The backbone consists of those networks not contained in any area, their attached routers, and those routers that belong to multiple areas. The backbone must be contiguous.
It is possible to define areas in such a way that the backbone is no longer contiguous. In this case the system administrator must restore backbone connectivity by configuring virtual links.
Virtual links can be configured between any two backbone routers that have an interface to a common non-backbone area. Virtual links belong to the backbone. The protocol treats two routers joined by a virtual link as if they were connected by an unnumbered point-to-point network. On the graph of the backbone, two such routers are joined by arcs whose costs are the intra-area distances between the two routers. The routing protocol traffic that flows along the virtual link uses intra- area routing only.
The backbone is responsible for distributing routing information between areas. The backbone itself has all of the properties of an area. The topology of the backbone is invisible to each of the areas, while the backbone itself knows nothing of the topology of the areas.