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eBGP vs iBGP: Inter-AS and Intra-AS Routing

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eBGP and iBGP: BGP in two different roles

BGP runs in two contexts: eBGP (between autonomous systems, like your network and your ISP) and iBGP (inside a single AS, distributing routes learned from eBGP peers to all routers internally).

The distinction matters because iBGP has a fundamental constraint — the split-horizon rule — that forces either a full mesh of sessions or a route reflector architecture. Understanding this explains why large ISPs have specific internal BGP topologies.

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