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Keywords: Border, Gateway, Protocol, Routing
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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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.
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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), 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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