MTU of the Uplink: The maximum transmission unit (MTU) of the uplink interface must match the MTU of the upstream router interface. Otherwise, OSPF packets may be fragmented or dropped, causing neighbor adjacency issues.
Subnet mask: The subnet mask of the uplink interface must match the subnet mask of the upstream router interface. Otherwise, OSPF packets may not reach the correct destination or be rejected by the upstream router.
Area ID: The area ID of the uplink interface must match the area ID of the upstream router interface. Otherwise, OSPF packets may be ignored or discarded by the upstream router.