Refer to the exhibit.

After two OSPF neighboring routers establish bi-directional communication and complete DR/BDR

election (on multi-access networks), the routers transition to the exstart state. In this state, the

neighboring routers establish a master/slave relationship and determine the initial database descriptor

(DBD) sequence number to use while exchanging DBD packets.

Neighbors Stuck in Exstart/Exchange State

The problem occurs most frequently when attempting to run OSPF between a Cisco router and another

vendor’s router. The problem occurs when the maximum transmission unit (MTU) settings for

neighboring router interfaces don’t match. If the router with the higher MTU sends a packet larger that

the MTU set on the neighboring router, the neighboring router ignores the packet.