Users Guide

Figure 90. Autonomous System Areas
Area Types
The 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 a link 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, networks not
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 information from
inter-area (IA) routes.
NOTE:
Configure all routers within an assigned stub area as stubby, and 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.
• A not-so-stubby area (NSSA) can import AS external route information and send it to the backbone. It cannot receive external AS
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.
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Open Shortest Path First (OSPFv2 and OSPFv3)