The design rules of OSPF state that every area must connect back to area 0. In the case where this not possible, OSPF virtual links (tunnels) can be configured so OSPF communication can be tunneled across a transit area so that an area that is not physically connected to area 0 can be joined. In the example below; area 1 would be the transit area between 0 and 2. The virtual link is formed using the router ID’s of the ABR’s of the areas concerned:
R4 (A0) ——- (A0) R3 (A1) —— (A1) R2) (A2) ——|
Command on router 3:
router ospf 1
area 1 virtual-link 2.2.2.2
Command on router 2:
router ospf 1
area 1 virutal-link 3.3.3.3
Result:
*Nov 10 12:15:32.883: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on OSPF_VL0 from LOADING to FULL, Loading Done
