iBGP vs eBGP: What is the Difference?
BGP operates in two modes: iBGP (internal BGP) for peering within the same autonomous system, and eBGP (external BGP) for peering between different autonomous systems. Understanding the distinction is fundamental to network design and troubleshooting.
Como Funciona
eBGP peers are typically directly connected and run between different ASNs, exchanging routing information across the internet. They automatically decrement TTL and change the NEXT_HOP attribute. iBGP peers can be anywhere within the same AS and are used to distribute externally learned routes internally. iBGP does not change AS_PATH (preventing loops using the iBGP split-horizon rule: routes learned via iBGP are not re-advertised to other iBGP peers). This requires either a full iBGP mesh or a route reflector/confederation design.
Por Que Importa
Misunderstanding iBGP vs eBGP leads to routing black holes and reachability issues. In large ISP networks, iBGP design (route reflectors, confederations) is critical for scalability. The iBGP split-horizon rule is a common source of bugs when engineers accidentally create iBGP loops.
Problemas Comuns
- iBGP routes not propagating due to missing full mesh or route reflector
- NEXT_HOP not reachable for iBGP sessions (requires IGP reachability)
- Routing loops from misconfigured iBGP confederations
- Asymmetric routing due to different LOCAL_PREF between iBGP peers