An OSPF virtual link is used for two primary purposes:
* Linking an area that does not have a physical connection to the backbone (Area 0): In cases where it is not possible to have a direct physical connection to the backbone area (Area 0), a virtual link allows an OSPF router in a non-backbone area to connect to the backbone through a transit area. This transit area must have full routing information and cannot be a stub area.
* Patching the backbone in case of discontinuity of Area 0: If there is a partitioned backbone (Area 0) due to network changes or other reasons, a virtual link can be used to connect the two parts of the partitioned backbone through a non-backbone area.
In summary, an OSPF virtual link helps maintain connectivity between areas when physical connections are not feasible or when the backbone itself is partitioned12345.
References:
* Cisco Support: Configure OSPF Connection in a Virtual Link Environment
* Network Engineering Stack Exchange: Other Uses of OSPF Virtual-Link
* CiscoZine: OSPF Virtual Link
* Orhan Ergun: OSPF Virtual Link
* Cisco: Understand OSPF Areas and Virtual Links