I just did a lab last night and got confused with the message I got from OSPF. Here's the setup.
The link between R1 and R2 is area 0 and the link between R2 and R5 is area 1. R5 and R4 segment is area 2. The task mentioned that I can use another IP subnet to connect area 2 to the backbone so it gave me the idea that I should be using tunnel interface. I created a tunnel interface between R2 and R5 and put it in area 0. The routes propagated properly but on R5, i keep on receiving this message on the console.
Apr 8 12:15:31.011: %OSPF-4-ERRRCV: Received invalid packet: mismatch area ID, from backbone area must be virtual-link but not found from 188.8.131.52, Serial0/0
R1 and R2 segment is a frame-relay link. That is the 184.108.40.206 IP. The routing table is stable but I just wonder why do I keep on getting these message. I tried to change the tunnel interface to area 2 but I still receive the same error message. Thanks in advance.
Oh wait...I checked out the solution guide and the one I did was correct. It didn't use virtual-link because area 1 is an NSSA.