Why MTU mismatch will cause the OSPF neighbor relationship down?

Hi,
What should be pointed out firstly is that the MTU affects the adjacency relationship establishment, rather than the neighbor relationship establishment.
When two OSPF peers exchange the DBD packets, in which the MTU value will be checked. As you said, Huawei wouldn’t check the MTU value by default, so, with some specific conditions, the adjacency relationship could establish successfully, we’ll talk that later.
Back to the DBD packets exhanging. As we know, the very first DBD packets carry no LSAs, for the later DBD packets, the LSAs headers in the LSDB will be encapsulated. Suppose that the total size of the LSA headers exceed the interface MTU, it will be dropped, which will cause the router stuck in waitting for the DBD packets. As a result, the adjacency relationship wouldn’t be established successfully. On the contrary, if the MTU doesn’t exceed the interface MTU, then, the OSPF adjacency relationship might establish successfully.