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Network Working Group                                          P. TrainaRequest for Comments: 1656                                 cisco SystemsCategory: Informational                                        July 1994BGP-4 Protocol Document Roadmap and Implementation ExperienceStatus of this Memo   This memo provides information for the Internet community.  This memo   does not specify an Internet standard of any kind.  Distribution of   this memo is unlimited.Introduction   Border Gateway Protocol v4 (BGP-4) [1] is an inter-Autonomous System   routing protocol.  It is built on experience gained with BGP as   defined inRFC-1267 [2] and BGP usage in the connected Internet as   described inRFC-1268 [3].   The primary function of a BGP speaking system is to exchange network   reachability information with other BGP systems.  This network   reachability information includes information on the list of   Autonomous Systems (ASs) that reachability information traverses.   This information is sufficient to construct a graph of AS   connectivity from which routing loops may be pruned and some policy   decisions at the AS level may be enforced.   BGP-4 provides a new set of mechanisms for supporting classless   inter-domain routing.  These mechanisms include support for   advertising an IP prefix and eliminates the concept of network   "class" within BGP.  BGP-4 also introduces mechanisms which allow   aggregation of routes, including aggregation of AS paths.  These   changes provide support for the proposed supernetting scheme [4].   The management information base has been defined [5] and security   considerations are discussed in the protocol definition document [1].Applicability Statement for BGP-4   BGP-4 is explicitly designed for carrying reachability information   between Autonomous Systems.  BGP-4 is not intended to replace   interior gateway protocols such as OSPF [7] or RIP [6].Implementations   Four vendors have developed independent implementations at the time   of this memo:Traina                                                          [Page 1]

RFC 1656                  BGP-4 Implementation                 July 1994        ANS (gated)        Europanet        3COM        cisco   The complete interoperability matrix between all known   implementations of various versions of BGP is available under   separate cover [9].Implementation Testing   One implementation has been extensively tested in a network designed   to mirror the complex connectivity present at many major Internet   borders.  This network consists of multiple BGP-3 and BGP-4 speakers   carrying full routing information injected from Alternet, EBone,   Sprint, CERFnet, and cisco.  In many cases additional AS adjacencies   are simulated via the use of IP over IP tunnels to increase the   complexity of the routing topology.   The primary feature of BGP-4 is the ability to carry network   reachability information without regard to classfull routing.  In   addition to canonical routing information,  CIDR prefixes (both   supernets and subnets) are being injected from IGP information and   aggregated using the methods described in BGP-4.  AS set aggregation   and policy decisions based upon AS sets have been tested.   Secondary extensions incorporated as part of version 4 of this   protocol include enhancements to use of the INTER_AS_METRIC (now   called MULTI_EXIT_DISC), the addition of a LOCAL_PREF parameter to   influence route selection within an AS,  and a specified method of   damping route fluctuations.  All of these features have been tested   in at least one implementation.Observations   All implementations, are able to carry and exchange network   reachability information.   Not all implementations are capable of generating aggregate   information based upon the existence of more specific routes.   No implementation supports automatic deaggregation (enumeration of   all networks in an aggregate block for backwards compatibility with   routing protocols that do not carry mask information (e.g. BGP-3)).   However, most implementations do allow for staticly configured   controlled deaggregation for minimal backwards compatibility with   non-CIDR capable routers.Traina                                                          [Page 2]

RFC 1656                  BGP-4 Implementation                 July 1994   At least one implementation capable of running earlier versions of   BGP deliberately does not automaticly negotiate to earlier versions.   Connections to BGP-4 peers must be explicitly configured as such.Conclusions   The ability to carry and inject natural networks and CIDR supernets   is the immediate requirement for BGP-4.  The ability to carry subnet   information (useful when reassigning parts of class A networks to   organizations with different routing policies) is of secondary   concern.   The ability to conditionally aggregate routing information may be   worked around by injecting static or IGP network information into   BGP, or aggregation may be performed by an upstream router that is   capable.   Deaggregation is dangerous.  It leads to information loss and unless   tightly controlled by a manual mechanism,  will create a routing   information explosion.   Automatic version negotiation is dangerous due to the state-less   nature.  Given packet losses or spontaneous restarts,  it is possible   for two BGP peers capable of BGP-4 to negotiate a BGP-3 or BGP-2   connection,  which is incapable of carrying super/subnet reachability   information and AS set information.Acknowledgments   The author would like to acknowledge Yakov Rekhter (IBM) and Tony Li   (cisco) for their advice, encouragement and insightful comments.References   [1] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4),RFC1654, cisco Systems, T.J. Watson Research Center, IBM Corp., July       1994.   [2] Lougheed K., and Y. Rekhter, "A Border Gateway Protocol 3 (BGP-       3)",RFC 1267, cisco Systems, T.J. Watson Research Center, IBM       Corp., October 1991.   [3] Gross P., and Y. Rekhter, "Application of the Border Gateway       Protocol in the Internet",RFC 1268, T.J. Watson Research Center,       IBM Corp., ANS, October 1991.Traina                                                          [Page 3]

RFC 1656                  BGP-4 Implementation                 July 1994   [4] Fuller V., Li. T, Yu J., and K. Varadhan, "Supernetting: an       Address Assignment and Aggregation Strategy", Work in Progress.       [Note: This is an expired draft, and is also referred to in       BGP4.6.]   [5] Willis S., Burruss J., and J. Chu, "Definitions of Managed       Objects for the Border Gateway Protocol (Version 4) using SMIv2",RFC 1657, Wellfleet Communications Inc., IBM Corp., July 1994.   [6] Hedrick, C., "Routing Information Protocol",RFC 1058, Rutgers       University, June 1988.   [7] Moy J., "Open Shortest Path First Routing Protocol (Version 2)",RFC 1583, Proteon, March 1994.   [8] Varadhan, K., Hares S., and Y. Rekhter, "BGP4/IDRP for IP---OSPF       Interaction", Work in Progress, September 1993.   [9] Li, T., and P. Traina,"BGP Interoperabilty Matrix", Work in       Progress, November 1993.Security Considerations   Security issues are not discussed in this memo.Author's  Address   Paul Traina   cisco Systems, Inc.   1525 O'Brien Drive   Menlo Park, CA 94025   EMail: pst@cisco.comTraina                                                          [Page 4]