respectively.


6.3. Advantages of Using BGP

   PE routers in an SP network typically run BGP v4.  This means that
   SPs are familiar with using BGP, and have already configured BGP on
   their PEs, so configuring and using BGP to signal Layer 2 VPNs is not
   much of an additional burden to the SP operators.  This is especially
   the case when the protocol of choice for signalling MPLS LSPs across
   the SP network is RSVP (perhaps for its Traffic Engineering
   properties); in this case, the SP may find using LDP to signal Layer
   2 VPN information undesirable.

   Another advantage of using BGP is that with BPG it is easier to build
   inter-provider VPNs.  Mechanisms for this will be described in a
   future version.


7. Acknowledgments

   The authors would like to thank Dennis Ferguson, Der-Hwa Gan, Dave
   Katz, Nischal Sheth, John Stewart, and Paul Traina for the
   enlightening discussions that helped shape the ideas presented here,
   and Ross Callon for his valuable comments.

   The idea of using extended communities for more general connectivity
   of a Layer 2 VPN was a contribution by Yakov Rekhter, who also gave
   many useful comments on the text; many thanks to him.
















Kompella et al.                                                [Page 21]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


8. Security Considerations

   The security aspects of this solution will be discussed at a later
   time.


9. IANA Considerations

   (To be filled in in a later revision.)


10. References

   [BGP] Rekhter, Y., and Li, T., "A Border Gateway Protocol 4 (BGP-4)",
   RFC 1771, March 1995.

   [BGP-CAP] Chandra, R., and Scudder, J., "Capabilities Advertisement
   with BGP-4", RFC 2842, May 2000.

   [BGP-MP] Bates, T., Rekhter, Y., Chandra, R., and Katz, D.,
   "Multiprotocol Extensions for BGP-4", RFC 2858, June 2000

   [BGP-ORF] Chen, E., and Rekhter, Y., "Cooperative Route Filtering
   Capability for BGP-4", March 2000 (work in progress).

   [BGP-RFSH] Chen, E., "Route Refresh Capability for BGP-4", draft-
   ietf-idr-bgp-route-refresh-01.txt, March 2000, (work in progress).

   [ENCAP] Rosen, E., Rekhter, Y., Tappan, D., Fedorkow, G., Farinacci,
   D., Li, T., and Conta, A., "MPLS Label Stack Encoding", draft-ietf-
   mpls-label-encaps-08.txt (work in progress)

   [IPVPN] Rosen, E., and Rekhter, Y., "BGP/MPLS VPNs", RFC 2547, March
   1999.

   [LDP] Andersson, L., Doolan, P., Feldman, N., Fredette, A., and
   Thomas, B., "LDP Specification", draft-ietf-mpls-ldp-11.txt, August
   2000 (work in progress).

   [MPLS] Callon, R., Doolan, P., Feldman, N., Fredette, A., Swallow,
   G., and Viswanathan, A., "A Framework for Multiprotocol Label
   Switching", draft-ietf-mpls-framework-05.txt, September 1999 (work in
   progress).

   [VPN] Kosiur, Dave, "Building and Managing Virtual Private Networks",
   Wiley Computer Publishing, 1998.





Kompella et al.                                                [Page 22]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


11. Intellectual Property Considerations

   Juniper Networks may seek patent or other intellectual property
   protection for some of all of the technologies disclosed in this
   document.  If any standards arising from this document are or become
   protected by one or more patents assigned to Juniper Networks,
   Juniper intends to disclose those patents and license them on
   reasonable and non-discriminatory terms.

   CoSine Communications may seek patent or other intellectual property
   protection for some of all of the technologies disclosed in this
   document.  If any standards arising from this document are or become
   protected by one or more patents assigned to CoSine Communications,
   CoSine intends to disclose those patents and license them on
   reasonable and non-discriminatory terms.


12. Full Copyright Statement

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   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.







Kompella et al.                                                [Page 23]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


13. Author Information


   Kireeti Kompella
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA 94089
   kireeti@juniper.net

   Manoj Leelanivas
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA 94089
   manoj@juniper.net

   Quaizar Vohra
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA 94089
   qv@juniper.net

   Javier Achirica
   Telefonica Data
   javier.achirica@telefonica-data.com

   Ronald P. Bonica
   WorldCom
   22001 Loudoun County Pkwy
   Ashburn, Virginia, 20147
   rbonica@mci.net

   Chris Liljenstolpe
   Cable & Wireless
   11700 Plaza America Drive
   Reston, VA 20190
   chris@cw.net

   Eduard Metz
   KPN Royal Dutch Telecom
   St. Paulusstraat 4
   2264 XZ Leidschendam
   The Netherlands
   e.t.metz@kpn.com

   Chandramouli Sargor
   CoSine Communications
   1200 Bridge Parkway
   Redwood City, CA 94065



Kompella et al.                                                [Page 24]


Internet Draft      draft-kompella-mpls-l2vpn-01.txt       November 2000


   csargor@cosinecom.com

   Vijay Srinivasan
   CoSine Communications
   1200 Bridge Parkway
   Redwood City, CA 94065
   vijay@cosinecom.com












































Kompella et al.                                                [Page 25]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/