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.