If the MTU settings on interfaces don't match, what is the impact on OSPF neighboring?

When the Maximum Transmission Unit (MTU) settings on interfaces do not match between OSPF neighbors, it can lead to issues with the formation of OSPF adjacencies. In this case, routers can initiate the neighbor relationship, but they will not reach a Full state due to discrepancies in the MTU size. OSPF requires that both routers agree on the MTU size to ensure that packets can be transmitted without fragmentation.

During the OSPF hello protocol exchanges, if one router has a different MTU setting than the other, it will not receive the OSPF packets correctly because the packet sizes exceed the configured MTU. This situation prevents the routers from synchronizing their LSDB (Link State Database) completely, thereby stopping them from achieving a Full state. Therefore, while they may initially recognize each other as neighbors, the inability to form a Full state means that they will have a limited connection and will not effectively share routing information. This underlines the critical importance of consistent MTU settings for successful OSPF operation.