About Bgp - Motorola BSR 2000 Configuration And Management Manual

BSR 2000 Configuration and Management Guide

About BGP

BGP, an Exterior Gateway Protocol (EGP) allows you to set up an inter-domain
routing system that automatically guarantees the loop-free exchange of routing
information between autonomous systems (ASs). An AS is a set of routers that use a
single routing policy running under a single technical administration. An AS runs
Interior Gateway Protocols (IGPs) such as Routing Information Protocol (RIP), and
Open Shortest Path First (OSPF) within its boundaries.
BGP is used by almost all routers to connect ASs to network backbones. With BGP,
each route comprises a network number, a list (AS path) of ASs that information
passed through, and a list of other path attributes. A BGP system exchanges network
reachability information with other BGP systems, including AS path information.
This information allows routing loops to be pruned and AS-level policy decisions to
be enforced.
Routers that belong to the same AS and exchange BGP updates run internal BGP
(IBGP). Routers that belong to different ASs and exchange BGP updates run external
BGP (EBGP). With few exceptions, the commands for configuring IBGP and EBGP
are identical.
routers.
BGP minimizes routing traffic outside ASs and manages the peer relationship
between border routers that connect ASs within a backbone of the network
infrastructure. IGPs concentrate on finding the shortest or quickest route between
endpoints within an AS; BGP is typically used between ASs. The Multi-Exit
Discriminator (MED) metric attribute value is configured using route maps. Updates
sent to an
the same AS to make a consistent path selection.
BGP supports classless inter-domain routing (CIDR). This allows reduction of the
routing table sizes by creating aggregate routes, which result in supernets. CIDR
eliminates the concept of network classes within BGP and supports the advertising of
IP prefixes. CIDR routes are also carried by OSPF and RIP.
12-2
Figure 12-1 shows exchanges with IBGP and EBGP running between
IBGP
peer also include unchanged MED information, enabling all peers in
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