BGP within autonomous systems is achieved by which type of BGP?

• A. iBGP
• B. eBGP
• C. MBGP
• D. BGP

Answer: (A)

Within an autonomous system, BGP is used as iBGP to exchange routing information among routers that share the same AS number. This keeps all devices inside the same administrative domain and allows them to learn about routes to external networks learned from outside the AS. A key rule of iBGP is that routes learned from one iBGP neighbor are not automatically re-advertised to other iBGP peers, which is why a full mesh of iBGP sessions or the use of route reflectors is often required to ensure all internal routers see the same reachability.

This contrasts with eBGP, which is used for routing between different autonomous systems, where peers have different AS numbers. MBGP is an extension of BGP that adds support for multiple address families (like IPv6, VPNs, etc.), but it isn’t specifically the mechanism that defines internal versus external routing within an AS. Therefore, internal routing within an autonomous system is accomplished by iBGP.