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Chapter 7
BGP's Basic Components
F I G U R E 7 . 5
iBGP information exchange
Configuring BGP will be covered in Chapter 8.
eBGP
The external Border Gateway Protocol (eBGP) is used to exchange route
information between different autonomous systems. When only one link
connects two autonomous systems, the IP addresses of the connected inter-
faces are used to establish a BGP session between the two.
You can use any other IP address on the interfaces, but the address must
be reachable without using an IGP. You can use a static route or a few other
commands, which will be discussed in the Chapter 9. If multiple links are
used to connect to the other autonomous systems, then using a loopback
addresses is your best option.
Outside of each AS, eBGP is used to inject routes owned by one AS
through the enterprise network and into another AS. Two prerequisites need
to be met for internal routes to be propagated via BGP:
In order for a router to advertise routes to BGP, the route must exist
in an IGP's routing table on the router.
The BGP must be able to learn the route.
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