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.