Backbone Areas

An OSPF backbone area consists of all networks in area ID 0.0.0.0, their attached routers, and all area border routers. The backbone itself does not have any area border routers. The backbone distributes routing information between areas. The backbone is simply another area, so the terminology and rules of areas apply: a router that is directly connected to the backbone is an internal router on the backbone, and the backbone’s topology is hidden from the other areas in the AS.

The routers that make up the backbone must be physically contiguous. If they are not, you must configure virtual links to create the appearance of backbone connectivity. You can create virtual links between any two area border routers that have an interface to a common nonbackbone area. OSPF treats two routers joined by a virtual link as if they were connected to an unnumbered point-to-point network.

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