Virtual Links

Sometimes the backbone area becomes divided, or an area loses contact with the backbone (generally, when a network outage has occurred). For these situations, the designers of OSPF provided the virtual link. The virtual link acts as a tunnel, allowing traffic to traverse to and from the backbone area to pass through another area.

Router A in Figure B-9 has gone down, effectively partitioning area 1 from the rest of the network, making it unreachable. The network administrator could, by configuring a virtual link between Routers B and C across the backup link, make area 1 accessible until Router A is repaired and restored to service.

One of the most confusing aspects of configuring virtual links is the mysterious area number included in the command. This is not the area you are trying to reach or repair, but rather the area through which the virtual link passes.