Virtual links are used to restore/increase connectivity of the backbone. Virtual links may be configured between any pair of area border routers having interfaces to a common (non-backbone) area. The virtual link appears as an unnumbered point-to-point link in the graph for the backbone. The virtual link must be configured in both of the area border routers.
A virtual link appears in router links advertisements (for the backbone) as if it were a separate router interface to the backbone. As such, it has all of the parameters associated with a router interface (see Section C.3). Although a virtual link acts like an unnumbered point-to-point link, it does have an associated IP interface address. This address is used as the IP source in OSPF protocol packets it sends along the virtual link, and is set dynamically during the routing table build process. Interface output cost is also set dynamically on virtual links to be the cost of the intra-area path between the two routers. The parameter RxmtInterval must be configured, and should be well over the expected round-trip delay between the two routers. This may be hard to estimate for a virtual link; it is better to err on the side of making it too large. Router Priority is not used on virtual links.
A virtual link is defined by the following two configurable parameters: the Router ID of the virtual link's other endpoint, and the (non-backbone) area through which the virtual link runs (referred to as the virtual link's Transit area). Virtual links cannot be configured through stub areas.