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OSI Layer 3: Routed and Routing Protocols

F I G U R E 2 . 1 8

iBGP information exchange

eBGP (External Border Gateway Protocol)

External BGP is used to exchange route information between two different
autonomous systems. When only one link connects two autonomous sys-
tems, the IP address of the connected interface is used to establish the BGP
session. It is also possible to use other IP addresses, but the address must be
reachable without using an IGP. This is accomplished by using static routes
or additional BGP commands when configuring eBGP. If multiple links are
used to connect to the other AS, using loopback addresses is the best option.

The purpose of eBGP is to inject routes owned by the enterprise network

into another AS. Two prerequisites need to be met for internal routes to be
propagated via BGP:

The route to be advertised must be present in the router's IGP route
table.

BGP must learn the route.

You can fulfill the first condition by using one of three methods: inject the

routes into a router's route table via an IGP, a static route, or directly con-
nected networks. BGP has a synchronization option that requires BGP and
the IGP routes to synchronize before BGP will advertise IGP learned net-
works. The no synchronization command indicates that BGP and the IGP
do not have to synchronize before BGP advertises the routes.

Router A networks sent to Router C
Router C networks sent to Router A

Router A networks sent to Router B
Router B networks sent to Router A

Router B networks sent
to Router C
Router C networks sent
to Router B

iBGP peers

FDDI

ring

Router A

Router C

AS500

Router B

iBGP peers

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