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Network Working Group P. Traina Request for Comments: 1656 cisco Systems Category: Informational July 1994

   BGP-4 Protocol Document Roadmap and Implementation Experience

Status 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 in RFC-1267 [2] and BGP usage in the connected Internet as
 described in RFC-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:

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      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.

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 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), RFC
     1654, 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.

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 [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.com

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