When OSPF routers on Cisco switches fail to receive “Hello” packets from their neighbors within the “Dead Interval,” indicating a potential loss of connectivity, the routers generate log messages to inform network administrators about the situation. These log messages can be helpful in diagnosing the cause of the adjacency removal. Here are some example log messages that may appear in the Cisco switch logs:
- Message: %OSPF-5-ADJCHG: Process <Process_ID>, Nbr <Neighbor_IP> on <Interface> from FULL to DOWN, Dead timer expired Explanation: This log message indicates that the OSPF neighbor adjacency on the specified interface is transitioning from FULL to DOWN state because the dead timer has expired. The OSPF router failed to receive “Hello” packets from the neighbor within the configured dead interval, leading to the removal of the adjacency.
- Message: %OSPF-5-NBRCHG: Process <Process_ID>, Nbr <Neighbor_IP> on <Interface> from FULL to DOWN, Neighbor dead Explanation: This log message suggests that the OSPF router detected that the neighbor on the specified interface is dead because it did not receive “Hello” packets within the dead interval. Consequently, the OSPF neighbor adjacency is transitioning from FULL to DOWN state.
- Message: %OSPF-4-TIMERLOST: Neighbor <Neighbor_IP> on <Interface> has been up for <Time_Duration> without becoming neighbors Explanation: This log message indicates that the OSPF router has detected that the neighbor on the specified interface has been up for a duration of time without successfully forming a neighbor adjacency. The failure to establish a neighbor relationship may be due to dead timer expiry.
- Message: %OSPF-5-NBRCHG: Process <Process_ID>, Nbr <Neighbor_IP> on <Interface> from FULL to INIT, Neighbor dead: dead timer expired Explanation: This log message suggests that the OSPF router is transitioning the OSPF neighbor adjacency on the specified interface from FULL to INIT state due to the death of the neighbor. The neighbor is considered dead because the dead timer has expired, and the router failed to receive “Hello” packets.
These log messages provide crucial insights into the potential “Dead Timer Expiry” issue and help network administrators identify the reason for the OSPF adjacency removal. Monitoring the switch logs and identifying these log messages can aid in troubleshooting OSPF neighbor issues related to dead timer expiry and assist in maintaining a stable and efficient OSPF network.
A crucial aspect of OSPF is the exchange of “Hello” packets between neighboring routers. If OSPF routers fail to receive “Hello” packets within the specified “Dead Interval,” they assume the neighbor is down and remove the adjacency. This situation can disrupt routing, cause connectivity issues, and impact network performance. In this article, we will explore the issue of Dead Timer Expiry in OSPF on Cisco 3850 switches and provide step-by-step solutions using relevant commands to restore neighbor adjacencies and ensure seamless OSPF operation.
- Understanding Dead Timer Expiry in OSPF: The “Dead Interval” is a timer set in OSPF to determine the maximum time between the receipt of consecutive “Hello” packets from a neighboring router. If a router does not receive “Hello” packets from its neighbor within this interval, it concludes that the neighbor is down and removes the adjacency.
- Causes of Dead Timer Expiry: a. Network Congestion: High network congestion can delay “Hello” packet transmission, leading to Dead Timer Expiry.
b. Interface Issues: Interface errors, flapping, or misconfigurations can disrupt “Hello” packet exchange.
c. Reachability Problems: Reachability issues between OSPF neighbors can result in Dead Timer Expiry.
- Solution for Dead Timer Expiry in OSPF on Cisco 3850 Switches:
Step 1: Verify Neighbor Status Check the status of OSPF neighbors to identify those with Dead Timer Expiry:
show ip ospf neighbor
Step 2: Review Interface Configurations Inspect the interface configurations of the affected OSPF neighbors to ensure they are correct:
show running-config interface <Interface>
Step 3: Check Reachability Verify reachability between OSPF neighbors by checking routing tables:
show ip route <Neighbor_IP>
Step 4: Adjust Dead Interval If the issue is caused by network congestion or reachability problems, consider increasing the Dead Interval to allow more time for “Hello” packet exchanges:
router ospf <Process_ID>
timers throttle spf <Time_in_milliseconds>
Replace “<Process_ID>” with the OSPF process ID and “<Time_in_milliseconds>” with the desired Dead Interval value.
Step 5: Clear OSPF Neighbors Clear the OSPF neighbors on the affected interface to force re-establishment of adjacencies:
clear ip ospf <Process_ID> <Interface>
Step 6: Verify Neighbor Adjacencies Monitor the OSPF neighbors to check if the Dead Timer Expiry issue has been resolved:
show ip ospf neighbor