Users Guide

Areas, networks, and neighbors
The backbone of the network is Area 0, also called Area 0.0.0.0, the core of any AS. All other areas must connect to Area 0. An
OSPF backbone distributes routing information between areas. It consists of all area border routers and networks not wholly
contained in any area and their attached routers.
The backbone is the only area with a default area number. You configure all other areas Area ID. If you configure two
nonbackbone areas, you must enable the B bit in OSPF. Routers, A, B, C, G, H, and I are the backbone, see Autonomous system
areas.
● A stub area (SA) does not receive external route information, except for the default route. These areas do receive
information from interarea (IA) routes.
● A not-so-stubby area (NSSA) can import AS external route information and send it to the backbone as type-7 LSA.
● Totally stubby areas are also known as no summary areas.
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.
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