L3VPN WG                                              Hamid Ould-Brahim
Internet Draft                                          Nortel Networks
Expiration Date: October 2004
                                                          Eric C. Rosen
                                                          Cisco Systems

                                                          Yakov Rekhter
                                                       Juniper Networks

                                                              (Editors)

                                                             April 2004


                     Using BGP as an Auto-Discovery
                 Mechanism for Layer-3 and Layer-2 VPNs

                  draft-ietf-l3vpn-bgpvpn-auto-03.txt



Status of this Memo

   This document is an Internet-Draft and is in full conformance with
      all provisions of Section 10 of RFC2026 [RFC-2026].

   Internet-Drafts are working documents of the Internet Engineering
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   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
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Abstract

   In any Provider Provisioned-Based VPN (PPVPN) scheme, the Provider
   Edge (PE) devices attached to a common VPN must exchange certain
   information as a prerequisite to establish VPN-specific
   connectivity. The purpose of this draft is to define a BGP based
   auto-discovery mechanism for both layer-2 VPN architectures and
   layer-3 VPNs (Virtual Routers ûVR [VPN-VR]). This mechanism is based
   on the approach used by BGP/MPLS-IP-VPN [BGP/MPLS-IP-VPN] for
   distributing VPN routing information within the service provider(s).

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   Each VPN scheme uses the mechanism to automatically discover the
   information needed by that particular scheme.


1. Introduction


   In any Provider Provisioned-Based VPN (PPVPN) scheme, the Provider
   Edge (PE) devices attached to a common VPN must exchange certain
   information as a prerequisite to establish VPN-specific
   connectivity. The purpose of this draft is to define a BGP based
   auto-discovery mechanism for both layer-2 VPN architectures (i.e.,
   [L2VPN-KOMP], [L2VPN-ROSEN]) and layer-3 VPNs Virtual Router(VR
   [VPN-VR]). This mechanism is based on the approach used by BGP/MPLS-
   IP-VPN for distributing VPN routing information within the service
   provider(s). Each VPN scheme uses the mechanism to automatically
   discover the information needed by that particular scheme.

   In BGP/MPLS-IP-VPN, VPN-specific routes are exchanged, along with
   the information needed to enable a PE to determine which routes
   belong to which VRFs. In VR model, virtual router (VR) addresses
   must be exchanged, along with the information needed to enable the
   PEs to determine which VRs are in the same VPN ("membership"), and
   which of those VRs are to have VPN connectivity ("topology"). Once
   the VRs are reachable through the tunnels, routes ("reachability")
   are then exchanged by running existing routing protocols per VPN
   basis.

   The BGP-4 multiprotocol extensions are used to carry various
   information about VPNs for both layer-2 and layer-3 VPN
   architectures. VPN-specific information associated with the NLRI is
   encoded either as attributes of the NLRI, or as part of the NLRI
   itself, or both.


2. Provider-Provisioned VPN Reference Model

   Both the layer-2 and layer-3 vpns architectures are using a network
   reference model as illustrated in figure 1.













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                     PE                         PE
               +--------------+             +--------------+
   +--------+  | +----------+ |             | +----------+ | +--------+
   |  VPN-A |  | |  VPN-A   | |             | |  VPN-A   | | |  VPN-A |
   |  Sites |--| |Database /| |  BGP route  | | Database/| |-|  sites |
   +--------+  | |Processing| |<----------->| |Processing| | +--------+
               | +----------+ | Distribution| +----------+ |
               |              |             |              |
   +--------+  | +----------+ |             | +----------+ | +--------+
   | VPN-B  |  | |  VPN-B   | |  --------   | |   VPN-B  | | |  VPN-B |
   | Sites  |--| |Database /| |-(Backbones)-| | Database/| |-|  sites |
   +--------+  | |Processing| |  --------   | |Processing| | +--------+
               | +----------+ |             | +----------+ |
               |              |             |              |
   +--------+  | +----------+ |             | +----------+ | +--------+
   | VPN-C  |  | |  VPN-C   | |             | |   VPN-C  | | |  VPN-C |
   | Sites  |--| |Database /| |             | | Database/| |-|  sites |
   +--------+  | |Processing| |             | |Processing| | +--------+
               | +----------+ |             | +----------+ |
               +--------------+             +--------------+


                Figure 1: Network based VPN Reference Model


   It is assumed that the PEs can use BGP to distribute information to
   each other. This may be via direct IBGP peering, via direct EBGP
   peering, via multihop BGP peering, through intermediaries such as
   Route Reflectors, through a chain of intermediate BGP connections,
   etc. It is assumed also that the PE knows what architecture it is
   supporting.


3. Carrying VPN information in BGP Multi-Protocol (BGP-MP) Attributes

   The BGP-4 multiprotocol extensions are used to carry various
   information about VPNs for both layer-2 and layer-3 VPN
   architectures. VPN-specific information associated with the NLRI is
   encoded either as attributes of the NLRI, or as part of the NLRI
   itself, or both.  The addressing information in the NLRI field is
   ALWAYS within the VPN address space, and therefore MUST be unique
   within the VPN. The address specified in the BGP next hop attribute,
   on the other hand, is in the service provider addressing space. In
   L3VPNs, the NLRI contains an address prefix which is within the
   VPN address space, and therefore must be unique within the VPN.


3.1 Carrying Layer-3 VPN Information in BGP-MP

   This is done as follows.  The NLRI is a VPN-IP address or a labeled
   VPN-IP address.

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   In the case of the virtual router, the NLRI address prefix is an
   address of one of the virtual routers configured on the PE. Thus
   this mechanism allows the virtual routers to discover each other, to
   set up adjacencies and tunnels to each other, etc. In the case of
   BGP/MPLS-IP-VPN, the NLRI prefix represents a route to an arbitrary
   system or set of systems within the VPN.

3.2 Carrying Layer-2 VPN Information in BGP-MP

   The NLRI carries VPN layer-2 addressing information called VPN-L2
   address. A VPN-L2 address is composed of a quantity beginning with
   an 8 bytes Route Distinguisher (RD) field and a variable length
   quantity encoded according to the layer-2 VPN architecture used.

   Different layer-2 VPN solutions use the same common AFI, but
   different SAFI. The AFI indicates that the NLRI is carrying a VPN-l2
   address, while the SAFI indicates solution-specific semantics and
   syntax of the VPN-l2 address that goes after the RD. The RD must be
   chosen so as it ensures that each NLRI is globally unique (i.e., the
   same NLRI does not appear in two VPNs).


   BGP Route target extended community is used to constrain route
   distribution between PEs. The BGP Next hop carries the service
   provider tunnel endpoint address.

   This draft doesn't preclude the use of additional extended
   communities for encoding specific l2vpn parameters.

4. Interpretation of VPN Information in Layer-3 VPNs

4.1 Interpretation of VPN Information in the BGP/MPLS-IP-VPN Model

   For details see [BGP/MPLS-IP-VPN].

4.2 Interpretation of VPN Information in the VR Model

4.2.1 Membership Discovery

   The VPN-ID format as defined in [RFC-2685] is used to identify a
   VPN. All virtual routers that are members of a specific VPN share
   the same VPN-ID. A VPN-ID is carried in the NLRI to make addresses
   of VRs globally unique. Making these addresses globally unique is
   necessary if one uses BGP for VRs' auto-discovery.

4.2.1.1 Encoding of the VPN-ID in the NLRI

   For the virtual router model, the VPN-ID is carried within the route
   distinguisher (RD) field. In order to hold the 7-bytes VPN-ID, the
   first byte of RD type field is used to indicate the existence of the

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   VPN-ID format. A value of 0x80 in the first byte of RD's type field
   indicates that the RD field is carrying the VPN-ID format. In this
   case, the type field range 0x8000-0x80ff will be reserved for the
   virtual router case.


4.2.1.2 VPN-ID Extended Community

   A new extended community is used to carry the VPN-ID format. This
   attribute is transitive across the Autonomous system boundary. The
   type field of the VPN-ID extended community is of regular type to be
   assigned by IANA [BGP-COMM]. The remaining 7 bytes hold the VPN-ID
   value field as per [RFC-2685]. The BGP UPDATE message will carry
   information for a single VPN. It is the VPN-ID Extended Community,
   or more precisely route filtering based on the Extended Community
   that allows one VR to find out about other VRs in the same VPN.


4.2.2 VPN Topology Information

   A new extended community is used to indicate different VPN topology
   values. This attribute is transitive across the Autonomous system
   boundary. The value of the type field for extended type is assigned
   by IANA. The first two bytes of the value field (of the remaining 6
   bytes) are reserved. The actual topology values are carried within
   the remaining four bytes. The following topology values are defined:

         Value    Topology Type

           1          "Hub"
           2          "Spoke"
           3          "Mesh"

   Arbitrary values can also be used to allow specific topologies to be
   constructed. VPN connectivity between two VRs within the same VPN is
   achieved if and only if at least one of them is a hub (the other is
   a hub or a spoke), or if both VRs are part of a full mesh VPN
   topology.

5. Interpretation of VPN Information in Layer-2 VPNs

   The interpretation of the VPN information carried in the VPN-L2
   address is to be specified as part of each L2VPN solution
   standardized by L2VPN working group.


6. Tunnel Discovery

   Layer-3 VPNs and Layer-2 VPNs must be implemented through some form
   of tunneling mechanism, where the packet formats and/or the
   addressing used within the VPN can be unrelated to that used to

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   route the tunneled packets across the backbone. There are numerous
   tunneling mechanisms that can be used by a network based VPN (e.g.,
   IP/IP [RFC-2003], GRE tunnels [RFC-1701], IPSec [RFC-2401], and MPLS
   tunnels [RFC-3031]). Each of these tunnels allows for opaque
   transport of frames as packet payload across the backbone, with
   forwarding disjoint from the address fields of the encapsulated
   packets. A provider edge router may terminate multiple type of
   tunnels and forward packets between these tunnels and other network
   interfaces in different ways.

   BGP can be used to carry tunnel endpoint addresses between edge
   routers. For scalability purposes, this draft recommends the use of
   tunneling mechanisms with demultiplexing capabilities such as IPSec,
   MPLS, and GRE (with respect to using GRE -the key field, it is no
   different than just MPLS over GRE, however there is no specification
   on how to exchange the key field, while there is a specification and
   implementations on how to exchange the label). Note that IP in IP
   doesn't have demultiplexing capabilities.

   The BGP next hop will carry the service provider tunnel endpoint
   address. As an example, if IPSec is used as tunneling mechanism, the
   IPSec tunnel remote address will be discovered through BGP, and the
   actual tunnel establishment is achieved through IPSec signaling
   protocol.

   When MPLS tunneling is used, the label carried in the NLRI field is
   associated with an address of a VR, where the address is carried in
   the NLRI and is encoded as a VPN-IP address.

7. Auto-Discovery and VR-BGP/MPLS-IP-VPN Interworking Scenarios

   Two interwoking scenarios are considered when the network is using
   both virtual routers and BGP/MPLS-IP-VPN. The first scenario is a
   CE-PE relationship between a PE (implementing BGP/MPLS-IP-VPN), and
   a VR appearing as a CE to the PE. The connection between the VR, and
   the PE can be either direct connectivity, or through a tunnel (e.g.,
   IPSec).

   The second scenario is when a PE is implementing both architectures.
   In this particular case, a single BGP session configured on the
   service provider network can be used to advertise either BGP/MPLS-
   IP-VPN VPN information or the virtual router related VPN
   information. From the VR and the BGP/MPLS-IP-VPN point of view there
   is complete separation from data path and addressing schemes.
   However the PE's interfaces are shared between both architectures.

   A PE implementing only BGP/MPLS-IP-VPN will not import routes from a
   BGP UPDATE message containing the VPN-ID extended community. On the
   other hand, a PE implementing the virtual router architecture will
   not import routes from a BGP UPDATE message containing the route
   target extended community attribute.

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   The granularity at which the information is either BGP/MPLS-IP-VPN
   related or VR-related is per BGP UPDATE message. Different SAFI
   numbers are used to indicate that the message carried in BGP
   multiprotocol extension attributes is to be handled by the VR or
   BGP/MPLS-IP-VPN architectures. SAFI number of 128 is used for
   BGP/MPLS-IP-VPN related format. A value of 129 for the SAFI number is
   for the virtual router (where the NLRI are carrying a labeled
   prefixes), and a SAFI value of 140 is for non labeled addresses.

8. Scalability Considerations

   In this section, we briefly summarize the main characteristics of
   our model with respect to scalability.

   Recall that the Service Provider network consists of (a) PE routers,
   (b) BGP Route Reflectors, (c) P routers (which are neither PE
   routers nor Route Reflectors), and, in the case of multi-provider
   VPNs, and (d) ASBRs.

   A PE router, unless it is a Route Reflector should not retain
   VPN-related information unless it has at least one VPN with an
   Import Target identical to one of the VPN-related information Route
   Target attributes.  Inbound filtering should be used to cause such
   information to be discarded.  If a new Import Target is later added
   to one of the PE's VPNs (a "VPN Join" operation), it must then
   acquire the VPN-related information it may previously have
   discarded.

   This can be done using the refresh mechanism described in [BGP-
   RFSH].

   The outbound route filtering mechanism of [BGP-ORF] can also be
   used to advantage to make the filtering more dynamic.

   Similarly, if a particular Import Target is no longer present in
   any of a PE's VPNs (as a result of one or more "VPN Prune"
   operations), the PE may discard all VPN-related information which,
   as a result, no longer have any of the PE's VPN's Import Targets as
   one of their Route Target Attributes.

   Note that VPN Join and Prune operations are non-disruptive, and do
   not require any BGP connections to be brought down, as long as the
   refresh mechanism of [BGP-RFSH] is used.

   As a result of these distribution rules, no one PE ever needs to
   maintain all routes for all VPNs; this is an important scalability
   consideration.

   Route reflectors can be partitioned among VPNs so that each
   partition carries routes for only a subset of the VPNs supported by
   the Service Provider. Thus no single route reflector is required to
   maintain VPN-related information for all VPNs.

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   For inter-provider VPNs, if multi-hop EBGP is used, then the ASBRs
   need not maintain and distribute VPN-related information at all.

   P routers do not maintain any VPN-related information.  In order
   to properly forward VPN traffic, the P routers need only maintain
   routes to the PE routers and the ASBRs.

   As a result, no single component within the Service Provider network
   has to maintain all the VPN-related information for all the VPNs.
   So the total capacity of the network to support increasing numbers
   of VPNs is not limited by the capacity of any individual component.

   An important consideration to remember is that one may have any
   number of INDEPENDENT BGP systems carrying VPN-related information.
   This is unlike the case of the Internet, where the Internet BGP
   system must carry all the Internet routes. Thus one significant
   (but perhaps subtle) distinction between the use of BGP for the
   Internet routing and the use of BGP for distributing VPN-related
   information, as described in this document is that the former is not
   amenable to partition, while the latter is.


9. Security Considerations


   This document describes a BGP-based auto-discovery mechanism which
   enables a PE router that attaches to a particular VPN to discover
   the set of other PE routers that attach to the same VPN.  Each PE
   router that is attached to a given VPN uses BGP to advertise that
   fact. Other PE routers which attach to the same VPN receive these
   BGP advertisements. This allows that set of PE routers to discover
   each other. Note that a PE will not always receive these
   advertisements directly from the remote PEs; the advertisements may
   be received from "intermediate" BGP speakers.

   It is of critical importance that a particular PE should not be
   "discovered" to be attached to a particular VPN unless that PE
   really is attached to that VPN, and indeed is properly authorized to
   be attached to that VPN.  If any arbitrary node on the Internet
   could start sending these BGP advertisements, and if those
   advertisements were able to reach the PE routers, and if the PE
   routers accepted those advertisements, then anyone could add any
   site to any VPN.  Thus the auto-discovery procedures described here
   presuppose that a particular PE trusts its BGP peers to be who they
   appear to be, and further that it can trusts those peers to be
   properly securing their local attachments.  (That is, a PE must
   trust that its peers are attached to, and are authorized to be
   attached to, the VPNs to which they claim to be attached.).

   If a particular remote PE is a BGP peer of the local PE, then the
   BGP authentication procedures of RFC 2385 can be used to ensure that

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   the remote PE is who it claims to be, i.e., that it is a PE that is
   trusted.

   If a particular remote PE is not a BGP peer of the local PE, then
   the information it is advertising is being distributed to the local
   PE through a chain of BGP speakers.  The local PE must trust that
   its peers only accept information from peers that they trust in
   turn, and this trust relation must be transitive.  BGP does not
   provide a way to determine that any particular piece of received
   information originated from a BGP speaker that was authorized to
   advertise that particular piece of information.  Hence the
   procedures of this document should be used only in environments
   where adequate trust relationships exist among the BGP speakers.

   Some of the VPN schemes which may use the procedures of this
   document can be made robust to failures of these trust
   relationships.  That is, it may be possible to keep the VPNs secure
   even if the auto-discovery procedures are not secure.  For example,
   a VPN based on the VR model can use IPsec tunnels for transmitting
   data and routing control packets between PE routers.  An
   illegitimate PE router which is discovered via BGP will not have the
   shared secret which makes it possible to set up the IPsec tunnel,
   and so will not be able to join the VPN.  Similarly, [IPSEC-2547]
   describes procedures for using IPsec tunnels to secure VPNs based on
   the BGP/MPLS-IP-VPN model.  The details for using IPsec to secure a
   particular sort of VPN depend on that sort of VPN and so are out of
   scope of the current document.

10. IANA Considerations

    New AFI value to be assigned by IANA to indicate that the NLRI is
    carrying VPN-L2 Address as described in section 3.2 to be used by
    all L2VPN solutions.

    SAFI number of "128" is used for BGP/MPLS-IP-VPN.

    SAFI number "129" for indicating that the NLRI is carrying
    information for VR-based solution.

    SAFI number "140" for indicating that the NLRI is carrying
    information for VR for non labeled prefixes.

    New Extended Community to be assigned by IANA and used for Topology
    values for VR-based L3VPN solution see section 4.2.2.

    New Extended Community to be assigned by IANA for carrying VPN-ID
    format based on RFC2685 format (see section 4.2.1.2)

11. Use of BGP Capability Advertisement

   A BGP speaker that uses VPN information as described in this
   document with multiprotocol extensions should use the Capability

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   Advertisement procedures [RFC-3392] to determine whether the speaker
   could use Multiprotocol Extensions with a particular peer.

12. Normative References


   [BGP-COMM] Ramachandra, Tappan, et al., "BGP Extended Communities
      Attribute", June 2001, work in progress

   [BGP-MP] Bates, Chandra, Katz, and Rekhter, "Multiprotocol
      Extensions for BGP4", February 1998, RFC 2283

   [RFC-3107] Rekhter Y, Rosen E., "Carrying Label Information in
      BGP4", January 2000, RFC3107

   [BGP/MPLS-IP-VPN] Rosen E., et al, "BGP/MPLS VPNs", Work in
      Progress.

   [RFC-2685] Fox B., et al, "Virtual Private Networks Identifier", RFC
      2685, September 1999.

   [RFC-3392] Chandra, R., et al., "Capabilities Advertisement with
      BGP-4", RFC3392, May 2002.

   [VPN-VR] Knight, P., Ould-Brahim H., Gleeson, B., "Network based IP
      VPN Architecture using Virtual Routers", Work in Progress.

13. Informative References

   [L2VPN-ROSEN] Rosen, E., Radoaca, V., "Provisioning Models and
       Endpoint Identifiers in L2VPN Signaling", Work in Progress.

   [L2VPN-KOMP] Kompella, K., et al., "Virtual Private LAN Service",
       Work in Progress.

   [L2VPN-VKOMP-LASS] Kompella, V., Lasserre, M., et al., "Transparent
       VLAN Services over MPLS", Work in Progress.

   [RFC-1701] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic
      Routing Encapsulation (GRE)", RFC 1701, October 1994.

   [RFC-2003] Perkins, C., "IP Encapsulation within IP", RFC2003,
      October 1996.

   [RFC-2026] Bradner, S., "The Internet Standards Process -- Revision
      3", RFC2026, October 1996.

   [RFC-2401] Kent S., Atkinson R., "Security Architecture for the
      Internet Protocol", RFC2401, November 1998.

   [RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
      Requirement Levels", RFC 2119, March 1997.

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   [TLS-TISSA] "BGP/MPLS Layer-2 VPN", draft-tsenevir-bgpl2vpn-01.txt,
      work in progress, July 2001.

   [IPSEC-2547] Rosen, E., et al., "Use of PE-PE IPsec in RFC2547
      VPNs", Work in Progress.

   [BGP-RFSH] Chen, A., "Route Refresh Capability for BGP-4", RFC2918,
      September 2000.

   [BGP-ORF] Chen, E., and Rekhter, Y., "Cooperative Route Filtering
      Capability for BGP-4", Work in Progress.


14. Intellectual Property Rights Notices

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication and any assurances
   of licenses to be made available, or the result of an attempt made
   to obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification
   can be obtained from the IETF Secretariat.

15. Contributors


   Bryan Gleeson
   Tahoe Networks
   3052 Orchard Drive
   San Jose, CA 95134 USA
   Email: bryan@tahoenetworks.com

   Peter Ashwood-Smith
   Nortel Networks
   P.O. Box 3511 Station C,
   Ottawa, ON K1Y 4H7, Canada
   Phone: +1 613 763 4534
   Email: petera@nortelnetworks.com


    Luyuan Fang
    AT&T
    200 Laurel Avenue
    Middletown, NJ 07748

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                 draft-ietf-l3vpn-bgpvpn-auto-03.txt       April 2004

    Email: Luyuanfang@att.com
    Phone: +1 (732) 420 1920

   Jeremy De Clercq
   Alcatel
   Francis Wellesplein 1
   B-2018 Antwerpen, Belgium
   Phone: +32 3 240 47 52
   Email: jeremy.de_clercq@alcatel.be

   Riad Hartani
   Caspian Networks
   170 Baytech Drive
   San Jose, CA 95143
   Phone: 408 382 5216
   Email: riad@caspiannetworks.com

   Tissa Senevirathne
   Force10 Networks
   1440 McCarthy Blvd,
   Milpitas, CA 95035.

   Phone: 408-965-5103
   Email: tsenevir@hotmail.com


16. Authors Information

   Hamid Ould-Brahim
   Nortel Networks
   P O Box 3511 Station C
   Ottawa, ON K1Y 4H7, Canada
   Email: hbrahim@nortelnetworks.com



   Eric C. Rosen
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA 01719
   E-mail: erosen@cisco.com


   Yakov Rekhter
   Juniper Networks
   1194 N. Mathilda Avenue
   Sunnyvale, CA 94089
   Email: yakov@juniper.net





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Ould-Brahim & Rosen & Rekhter      April 2004                [Page 13]