Which statement best describes BGP's role on the Internet?

• A. It is used only inside a single AS.
• B. It is the exterior gateway protocol that exchanges routing information between autonomous systems.
• C. It is a link-state protocol.
• D. It is not scalable.

Answer: (B)

BGP’s role on the Internet is to serve as the exterior gateway protocol that exchanges routing information between autonomous systems. The Internet is made up of many ASes, each running its own interior routing protocol to learn routes inside the AS. To reach destinations outside, an AS uses BGP to share reachability with neighboring ASes and to learn routes from them. This inter-domain routing, with policy controls and attributes like AS-Path, is what lets different networks connect and decide which paths to export or accept.

It isn’t limited to a single AS, and it isn’t a link-state protocol (it’s a path-vector protocol). BGP is designed to scale to the size of the global Internet, unlike the idea that it isn’t scalable. Inside an AS, other protocols such as OSPF or IS-IS handle internal routing, while BGP focuses on routing between ASes.