Virtual Links
There are times when the core area becomes divided, or an area loses contact with the core—generally, when there is some network outage. For these situations, the designers of OSPF provided the virtual link. The virtual link acts as a tunnel, allowing traffic that needs to traverse to and from the core area to pass through another area.
Router A in Figure A-10 has gone down for some reason, effectively partitioning area 1 from the rest of the network (making it unreachable). The network administrator could, by configuring a virtual link between Router C and Router B across the backup link, make area 1 accessible until Router A could be repaired and restored to service.
Note
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.
Virtual links are typically a sign of poor network design; rather than using them, you should evaluate your network design and attempt to eliminate them where you can.
Figure A-10. Virtual Links
