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 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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. This Internet-Draft will expire on December 24, 2005. Copyright Notice Copyright (C) The Internet Society (2005). Abstract This document defines MP-BGP procedures that allow BGP speakers to Marques, et al. Expires December 24, 2005 [Page 1] Internet-Draft rt-constrain June 2005 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 Marques, et al. Expires December 24, 2005 [Page 2] Internet-Draft rt-constrain June 2005 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]. Marques, et al. Expires December 24, 2005 [Page 3] Internet-Draft rt-constrain June 2005 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. Marques, et al. Expires December 24, 2005 [Page 4] Internet-Draft rt-constrain June 2005 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. Marques, et al. Expires December 24, 2005 [Page 5] Internet-Draft rt-constrain June 2005 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 Marques, et al. Expires December 24, 2005 [Page 6] Internet-Draft rt-constrain June 2005 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 Marques, et al. Expires December 24, 2005 [Page 7] Internet-Draft rt-constrain June 2005 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 Marques, et al. Expires December 24, 2005 [Page 8] Internet-Draft rt-constrain June 2005 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. Marques, et al. Expires December 24, 2005 [Page 9] Internet-Draft rt-constrain June 2005 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. Marques, et al. Expires December 24, 2005 [Page 10] Internet-Draft rt-constrain June 2005 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. Marques, et al. Expires December 24, 2005 [Page 11] Internet-Draft rt-constrain June 2005 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. Marques, et al. Expires December 24, 2005 [Page 12] Internet-Draft rt-constrain June 2005 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. Marques, et al. Expires December 24, 2005 [Page 13] Internet-Draft rt-constrain June 2005 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. Marques, et al. Expires December 24, 2005 [Page 14] Internet-Draft rt-constrain June 2005 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. Marques, et al. Expires December 24, 2005 [Page 15] Internet-Draft rt-constrain June 2005 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 Marques, et al. Expires December 24, 2005 [Page 16] Internet-Draft rt-constrain June 2005 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 Marques, et al. Expires December 24, 2005 [Page 17] Internet-Draft rt-constrain June 2005 Jim Guichard Cisco Systems, Inc. 300 Beaver Brook Road Boxborough, MA 01719 US Email: jguichar@cisco.com Marques, et al. Expires December 24, 2005 [Page 18] Internet-Draft rt-constrain June 2005 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. 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Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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. Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Marques, et al. Expires December 24, 2005 [Page 19]