Reference Guide

Figure 83. 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.
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.
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