Resolving OSPF Adjacency Stuck in ExStart State

D

OSPF (Open Shortest Path First) is a popular link-state routing protocol used in large-scale enterprise networks. The ExStart state is a phase of OSPF adjacency establishment in which two routers exchange database summary information to compare their Link State Databases (LSDBs). If the adjacency remains stuck in the ExStart state, it indicates that there is a problem with the communication between the two routers.

To troubleshoot an OSPF adjacency stuck in the ExStart state, we can follow the steps below:

Step 1: Verify physical connectivity Check if the physical links between the two routers are up and operational. Verify if the physical interfaces are enabled, have the correct speed and duplex settings, and are not administratively shut down.

Step 2: Verify OSPF configuration Check the OSPF configuration on both routers and verify if the OSPF process ID, area ID, and router ID are configured correctly. Also, check if the OSPF interface is configured with the correct network type (point-to-point, broadcast, etc.) and authentication settings.

Step 3: Verify OSPF parameters Verify that the OSPF parameters such as the hello and dead intervals, MTU, and authentication settings are the same on both routers. These parameters should match to establish OSPF adjacency.

Step 4: Verify OSPF neighborship state Check the OSPF neighborship state of the router that is stuck in the ExStart state. Verify if the router has formed OSPF adjacency with other routers in the same network segment. If not, there may be a problem with the local router's OSPF configuration.

Step 5: Verify LSDB Verify if the Link State Databases (LSDBs) on both routers are identical. Check if there are any missing or mismatched LSAs (Link State Advertisements).

Step 6: Check logs and debug output Check the logs and debug output on both routers to identify any error messages or events related to the OSPF adjacency.

Based on the answers provided in the exam question, the correct step to resolve the problem would be:

C. Determine whether the hello and dead intervals are the same on both sides.

The hello and dead intervals are OSPF parameters that determine how frequently the routers exchange hello packets and the amount of time before a router declares a neighbor as dead. If the hello and dead intervals are not the same on both routers, the OSPF adjacency may fail to establish or get stuck in the ExStart state. Therefore, checking and ensuring the hello and dead intervals are the same on both sides would be the correct step to resolve the problem.