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:

        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.

   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.

   [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