Configuration Example OSPF Adjacency and MTU Mismatch

In Figure 19-9, Routers A and B have an MTU of 1500 on their serial interfaces and Router C has an MTU of 1490. When Routers A and C reach the EXSTART state in the forming of the adjacency, they will attempt to exchange their OSPF database description packets. An OSPF router will ignore a database description packet if there is an MTU mismatch. When there is a mismatch, the database description packet will not be acknowledged and the sending router will continue to send the packets until they are acknowledged. The acknowledgement will never come and the routers will be stuck in the EXSTART state. To demonstrate this, Router C has been configured with an MTU of 1490 as shown in the following configurations.

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