What is ASBR?

An ASBR (Autonomous System Boundary Router) is an OSPF router that brings routes from outside OSPF, such as static, connected or BGP routes, into the OSPF domain.

A router becomes an ASBR the moment you configure redistribution on it, for example redistribute static or redistribute connected, or when it originates a default route into OSPF. Each external prefix is described in its own Type 5 (AS-External) LSA, which is flooded to every normal area. The ASBR also sets a flag in its own router LSA to announce that it is a boundary router. Routers in other areas can't see the ASBR's router LSA directly, so each ABR generates a Type 4 (ASBR-Summary) LSA that tells its area how to reach the ASBR. External routes come in two flavours: E2, the usual default, keeps the external metric fixed wherever you are in the network, while E1 adds the internal cost of reaching the ASBR. Inside an NSSA, where Type 5 LSAs aren't allowed, the ASBR uses Type 7 LSAs instead and the ABR translates them.

Despite the name, an ASBR has nothing to do with BGP autonomous systems directly. "Autonomous system" here means the OSPF routing domain, and the boundary is simply the edge between OSPF and any other source of routes. An edge router that redistributes BGP into OSPF would be an ASBR, but most well-run networks avoid that and redistribute only a default route or a few static and connected prefixes, because pushing a full BGP table into an IGP floods enormous numbers of LSAs. Unfiltered redistribution in both directions between two protocols is a classic way to create routing loops. The OSPF Fundamentals lesson covers the ASBR alongside areas and every LSA type.