iBGP

iBGP, or internal Border Gateway Protocol, is the version of BGP used for exchanging routing information between routers within the same autonomous system (AS). It is distinguished from eBGP, which operates between different ASes. All iBGP peers share the same AS number in their BGP configuration and establish TCP sessions on port 179 to exchange updates.

In an iBGP deployment, routes learned from eBGP are redistributed to iBGP peers, but routes learned from one iBGP neighbor are not by default advertised to other iBGP neighbors. This behavior, known as split-horizon, prevents routing loops but requires a full mesh of iBGP peers within the AS to ensure all routers learn external routes. If a full mesh is impractical, networks deploy route reflectors or confederations to reduce the number of iBGP sessions while preserving reachability.

Next-hop handling is an important consideration in iBGP. When a router advertises a route learned from eBGP to iBGP peers, the next-hop is preserved as the eBGP next-hop by default. Operators commonly configure next-hop-self on certain iBGP speakers to ensure all participants can reach the next hop, especially across multi-hop or non-directly connected topologies.

Common use cases for iBGP include distributing external routes learned from an eBGP border router to all internal routers, and maintaining a consistent view of external routing inside an AS. iBGP is typically combined with an interior routing protocol such as OSPF or IS-IS to ensure reachability between iBGP peers.