Two IOS XE routers on a cloud GRE tunnel never become OSPF neighbors. Area, network type, and MTU match. The hello and dead timers do not. What must match?
Select an answer to reveal the explanation.
Short Explanation
The two routers are shouting on different schedules, so neither accepts the other. You make the hello and dead timers match on the tunnel. Area, network type, and MTU are already fine, and the IKE lifetime is not the hello timer.
Full Explanation
OSPF will not form an adjacency when the hello or dead intervals differ. These routers already agree on area, network type, and MTU, and they disagree on the timers, so the neighbor stays down. Set the same hello and dead values on both tunnel interfaces. The IKE lifetime is not copied into the OSPF hello. The BGP hold timer is unrelated, and BGP does not have to be shorter than the OSPF dead interval for hellos to work. Tunnel interface numbers are local labels. OSPF does not require them to match. Exam caveat: the dead interval is normally a multiple of the hello. If you match the hello and leave the dead different, the adjacency still fails. Set both. Operational check: show ip ospf interface on each tunnel should list the same hello and dead values, and the neighbor should reach Full.