In network routing, BGP confederation is a method to use Border Gateway Protocol (BGP) to subdivide a single autonomous system (AS) into multiple internal sub-AS's, yet still advertise as a single AS to external peers. The intent is to reduce IBGP mesh size.
The confederated AS is composed of multiple ASs. Each confederated AS alone has iBGP fully meshed and has connections to other ASs inside the confederation. Even though these ASs have eBGP peers to ASs within the confederation, the ASs exchange routing as if they used iBGP. In this way, the confederation preserves next hop, metric, and local preference information. To the outside world, the confederation appears to be a single AS. From this solution, iBGP Transit AS problems can be resolved as iBGP requires a full mesh between all BGP routes: Large number of TCP sessions and Unnecessary duplication of routing traffic.
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