System information

BSR 64000 Configuration and Management Guide
12-2
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. Figure 12-1 shows exchanges with IBGP and EBGP running between
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
IBGP peer also include unchanged MED information, enabling all peers in
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.
BGP uses AS path information to prevent routing loops. In BGP, each AS delineates
the route in the path. This enables routers to look for loops by examining the
information sent to them about the path. Figure 12-1 shows how BGP advertises
routes to neighbors in an AS path.
1. Router Boston originates a route to Router New York.
2. Router New York forwards the route to Router Los Angeles, after adding its AS
to the AS path.
3. Router Los Angeles receives the route and ascertains that it comes from another
AS.
Note: Set up your router with BGP if your AS or router is dual or multihomed
(if it has two or more backbone connections, either direct or daisy-chained
thorough another point of presence). Also, use BGP if the BSR provides IP
routing to a downstream site or customer or if you must preserve AS path
information in your network. If your site is single-homed and you do not
provide IP services or AS paths, consider using static routes instead.
Because external routing requirements are relatively simple, static routing is
easier to set up and to maintain, and it requires less overhead.