How do ISPs and their customers communicate with one another?

• A. The ISP and customer use an interior gateway protocol such as OSPF or EIGRP to communicate within their own network. Any communication between these two however is accomplished via BGP.
• B. ISPs and customers communicate exclusively via static routes.
• C. BGP is used inside each network instead of an interior gateway protocol.
• D. Customers connect directly to the Internet without any routing protocols.

Answer: (C)

Inter-domain routing between an ISP and a customer is driven by BGP. This protocol is designed to exchange reachability information and apply policies across autonomous systems, so the path to reach the customer’s networks (and to reach the Internet from the customer) is learned and controlled at the border between the two networks. Inside each network, routers still need to distribute knowledge about internal destinations and reachability to the BGP next-hops, which is typically handled by an interior gateway protocol. In some designs, BGP can be used inside a network as well (iBGP) to propagate routes, but the critical function that enables the ISP and its customer to exchange routes and enforce policies across their boundary is BGP.