On an OSPF network, a switch that acts as an ABR must connect directly to the backbone. If no physical connection is available, you can automatically or manually configure a virtual link.
An automatic virtual link can provide redundancy support for critical network connections. Automatic virtual linking creates virtual paths for vital traffic paths in your OSPF network. If a connection fails on the network, such as after an interface cable that provides connection to the backbone (either directly or indirectly) disconnects from the switch, the virtual link is available to maintain connectivity.
Use automatic virtual linking to ensure that a link is created to another router. If automatic virtual linking uses more resources than you want to expend, creating a manual virtual link can be the better solution. Use this approach to conserve resources and control virtual links in the OSPF configuration.
On the switch, OSPF behavior follows OSPF standards; the router cannot learn OSPF routes through an ABR unless the ABR connects to the backbone or through a virtual link.
The following figure shows how to configure a virtual link between the ABR in area 2.2.2.2 and the ABR in area 0.0.0.0.
To configure a virtual link between the ABRs in area 1 and area 3, define area 2 as the transit area between the other two areas, and identify R2 as the neighbor router through which R2 must send information to reach the backbone through R1.