Users Guide

routing tables on all routers. An area within the AS may not see the details of another area’s topology. AS
areas are known by their area number or the router’s IP address.
Figure 101. 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.
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.
Open Shortest Path First (OSPFv2) 733