Users Guide

Configure 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 A.B.C.D format. 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 can connect to many areas in an AS and is considered a member of each
area it connects to—shown as Router H in the example.
Autonomous
system border
router
The autonomous system border router (ASBR) connects to more than one AS and exchanges information
with the routers in other ASs. The ASBR connects to a non-IGP such as BGP or uses static routes—
shown as Router N in the example.
Layer 3 927