Figure 101. Autonomous System AreasArea TypesThe backbone of the network is Area 0. It is also called Area 0.0.0.0 and is the core of any AS.All other areas must connect to Area 0. Areas can be defined in such a way that the backbone is not contiguous. In this case,backbone connectivity must be restored through virtual links. Virtual links are configured between any backbone routers that share alink to a non-backbone area and function as if they were direct links.An OSPF backbone is responsible for distributing routing information between areas. It consists of all area border routers, networksnot wholly contained in any area, and their attached routers.NOTE: If you configure two non-backbone areas, then you must enable the B bit in OSPF.The backbone is the only area with a default area number. All other areas can have their Area ID assigned in the configuration.In the previous example, Routers A, B, C, G, H, and I are the Backbone.• A stub area (SA) does not receive external route information, except for the default route. These areas do receive informationfrom inter-area (IA) routes.NOTE: Configure all routers within an assigned stub area as stubby, and not generate LSAs that do not apply. Forexample, a Type 5 LSA is intended for external areas and the Stubby area routers may not generate external LSAs. Avirtual link cannot traverse stubby areas.• A not-so-stubby area (NSSA) can import AS external route information and send it to the backbone. It cannot receive externalAS information from the backbone or other areas. However, a virtual link can traverse it.• Totally stubby areas are referred to as no summary areas in the Dell Networking OS.Open Shortest Path First (OSPFv2 and OSPFv3) 577
