The backbone area (area 0.0.0.0) must be contiguous and all other areas must be connected to the backbone area. If it is not practical to connect an area to the backbone then the area border routers must be connected via a virtual link. The two area border routers form a point-to-point-like adjacency across the transit area. A virtual link can only be configured while in the area 0.0.0.0 context.
The router-id parameter specified in the virtual-link command must be associated with the virtual neighbor, that is, enter the virtual neighbor router ID, not the local router ID. The transit area cannot be a stub area or an NSSA.
Use the following syntax to configure stub areas.
ospf ospf-instance
area area-id
virtual-link router-id transit-area area-id
authentication-key [authentication-key | hash-key] [hash]
authentication-type [password | message-digest]
dead-interval seconds
hello-interval seconds
message-digest-key key-id md5 [key | hash-key] [hash|hash2]
retransmit-interval seconds
transit-delay
no shutdownA:ALA-49>config>router>ospf# info
----------------------------------------------
asbr
overload
overload-on-boot timeout 60
traffic-engineering
export "OSPF-Export"
graceful-restart
helper-disable
exit
area 0.0.0.0
virtual-link 1.2.3.4 transit-area 1.2.3.4
hello-interval 9
dead-interval 40
exit
exit
area 0.0.0.20
stub
exit
exit
area 0.0.0.25
nssa
exit
exit
area 1.2.3.4
exit
----------------------------------------------
A:ALA-49>config>router>ospf#