VIRTUAL LINKS AND BACKBONES
OSPF requires that all areas be connected together by a backbone area (designated as area 0). Usually, a backbone is considered as a set of contiguous nodes and their interconnecting links that allow the tertiary networks (or areas) to communicate through the backbone with each other. The idea of a backbone forces a simple hub topology wherein the area's routing information goes through the backbone to get to another area. Conventional distance vector protocols can operate with this arrangement.
However, this arrangement might be too restrictive, since it implies that the areas attached to the backbone must be stub areas. Therefore, OSPF supports the use of virtual links in which routing packets can be sent through ...
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