OSPF Virtual Links
In OSPF there is a basic requirement while designing a network with multiple areas that every area is directly connected to backbone area 0. Virtual link comes when some area e.g. area-2 in figure is not directly connected to area 0. There is area-1 in between area-2 and backbone area-0, so in this case communication between area-0 and area-2 is not possible. To make communication between these areas you need to create virtual-links.
How to create OSPF Virtual-links:
For creating virtual- links you need to connect the area which is not directly connect to area-0 to area which is directly connected to area-0. In figure you can see that Area-2 is not directly connect to area-0 to connect this area-2 to area-0 you need to create a virtual link between area-1 & area-2 by using commands as
R2(config)#router ospf 1
R2(config-rtr)#area 1 virtual-link 126.96.36.199
R3(config)#router ospf 1
R3(config-rtr)#area 1 virtual-link 188.8.131.52
Where 184.108.40.206 and 220.127.116.11 are router IDs.
OSPF Virtual link configuration Example
Follow this link for OSPF virtual link configuration example on Cisco router.