Routers within an AS advertise updates internally and only to each other. However, routers on the edge of the AS must communicate both with routers inside their AS and with routers external to their AS, often running a different routing protocol. These routers are called Area Border Routers (ABRs) or edge routers. Often ABRs run multiple routing protocols to be able to redistribute traffic between different ASes that are running different protocols, such as the edge between an ISP’s IS-IS routing network and a large company’s OSPF network.

OSPF defines ABRs differently from other routers. In OSPF, an ABR is an OSPF router that connects another AS to the backbone AS, and is a member of all the areas it connects to. An OSPF ABR maintains a LSA database for each area that it is connected to. The concept of the edge router is present, but its the edge of the backbone instead of the edge of the OSPF supported ASes.