What is iBGP and what is it used for?

• A. When BGP is run between routers within the same AS. It is run within an AS to exchange BGP information so that all internal BGP routers have the same BGP routing information about outside AS.
• B. When BGP is run between routers in different AS
• C. When BGP is used for external routing only
• D. When BGP is ran between routers within the same AS. It is run within an AS to exchange BGP information so that all internal BGP routers have the same BGP routing information about outside AS.

Answer: (D)

iBGP is BGP between routers inside the same AS, and its purpose is to distribute knowledge about routes to destinations outside that AS so all internal routers share a consistent view of how to reach networks in other ASes. This differs from eBGP, which runs between routers in different ASes and is how routes to external networks first enter the AS. The reason this option is best is that it correctly states the scope (within an AS) and the goal (to have internal routers know how to reach outside destinations). In practice, to ensure all internal routers learn the same external routes, iBGP peers are typically fully meshed or use route reflectors, because iBGP learned routes aren’t guaranteed to be re-advertised to other iBGP peers without those mechanisms.