What is ABR?

An ABR (Area Border Router) is an OSPF router that connects the backbone, area 0, to one or more other areas and passes routing information between them.

OSPF areas exist to keep the link-state database small: routers only flood and store full topology for the areas they belong to. An ABR sits on the boundary, so it runs a separate LSDB and a separate SPF calculation for each of its areas. After working out the best paths inside one area, it describes the resulting prefixes to its other areas as Type 3 (Summary) LSAs. Those LSAs carry a prefix and a cost, not the links and routers behind it, so a router in another area knows what is reachable through the ABR without knowing the internal shape of the area. The ABR also originates Type 4 LSAs so other areas can find an ASBR, and in stub and NSSA areas it can inject a default route instead of the full set of external routes.

The ABR is also where you summarise. With an area ... range statement it can advertise one aggregate, such as 198.51.100.0/24, in place of many smaller subnets, which hides flapping links inside the area from everyone else. Two rules catch people out: all inter-area traffic must pass through area 0, so an ABR needs at least one interface in the backbone, and a router with interfaces in two non-backbone areas only is not a working ABR. Inter-area routes appear in the routing table marked IA. Don't confuse an ABR with an ASBR, which brings in routes from outside OSPF altogether; one router can be both. The OSPF Fundamentals lesson covers areas, ABRs and every LSA type in detail.