Areas allow you to further organize your routers within in the AS. One or more areas are required within the AS. Areas are valuable in that
they allow sub-networks to "hide" within the AS, thus minimizing the size of the 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 86. 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.
NOTE
: If you congure 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 conguration.
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
: Congure 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.
540 Open Shortest Path First (OSPFv2 and OSPFv3)
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