As defined in RFC 2328, the Interface Maximum Transmission Unit (MTU) of a DD packet should be set to the MTU (0 has to be set for a virtual link) of the link. If the Interface MTU of the DD packet received from the peer is greater than Interface MTU at the local end, the DD packet is discarded and the neighbor state remains in the Exchange state.

In the switch, you can run the ospf mtu-enable command on the interface to check the MTU of a received DD packet. By default, OSPF does not check the MTU of DD packets. If the interface MTU check is ignored, the OSPF neighbor relationship can reach the Full state normally and complete database synchronization even if the MTUs configured on both ends of the link are the same. Note that the difference between the MTUs of both ends of the link cannot be too great.