* The exhibit shows the output of the command show ospf neighbor , which displays information about the OSPF neighbors on a router 1 .
* The output shows that the OSPF neighbor with the address 172.26.1.1 and the interface ge-0/0/3.0 is in the Exstart state 1 .
* The Exstart state is the fourth state in the OSPF neighbor formation process, after Down, Init, and 2- Way states 2 . In this state, the OSPF neighbors establish a master-slave relationship and exchange database description (DBD) packets, which contain summaries of their link-state databases 2 .
* The most common reason for OSPF neighbors to be stuck in the Exstart state is an MTU mismatch between the interfaces 3 . MTU stands for maximum transmission unit, which is the largest size of a packet that can be transmitted on a network segment 4 . If the MTU values of two OSPF neighbors are different, they may not be able to exchange DBD packets successfully, as some packets may be dropped or fragmented due to their size exceeding the MTU limit 3 .
* To solve this problem, you need to ensure that the MTU values of both OSPF neighbors are the same or compatible. You can use the command show interfaces to display the MTU value of an interface 5 .
You can also use the command ping with the do-not-fragment option to test the MTU size between two routers. You can change the MTU value of an interface by using the command set interfaces interface-name mtu mtu-value in configuration mode 5 .