Network Working Group Pedro Marques (Ed.) Internet Draft Juniper Networks Expiration Date: March 2005 Robert Raszuk Luyuan Fang Luca Martini AT&T Keyur Patel Jim Guichard Ronald Bonica Cisco Systems Inc. MCI September 2004 Constrained VPN route distribution draft-ietf-l3vpn-rt-constrain-01.txt Status of this Memo By submitting this Internet-Draft, we certify that any applicable patent or other IPR claims of which we are aware have been disclosed, or will be disclosed, and any of which we become aware will be disclosed, in accordance with RFC 3668. 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. Marques, et al. [Page 1] Internet Draft draft-ietf-l3vpn-rt-constrain-01.txt September 2004 Abstract This document defines MP-BGP procedures that allow BGP speakers to 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 ............................. 2 2 Intellectual Property Statement ........................... 3 3 Introduction .............................................. 3 4 NLRI DIstribution ......................................... 4 4.1 Inter-AS VPN route distribution. .......................... 4 4.2 Intra-AS VPN route distribution ........................... 6 5 Route Target membership NLRI advertisements ............... 7 6 Capability Advertisement .................................. 8 7 Operation ................................................. 8 8 Deployment considerations ................................. 9 9 Security considerations ................................... 10 10 Acknowledgments ........................................... 10 11 Normative References ...................................... 10 12 Informative References .................................... 11 13 Author Information ........................................ 11 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 Marques, et al. [Page 2] Internet Draft draft-ietf-l3vpn-rt-constrain-01.txt September 2004 2. Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights 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; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assur- ances 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 implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- ipr@ietf.org. 3. Introduction In BGP/MPLS IP VPNs, PE routers use Route Target (RT) extended commu- nities 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 use route reflection [BGP-RR] 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 con- sideration. In order to limit the VPN routing information that is maintained at a given RR, RFC2547bis [RFC2547bis] suggests, in section 4.3.3., the usage of "Cooperative Route Filtering" [ORF] between route reflec- tors. This proposal extends [RFC2547bis] ORF work to include support for multiple autonomous systems, and asymetric VPN topologies such as hub-and-spoke. While it Marques, et al. [Page 3] Internet Draft draft-ietf-l3vpn-rt-constrain-01.txt September 2004 would be possible to extend the encoding currently defined for extended-community ORF in order to achieve this purpose, BGP itself already has all the necessary machinery for dissemination of arbi- trary 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] and on concept of cooperative route filtering by adding the ability to prop- agate Route Target membership information between iBGP meshes. 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 detec- tion. 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 distri- bution graph. In this model, VPN NLRI routing information flows in the inverse direction of Route Target membership information. This mechanism is applicable to any BGP NLRI that controls the dis- tribution of routing information based on Route Targets, such as BGP L2VPNs [L2VPN] and VPLS [VPLS]. 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 will be referred to as RT membership information for the purpose of the explanation in this document. 4. NLRI DIstribution 4.1. 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 repre- sents an arbitrary graph of autonomous systems (a through j) inter- connected in an ad-hoc fashion. The following discussion ignores the complexity of intra-AS route distribution. Marques, et al. [Page 4] Internet Draft draft-ietf-l3vpn-rt-constrain-01.txt September 2004 +----------------------------------+ | +---+ +---+ +---+ | | | 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). In order to achieve this we will rely on ASa and ASi generating a NLRI consisting of ( 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 selec- tion rules concerning as-path length (and other policy mechanisms) to prune duplicate paths in the RT membership information 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 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, lets assume (f j) is selected, would be sufficient to guarantee that VPN information flows to all VPN mem- ber ASes, the information concerning the path (f i) is necessary to prune the arc (e g h i) from the route distribution graph. Marques, et al. [Page 5] Internet Draft draft-ietf-l3vpn-rt-constrain-01.txt September 2004 As with other approaches for building distribution graphs, the bene- fits 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. 4.2. 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 con- taining an NLRI of the form . 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 [BGP-BASE] will allow a tran- sit AS to create the necessary flooding state. Consider a IPv4 NLRI prefix, sourced by a single AS, which dis- tributed via BGP within a given transit AS. BGP protocol rules guar- antee that a BGP speaker has a valid route that can be used for for- warding of data packets for that destination prefix, in the inverse path of received routing updates. By the same token, and given that a key provides uniqueness between several ASes that may be sourcing this route-tar- get, BGP route selection and advertisement procedures guarantee that a valid VPN route distribution path exists to the origin of the Route Target membership information advertisement. Route Target membership information that are originated within the autonomous-system however require more careful examination. Several PE routers within a given autonomous-system may source the the same NLRI , 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 availiable 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: Marques, et al. [Page 6] Internet Draft draft-ietf-l3vpn-rt-constrain-01.txt September 2004 -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 [BGP-BASE], 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 is 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 cor- rectly. For instance, route reflection clusters must be contiguous An alternative solution to the procedure given above would have been to source different routes per PE, such as NLRI of the form , 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. 5. 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 [BGP-MP]. The 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 lenght of NextHop address is 4 octects, and as a IPv6 address, whenever the lenght 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 [BGP-MP]. Marques, et al. [Page 7] Internet Draft draft-ietf-l3vpn-rt-constrain-01.txt September 2004 This prefix is structured as follows: +-------------------------------+ | origin as (4 octects) | +-------------------------------+ | route target (8 octects) | + + | | +-------------------------------+ 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 [BGP-EXTCOMM]. The default route target can be used to indicate to a peer the will- ingness to receive all VPN route advertisements such as, for instance, the case of route reflector speaking to one of its PE router clients. 6. Capability Advertisement A BGP speaker that wishes to exchange Route Target membership infor- mation must use the the Multiprotocol Extensions Capability Code as defined in [BGP-MP], 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. 7. 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 reevaluate the advertisement status of routes that Marques, et al. [Page 8] Internet Draft draft-ietf-l3vpn-rt-constrain-01.txt September 2004 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 Tar- get membership information. In order to avoid VPN route churn when a BGP session is established, implementations SHOULD generate an End-of-RIB marker, as defined in [BGP-GR], 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. 8. Deployment considerations This mecanism 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 impossed on a given reflector by dividing the PE routers present on its clus- ter 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. 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 cur- rent mechanism to limit the scope of VPN route flooding is through manual filtering on the EBGP border routers. With the current pro- posal such filtering can be performed based on the dynamic Route Tar- get 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 commu- nity 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. Marques, et al. [Page 9] Internet Draft draft-ietf-l3vpn-rt-constrain-01.txt September 2004 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. 9. 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 mean in addition to RT membership information. Implementations SHOULD also provide means to filter RT membership information. 10. Acknowledgments This proposal is based on the extended community route filtering mechanism defined in [ORF]. Ahmed Guetari was instrumental in defining requirements for this pro- posal. The authors would also like to thank Yakov Rekhter, Dan Tappan, Dave Ward, John Scudder, and Jerry Ash for their comments and suggestions. 11. Normative References [BGP-BASE] Y. Rekhter, T. Li, S. Hares, "A Border Gateway Protocol 4 (BGP-4)", draft-ietf-idr-bgp4-20.txt, 03/03 [BGP-RR] Bates, Chandra, and Chen, "BGP Route Reflection: An alternative to full mesh IBGP", RFC 2796. [BGP-CAP] R. Chandra, J. Scudder, "Capabilities Advertisement with BGP-4", RFC2842. [BGP-MP] T. Bates, R. Chandra, D. Katz, Y. Rekhter, "Multiprotocol Extensions for BGP-4", RFC2858. [BGP-GR] S. Sangli, Y. Rekhter, R. Fernando, J. Scudder, E. Chen, "Graceful Restart Mechanism for BGP", draft-ietf-idr-restart-10.txt, 06/04. Marques, et al. [Page 10] Internet Draft draft-ietf-l3vpn-rt-constrain-01.txt September 2004 12. Informative References [RFC2547bis] "BGP/MPLS VPNs", Rosen et. al., draft-ietf-ppvpn-rfc2547bis-03.txt, 10/02. [ORF] E. Chen, Y. Rekhter, "Cooperative Route Filtering Capability for BGP-4", draft-ietf-idr-route-filter-09.txt, 08/03. [BGP-EXTCOMM] S. Sangli, D. Tappan, Y. Rekhter, "BGP Extended Communities Attribute", draft-ietf-idr-bgp-ext-communities-05.txt, 05/02. [L2VPN] K. Kompella et al., "Layer 2 VPNs Over Tunnels", draft-kompella-ppvpn-l2vpn-02.txt, 11/01. [VPLS] K Kompella (Ed.), "Virtual Private LAN Service", draft-kompella-ppvpn-vpls-01.txt, 11/02 13. Author Information Ronald P. Bonica MCI 22001 Loudoun County Pkwy Ashburn, Virginia, 20147 Phone: 703 886 1681 Email: ronald.p.bonica@mci.com Luyuan Fang AT&T 200 Laurel Avenue, Room C2-3B35 Middletown, NJ 07748 Phone: 732-420-1921 Email: luyuanfang@att.com Luca Martini Cisco Systems, Inc. 9155 East Nichols Avenue, Suite 400 Englewood, CO, 80112 e-mail: lmartini@cisco.com Marques, et al. [Page 11] Internet Draft draft-ietf-l3vpn-rt-constrain-01.txt September 2004 Pedro Marques Juniper Networks 1194 N. Mathilda Ave. Sunnyvale, CA 94089 Email: roque@juniper.net Robert Raszuk Cisco Systems, Inc. 170 West Tasman Dr San Jose, CA 95134 Email: rraszuk@cisco.com Keyur Patel Cisco Systems, Inc. 170 West Tasman Dr San Jose, CA 95134 Email: keyupate@cisco.com Jim Guichard Cisco Systems, Inc. 300 Beaver Brook Road Boxborough, MA, 01719 Email: jguichar@cisco.com Marques, et al. [Page 12]