Cisco CCNA (200-301)IP ConnectivityMedium
Two OSPF routers, R1 and R2, are connected via a serial link and remain stuck in the INIT state. R1 is configured with 'hello-interval 10' and 'dead-interval 40' (defaults), while R2 has been changed to 'hello-interval 5' and 'dead-interval 20'. What is the most likely cause of the adjacency failure?
- AThe subnet masks on the serial link do not match
- BHello and dead timer mismatches prevent the routers from reaching the FULL state
- CThe OSPF process IDs on R1 and R2 are different
- DR1 and R2 are in different OSPF areas
Show answer & explanationAnswer & explanation
Correct answer: B. Hello and dead timer mismatches prevent the routers from reaching the FULL state
OSPF requires matching hello and dead intervals between neighbors on the same link; mismatched timers cause neighbors to reject hello packets, preventing the adjacency from progressing beyond INIT/2-WAY toward FULL. Process IDs and areas (if matched elsewhere) are locally significant and don't need to match, and this scenario states the link is up, ruling out subnet mismatch as the described cause.
Why the other options are wrong
- A. Subnet mismatches typically prevent neighbors from forming at all (no hello received), not get stuck specifically at INIT with visible mismatched timers.
- C. OSPF process IDs are only locally significant and do not need to match between routers.
- D. Area mismatch would cause a distinct error but the scenario emphasizes timer differences.
OSPF Hello/Dead Timer Matching
Neighboring OSPF routers on the same segment must have identical hello and dead intervals or the adjacency will not form correctly.
- Default hello=10s/dead=40s on broadcast and point-to-point links
- Mismatched timers show as a debug/log error and adjacency stalls
- Process ID and router-id do not need to match between neighbors
Memory trick: Clocks must tick together or the handshake breaks