In the situation when a new area is introduced that does not have a direct physical attachment to the backbone, a virtual link is used.
A virtual link provides a logical path between the ABR of the disconnected area and the ABR of the normal area that connects to the backbone. A virtual link must be established between two ABRs that have a common area, with one ABR connected to the backbone. Virtual Link Using Area 1 as a Transit Area illustrates a virtual link.
NoteVirtual links cannot be configured through a stub or NSSA area.
Virtual links are also used to repair a discontiguous backbone area. For example, in Virtual Link Providing Redundancy, if the connection between ABR 1 and the backbone fails, the connection using ABR 2 provides redundancy so that the discontiguous area can continue to communicate with the backbone using the virtual link.