Two switches, SW1 and SW2, are connected by an 802.1Q trunk and both are configured for MST. All VLANs are allowed on the trunk, but you notice that SW1 treats SW2 as a boundary and runs a separate MST instance toward it rather than forming a single common MST region. You confirm both switches have 'spanning-tree mode mst' and identical instance-to-VLAN mappings. Which additional configuration mismatch most likely prevents them from being in the same MST region?