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.
Border Gateway Protocol v4 (BGP-4)  is an inter-Autonomous System
routing protocol. It is built on experience gained with BGP as
defined in RFC-1267  and BGP usage in the connected Internet as
described in RFC-1268 .
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 .
The management information base has been defined  and security
considerations are discussed in the protocol definition document .
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  or RIP .
Four vendors have developed independent implementations at the time
of this memo:
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RFC 1656 BGP-4 Implementation July 1994
The complete interoperability matrix between all known
implementations of various versions of BGP is available under
separate cover .
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.
All implementations, are able to carry and exchange network
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.
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