OS10 Enterprise Edition User Guide Release 10.4.0E(R3)


Areas, networks, and neighbors

The backbone of the network is Area 0, also called Area, 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 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.

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