Reference Guide
Congure all routers within an assigned stub area as stubby and do not generate LSAs that do not apply. For example, a Type 5 LSA is
intended for external areas and the stubby area routers may not generate external LSAs. A virtual link cannot traverse stubby areas.
Networks and neighbors
As a link-state protocol, OSPF sends routing information to other OSPF routers concerning the state of the links between them. The up or
down state of those links is important. Routers that share a link become neighbors on that segment. OSPF uses the hello protocol as a
neighbor discovery and keepalive mechanism. After two routers are neighbors, they may proceed to exchange and synchronize their
databases, which creates an adjacency.
Router types
Router types are attributes of the OSPF process—multiple OSPF processes may run on the same router. A router connected to more than
one area, receiving routing from a BGP process connected to another AS, acts as both an area border router and an autonomous system
border router.
Each router has a unique ID, written in decimal format—A.B.C.D. You do not have to associate the router ID with a valid IP address. To
make troubleshooting easier, ensure the router ID is identical to the router’s IP address.
Backbone router A backbone router (BR) is part of the OSPF Backbone, Area 0, and includes all ABRs. The BR includes routers
connected only to the backbone and another ABR, but are only part of Area 0—shown as Router I in the example.
Area border router Within an AS, an area border router (ABR) connects one or more areas to the backbone. The ABR keeps a copy of
the link-state database for every area it connects to. It may keep multiple copies of the link state database. An ABR
summarizes learned information from one of its attached areas before it is sent to other connected areas. An ABR
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