Constrained Route Distribution for Border Gateway Protocol/MultiProtocol Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual Private Networks (VPNs)
draft-ietf-l3vpn-rt-constrain-02

L3VPN Working Group                                           P. Marques
Internet-Draft                                                 R. Bonica
Expires: December 24, 2005                              Juniper Networks
                                                                 L. Fang
                                                                    AT&T
                                                              L. Martini
                                                               R. Raszuk
                                                                K. Patel
                                                             J. Guichard
                                                     Cisco Systems, Inc.
                                                           June 22, 2005

                   Constrained VPN Route Distribution
                    draft-ietf-l3vpn-rt-constrain-02

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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document defines MP-BGP procedures that allow BGP speakers to

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   exchange Route Target reachability information.  This information can
   be used to build a route distribution graph in order to limit the
   propagation of VPN NLRI (such as VPN-IPv4, VPN-IPv6 or L2-VPN NLRI)
   between different autonomous systems or distinct clusters of the same
   autonomous system.

Table of Contents

   1.   Specification of Requirements  . . . . . . . . . . . . . . .   3
   2.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.   NLRI Distribution  . . . . . . . . . . . . . . . . . . . . .   6
     3.1  Inter-AS VPN Route Distribution  . . . . . . . . . . . . .   6
     3.2  Intra-AS VPN Route Distribution  . . . . . . . . . . . . .   7
   4.   Route Target membership NLRI advertisements  . . . . . . . .  10
   5.   Capability Advertisement . . . . . . . . . . . . . . . . . .  11
   6.   Operation  . . . . . . . . . . . . . . . . . . . . . . . . .  12
   7.   Deployment Considerations  . . . . . . . . . . . . . . . . .  13
   8.   Security Considerations  . . . . . . . . . . . . . . . . . .  14
   9.   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  15
   10.  References . . . . . . . . . . . . . . . . . . . . . . . . .  16
     10.1   Normative References . . . . . . . . . . . . . . . . . .  16
     10.2   Informative References . . . . . . . . . . . . . . . . .  16
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  16
        Intellectual Property and Copyright Statements . . . . . . .  19

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1.  Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].

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2.  Introduction

   In BGP/MPLS IP VPNs, PE routers use Route Target (RT) extended
   communities to control the distribution of routes into VRFs.  Within
   a given iBGP mesh, PE routers need only to hold routes marked with
   Route Targets pertaining to VRFs that have local CE attachments.

   It is common, however, for an autonomous system to use route
   reflection [2] in order to simplify the process of bringing up a new
   PE router in the network and to limit the size of the iBGP peering
   mesh.

   In such a scenario, as well as when VPNs may have members in more
   than one autonomous system, the number of routes carried by the
   inter-cluster or inter-as distribution routers is an important
   consideration.

   In order to limit the VPN routing information that is maintained at a
   given route reflector, RFC2547bis [3] suggests, in section 4.3.3.,
   the use of "Cooperative Route Filtering" [4] between route
   reflectors.  This proposal extends the RFC2547bis [3] ORF work to
   include support for multiple autonomous systems, and asymmetric VPN
   topologies such as hub-and-spoke.

   While it would be possible to extend the encoding currently defined
   for the extended-community ORF in order to achieve this purpose, BGP
   itself already has all the necessary machinery for dissemination of
   arbitrary information in a loop free fashion, both within a single
   autonomous system, as well as across multiple autonomous systems.

   This document builds on the model described in RFC2547bis [3] and on
   concept of cooperative route filtering by adding the ability to
   propagate Route Target membership information between iBGP meshes.
   It is designed to supersede "cooperative route filtering" for VPN
   related applications.

   By using MP-BGP UPDATE messages to propagate Route Target membership
   information it is possible to reuse all this machinery including
   route reflection, confederations and inter-as information loop
   detection.

   Received Route Target membership information can then be used to
   restrict advertisement of VPN NLRI to peers that have advertised
   their respective Route Targets, effectively building a route
   distribution graph.  In this model, VPN NLRI routing information
   flows in the inverse direction of Route Target membership
   information.

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   This mechanism is applicable to any BGP NLRI that controls the
   distribution of routing information based on Route Targets, such as
   BGP L2VPNs [?] and VPLS [9].

   Throughout this document, the term NLRI, which originally expands to
   "Network Layer Reachability Information" is used to describe routing
   information carried via MP-BGP updates without any assumption of
   semantics.

   An NLRI consisting of {origin-as#, route-target} will be referred to
   as RT membership information for the purpose of the explanation in
   this document.

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3.  NLRI Distribution

3.1  Inter-AS VPN Route Distribution

   In order to better understand the problem at hand, it is helpful to
   divide it in its inter-AS and intra-AS components.  Figure 1
   represents an arbitrary graph of autonomous systems (a through j)
   interconnected in an ad-hoc fashion.  The following discussion
   ignores the complexity of intra-AS route distribution.

                     +----------------------------------+
                     | +---+    +---+    +---+          |
                     | | a | -- | b | -- | c |          |
                     | +---+    +---+    +---+          |
                     |   |        |                     |
                     |   |        |                     |
                     | +---+    +---+    +---+    +---+ |
                     | | d | -- | e | -- | f | -- | j | |
                     | +---+    +---+    +---+    +---+ |
                     |        /            |            |
                     |       /             |            |
                     | +---+    +---+    +---+          |
                     | | g | -- | h | -- | i |          |
                     | +---+    +---+    +---+          |
                     +----------------------------------+

                 Figure 1: Topology of autonomous systems

   Lets consider the simple case of a VPN with CE attachments in ASes a
   and i using a single Route Target to control VPN route distribution.
   Ideally we would like to build a flooding graph for the respective
   VPN routes that would not include nodes (c, g, h, j).  Nodes (c, j)
   are leafs ASes that do not require this information while nodes (g,
   h) are not in the shortest inter-as path between (e) and (i) and thus
   should be excluded via standard BGP path selection.

   In order to achieve this we will rely on ASa and ASi generating a
   NLRI consisting of {origin-as#, route-target} ( RT membership
   information ).  Receipt of such an advertisement by one of the ASes
   in the network will signal the need to distribute VPN routes
   containing this Route Target community to the peer that advertised
   this route.

   Using RT membership information that includes both route-target and
   originator AS number, allows BGP speakers to use standard path
   selection rules concerning as-path length (and other policy
   mechanisms) to prune duplicate paths in the RT membership information

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   flooding graph, while maintaining the information required to reach
   all autonomous systems advertising the Route Target.

   In the example above, AS e needs to maintain a path to AS a in order
   to flood VPN routing information originating from AS i and vice-
   versa.  It should however, as default policy, prune less preferred
   paths such as the longer path to ASi with as-path (g h i).

   Extending the example above to include AS j as a member of the VPN
   distribution graph would cause AS f to advertise 2 RT Membership NLRI
   to AS e, one containing origin AS i and one containing origin AS j.
   While advertising a single path would be sufficient to guarantee that
   VPN information flows to all VPN member ASes, this is not enough for
   the desired path selection choices.  In the example above, assume (f
   j) is selected and advertised.  Where that to be the case the
   information concerning the path (f i), which is necessary to prune
   the arc (e g h i) from the route distribution graph, would be
   missing.

   As with other approaches for building distribution graphs, the
   benefits of this mechanism are directly proportional to how "sparse"
   the VPN membership is.  Standard RFC2547 inter-AS behavior can be
   seen as a dense-mode approach, to make the analogy with multicast
   routing protocols.

3.2  Intra-AS VPN Route Distribution

   As indicated above, the inter-AS VPN route distribution graph, for a
   given route-target, is constructed by creating a directed arc on the
   inverse direction of received Route Target membership UPDATEs
   containing an NLRI of the form {origin-as#, route-target}.

   Inside the BGP topology of a given autonomous-system, as far as
   external RT membership information is concerned (route-targets where
   the as# is not the local as), it is easy to see that standard BGP
   route selection and advertisement rules [5] will allow a transit AS
   to create the necessary flooding state.

   Consider a IPv4 NLRI prefix, sourced by a single AS, which is
   distributed via BGP within a given transit AS.  BGP protocol rules
   guarantee that a BGP speaker has a valid route that can be used for
   forwarding of data packets for that destination prefix, in the
   inverse path of received routing updates.

   By the same token, and given that a {origin-as#, route-target} key
   provides uniqueness between several ASes that may be sourcing this
   route-target, BGP route selection and advertisement procedures
   guarantee that a valid VPN route distribution path exists to the

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   origin of the Route Target membership information advertisement.

   Route Target membership information that is originated within the
   autonomous-system however requires more careful examination.  Several
   PE routers within a given autonomous-system may source the same NLRI
   {origin-as#, route-target}, thus default route advertisement rules
   are no longer sufficient to guarantee that within the given AS each
   node in the distribution graph has selected a feasible path to each
   of the PEs that import the given route-target.

   When processing RT membership NLRIs received from internal iBGP
   peers, it is necessary to consider all available iBGP paths for a
   given RT prefix, when building the outbound route filter, and not
   just the best path.

   In addition, when advertising Route Target membership information
   sourced by the local autonomous system to an iBGP peer, a BGP speaker
   shall modify its procedure to calculate the BGP attributes such that:

      i.  When advertising RT membership NLRI to a route-reflector
      client, the Originator attribute shall be set to the router-id of
      the advertiser and the Next-hop attribute shall be set of the
      local address for that session.

      ii.  When advertising a RT membership NLRI to a non client peer,
      if the best path as selected by path selection procedure described
      in section 9.1 of the base BGP specification [5] is a route
      received from a non-client peer, and there is an alternative path
      to the same destination from a client, the attributes of the
      client path are advertised to the peer.

   The first of these route advertisement rules is designed such that
   the originator of RT membership NLRI does not drop a RT membership
   NLRI which is reflected back to it, thus allowing the route reflector
   to use this RT membership NLRI in order to signal the client that it
   should distribute VPN routes with the specific target torwards the
   reflector.

   The second rule makes it such that any BGP speaker present in an iBGP
   mesh can signal the interest of its route reflection clients in
   receiving VPN routes for that target.

   These procedures assume that the autonomous-system route reflection
   topology is configured such that IPv4 unicast routing would work
   correctly.  For instance, route reflection clusters must be
   contiguous.

   An alternative solution to the procedure given above would have been

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   to source different routes per PE, such as NLRI of the form {origina-
   tor-id, route-target}, and aggregate them at the edge of the network.
   The solution adopted is considered to be advantageous over the former
   given that it requires less routing-information within a given AS.

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4.  Route Target membership NLRI advertisements

   Route Target membership NLRI is advertised in BGP UPDATE messages
   using the MP_REACH_NLRI and MP_UNREACH_NLRI attributes [6].  The
   [AFI, SAFI] value pair used to identify this NLRI is (AFI=1,
   SAFI=132).

   The Next Hop field of MP_REACH_NLRI attribute shall be interpreted as
   an IPv4 address, whenever the length of NextHop address is 4 octets,
   and as a IPv6 address, whenever the length of the NextHop address is
   16 octets.

   The NLRI field in the MP_REACH_NLRI and MP_UNREACH_NLRI is a prefix
   of 0 to 96 bits encoded as defined in section 4 of [6].

   This prefix is structured as follows:

        +-------------------------------+
        | origin as        (4 octets)  |
        +-------------------------------+
        | route target     (8 octets)  |
        +                               +
        |                               |
        +-------------------------------+

   Except for the default route target, which is encoded as a 0 length
   prefix, the minimum prefix length is 32 bits.  As the origin-as field
   cannot be interpreted as a prefix.

   Route targets can then be expressed as prefixes, where for instance,
   a prefix would encompass all route target extended communities
   assigned by a given Global Administrator [7].

   The default route target can be used to indicate to a peer the
   willingness to receive all VPN route advertisements such as, for
   instance, the case of a route reflector speaking to one of its PE
   router clients.

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5.  Capability Advertisement

   A BGP speaker that wishes to exchange Route Target membership
   information must use the Multiprotocol Extensions Capability Code as
   defined in RFC 2858 [6], to advertise the corresponding (AFI, SAFI)
   pair.

   A BGP speaker MAY participate in the distribution of Route Target
   information while not using the learned information for purposes of
   VPN NLRI output route filtering, although the latter is discouraged.

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6.  Operation

   A VPN NLRI route should be advertised to a peer that participates in
   the exchange of Route Target membership information if that peer has
   advertised either the default Route Target membership NLRI or a Route
   Target membership NLRI containing any of the targets contained in the
   extended communities attribute of the VPN route in question.

   When a BGP speaker receives a BGP UPDATE that advertises or withdraws
   a given Route Target membership NLRI, it should examine the RIB-OUTs
   of VPN NLRIs and re-evaluate the advertisement status of routes that
   match the Route Target in question.

   A BGP speaker should generate the minimum set of BGP VPN route
   updates necessary to transition between the previous and current
   state of the route distribution graph that is derived from Route
   Target membership information.

   An an hint that initial RT membership exchange is complete
   implementations SHOULD generate an End-of-RIB marker, as defined in
   [8], for the Route Target membership (afi, safi).  Regardless of
   whether graceful-restart is enabled on the BGP session.  This allows
   the receiver to know when it has received the full contents of the
   peers membership information.  The exchange of VPN NLRI should follow
   the receipt of the End-of-RIB markers.

   If a BGP speaker chooses to delay the advertisement of BGP VPN route
   updates until it receives this End-of-RIB marker, it MUST limit that
   delay to an upper bound.  By default, a 60 second value should be
   used.

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7.  Deployment Considerations

   This mechanism reduces the scaling requirements that are imposed on
   route reflectors by limiting the number of VPN routes and events that
   a reflector has to process to the VPN routes used by its direct
   clients.  By default, a reflector must scale in terms of the total
   number of VPN routes present on the network.

   This also means that its is now possible to reduce the load imposed
   on a given reflector by dividing the PE routers present on its
   cluster into a new set of clusters.  This is a localized
   configuration change that need not affect any system outside this
   cluster.

   The effectiveness of RT-based filtering depends on how sparse the VPN
   membership is.

   The same policy mechanisms applicable to other NLRIs are also
   applicable to RT membership information.  This gives a network
   operator the option of controlling which VPN routes get advertised in
   an inter-domain border by filtering the acceptable RT membership
   advertisements inbound.

   For instance, in the inter-as case, it is likely that a given VPN is
   connected to only a subset of all participating ASes.  The only
   current mechanism to limit the scope of VPN route flooding is through
   manual filtering on the EBGP border routers.  With the current
   proposal such filtering can be performed based on the dynamic Route
   Target membership information.

   In some inter-as deployments not all RTs used for a given VPN have
   external significance.  For example, a VPN can use an hub RT and a
   spoke RT internally to an autonomous-system.  The spoke RT does not
   have meaning outside this AS and so it may be stripped at an external
   border router.  The same policy rules that result in extended
   community filtering can be applied to RT membership information in
   order to avoid advertising an RT membership NLRI for the spoke-RT in
   the example above.

   Throughout this document, we assume that autonomous-systems agree on
   an RT assignment convention.  RT translation at the external border
   router boundary, is considered to be a local implementation decision,
   as it should not affect inter-operability.

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8.  Security Considerations

   This document does not alter the security properties of BGP-based
   VPNs.  However it should be taken into consideration that output
   route filters built from RT membership information NLRI are not
   intended for security purposes.  When exchanging routing information
   between separate administrative domains, it is a good practice to
   filter all incoming and outgoing NLRIs by some other means in
   addition to RT membership information.  Implementations SHOULD also
   provide means to filter RT membership information.

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9.  Acknowledgments

   This proposal is based on the extended community route filtering
   mechanism defined in [4].

   Ahmed Guetari was instrumental in defining requirements for this
   proposal.

   The authors would also like to thank Yakov Rekhter, Dan Tappan, Dave
   Ward, John Scudder, and Jerry Ash for their comments and suggestions.

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10.  References

10.1  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [2]  Bates, T., Chandra, R., and E. Chen, "BGP Route Reflection - An
        Alternative to Full Mesh IBGP", RFC 2796, April 2000.

   [3]  Rosen, E., "BGP/MPLS IP VPNs", draft-ietf-l3vpn-rfc2547bis-03
        (work in progress), October 2004.

   [4]  Chen, E. and Y. Rekhter, "Cooperative Route Filtering Capability
        for BGP-4", draft-ietf-idr-route-filter-11 (work in progress),
        December 2004.

   [5]  Rekhter, Y., "A Border Gateway Protocol 4 (BGP-4)",
        draft-ietf-idr-bgp4-26 (work in progress), October 2004.

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

   [7]  Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
        Communities Attribute", draft-ietf-idr-bgp-ext-communities-08
        (work in progress), February 2005.

   [8]  Sangli, S., Rekhter, Y., Fernando, R., Scudder, J., and E. Chen,
        "Graceful Restart Mechanism for BGP", draft-ietf-idr-restart-10
        (work in progress), June 2004.

10.2  Informative References

   [9]  Kompella, K. and Y. Rekhter, "Virtual Private LAN Service",
        draft-ietf-l2vpn-vpls-bgp-05 (work in progress), April 2005.

Authors' Addresses

   Pedro Marques
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   US

   Email: roque@juniper.net

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   Ronald Bonica
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   US

   Email: rbonica@juniper.net

   Luyuan Fang
   AT&T
   200 Laurel Avenue, Room C2-3B35
   Middletown, NJ  07748
   US

   Email: luyuanfang@att.com

   Luca Martini
   Cisco Systems, Inc.
   9155 East Nichols Avenue, Suite 400
   Englewood, CO  80112
   US

   Email: lmartini@cisco.com

   Robert Raszuk
   Cisco Systems, Inc.
   170 West Tasman Dr
   San Jose, CA  95134
   US

   Email: rraszuk@cisco.com

   Keyur Patel
   Cisco Systems, Inc.
   170 West Tasman Dr
   San Jose, CA  95134
   US

   Email: keyupate@cisco.com

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   Jim Guichard
   Cisco Systems, Inc.
   300 Beaver Brook Road
   Boxborough, MA  01719
   US

   Email: jguichar@cisco.com

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