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Three Cisco routers form a triangle with two paths from router one to router three.
Both OSPF neighbors start in the Full state.
The route to the remote loopback uses the direct link because it has the lower cost.
A ping from the local loopback confirms the starting path.
Change the area on one end of the direct link to create a mismatch.
The direct adjacency drops, while the neighbor on the backup path stays Full.
The direct neighbor still replies to a ping, so the link itself is up.
The route to the remote loopback now points through router two.
Ping the remote loopback again to verify reachability over the backup path.
Set the interface back to area zero and watch both neighbors return to Full.
The lower cost route returns, and the remote loopback still replies.