12.1.6 Virtual Link Behavior
A virtual link must be configured in both routers that serve as the endpoints.
The routers at each end of the virtual link exchange OSPF packets over the virtual link and learn routing information for the backbone area.
Note the following when operating a virtual link:
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The cost of a virtual link is the cost of the route between the routers at the two endpoints over the transit area.
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The route for normal traffic might differ from that of routing information traffic over the virtual link if the route between the two endpoints over the transit area is an equal-cost multipath.
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(1) Connecting to neighboring routers
When the virtual link is active, hello packets are sent to the neighboring routers on the virtual link to detect connectivity. A virtual link becomes active when the transit area contains a path to the router at the other end of the virtual link.
A hello packet is the means by which a router recognizes other routers running OSPF.
Use the area virtual-link command to configure hello packets in the context of the virtual link. The dead-interval parameter must be longer than any interval value (as set by the ip ospf dead-interval command) set for an interface in any network that forms the route between the end points of the virtual link in the transit area. If you specify a shorter value and a network failure occurs that affects routes in the transit area, the virtual link might be shut down before the routers have time to switch to alternate routes.
The resending interval for LSAs (as set by the retransmit-interval parameter of the area virtual-link command) must comfortably exceed the expected round-trip time for packets between the endpoint routers of the virtual link.