Cisco CCNA (200-301)IP ConnectivityMedium
A network engineer is troubleshooting an OSPF network. R1 and R2 are directly connected via a GigabitEthernet0/1 interface. The engineer configures `ip ospf hello-interval 5` on R1's GigabitEthernet0/1 interface. What command should the engineer configure on R2's directly connected interface to ensure OSPF adjacency forms successfully?
- A`ip ospf hello-interval 10`
- B`ip ospf dead-interval 5`
- C`ip ospf hello-interval 5`
- D`ip ospf dead-interval 20`
Show answer & explanationAnswer & explanation
Correct answer: C. `ip ospf hello-interval 5`
For OSPF adjacency to form and remain stable, the Hello interval and Dead interval must match between directly connected neighbors. If R1's Hello interval is set to 5 seconds, R2's directly connected interface must also have its Hello interval set to 5 seconds. The Dead interval is automatically set to four times the Hello interval by default, so configuring the Hello interval correctly will also align the Dead interval.
Why the other options are wrong
- A. This would cause a Hello interval mismatch, preventing adjacency.
- B. This would cause a Dead interval mismatch if the Hello interval is 5 (which implies a Dead interval of 20).
- D. While Dead intervals must match, changing only the Dead interval without matching the Hello interval will still cause an issue. By default, Dead is 4x Hello, so setting Hello to 5 on both will automatically set Dead to 20 on both.
OSPF Timer Matching
OSPF Hello and Dead intervals must match between directly connected neighbors for adjacency to form and be maintained.
- Hello interval: frequency of sending Hello packets.
- Dead interval: time to wait for a Hello before declaring a neighbor down.
- Default Dead interval is 4 times the Hello interval.
- Mismatch prevents neighbor state from reaching FULL.
Memory trick: To be OSPF friends, timers must match from ends to ends.