How was BGP originally intended to be run?

• A. Within the core of the AS with border routers ignoring BGP
• B. Only EBGP is used between adjacent routers
• C. BGP updates are distributed via multicast to all routers
• D. Along the borders of the AS with the routers in the middle of the AS ignoring the existence of BGP

Answer: (D)

BGP was designed as a protocol to exchange reachability and policies between autonomous systems, so it was intended to run at the borders of an AS. The routers inside an AS would use an interior gateway protocol to reach the border routers, while the border routers speak BGP with external neighbors. This separation keeps BGP’s complex inter-domain routing at the edges and lets the interior use a faster, simpler IGP to reach those edge devices. In practice, iBGP is used inside an AS to spread the external routes learned from EBGP among the border routers, but the original idea centers on BGP operating along the borders, with interior routers not running BGP. The other options don’t fit because they describe multicast updates, running EBGP everywhere inside the AS, or interior routers ignoring BGP in a way that doesn’t reflect this edge-focused design.