OSPF Virtual Links: Bane or Benefit?
OSPF includes the concept of virtual links. Virtual links mend the OSPF backbone area (area 0). The backbone should never be intentionally partitioned, but if partitioning occurs, consider using a virtual link to temporarily repair the backbone area. Virtual links are logical connections that are vaguely analogous to a tunnel. Virtual links are not a true tunnel in the sense that one protocol is encapsulated by another. You can use a virtual link in the following cases:
Area 0 becomes partitioned
A nonbackbone area does not have a physical connection to area 0
Accepted network design theory considers the use of virtual links a result of a poorly designed backbone or network.