3Com Switch 7750 Configuration Guide

36
BGP CONFIGURATION
BGP Overview
Introduction to BGP Border gateway protocol (BGP) is a dynamic routing protocol designed to be used
between autonomous systems (AS). An AS is a group of routers that adopt the
same routing policy and belong to the same technical management department.
Four versions of BGP exist: BGP-1 (described in RFC1105), BGP-2 (described in
RFC1163), BGP-3 (described in RFC1267), and BGP-4 (described in RFC1771). As
the actual internet exterior routing protocol standard, BGP-4 is widely employed
between internet service providers (ISP).
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Unless otherwise noted, BGP in the following sections refers to BGP-4.
BGP is featured by the following.
■ Unlike interior gateway protocols (IGP) such as OSPF (open shortest path first),
RIP (routing information field), and so on, BGP is an exterior gateway protocol
(EGP). It does not focus on discovering and computing routes but controlling
the route propagation and choosing the optimal route.
■ BGP uses TCP as the transport layer protocol (with the port number being 179)
to ensure reliability.
■ BGP supports classless inter-domain routing (CIDR).
■ With BGP employed, only the changed routes are propagated. This saves
network bandwidth remarkably and makes it feasible to propagate large
amount of route information across the Internet.
■ The AS path information used in BGP eliminates routing loops thoroughly.
■ In BGP, multiple routing policies are available for filtering and choosing routes
in a flexible way.
■ BGP is extendible to allow for new types of networks.
In BGP, the routers that send BGP messages are known as BGP speakers. A BGP
speaker receives and generates new routing information and advertises the
information to other BGP speakers. When a BGP speaker receives a route from
other AS, if the route is better than the existing routes or the route is new to the
BGP speaker, the BGP speaker advertises the route to all other BGP speakers in the
AS it belongs to.
A BGP speaker is known as the peer of another BGP speaker if it exchanges
messages with the latter. A group of correlated peers can form a peer group.
BGP can operate on a router in one of the following forms.