14 Replies Latest reply: Oct 20, 2011 7:21 PM by Scott Morris - CCDE/4xCCIE/2xJNCIE RSS

    What do OSPF virtual links do?

    Mike Gannon


      What exactly do OSPF virtual links do?

      I've set up a lab on GNS where R4 and R6 have subnets in area 0 (172.16.x.0/24), and area 1 is the backbone across R5 (10.10.10.x/30).


      I was suprised to find that R4 still learned about routes on R6 without a virtual link. I thought only area 0 could be used as a transit network?


      I then configured a virtual link between R4 and R6 to see what the effect was on the routing table.

      I noticed that the only change appeared to be that R4 treated R6's networks as intra area routes (O) instead of inter area routes (O-IA), but there was no effect on the cost of the route (still 21).


      So all that virtual links do is allow routers to exchange type 1 LSAs when two routers use the same area ID, but the areas have no direct connection.

      The virtual link is not used for routing network traffic. Network traffic is still routed through two hops?

      Is that right.