Reference Guide
Areas allow you to further organize routers within the AS with one or more areas within the AS. Areas are valuable in that they allow
subnetworks to hide within the AS—minimizing the size of the routing tables on all routers. An area within the AS may not see the details
of another area’s topology. An area number or the router’s IP address identies AS areas.
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 is responsible for distributing routing information between areas. It consists of all area border routers, networks not wholly
contained in any area and their attached routers.
The backbone is the only area with a default area number. You congure all other areas Area ID. If you congure 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.
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.
Layer 3
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