Explanation

From the second command output (show spanning-tree mst) we learn that MST1 includes VLANs 10 &
20.Therefore if we want DSW1 to become root bridge for these VLANs we need to set the MST 1 region to root ->The command "spanning-tree mst 1 root primary" can do the trick. In fact, this command runs a macro and setsthe priority lower than the current root.
Also we can see the current root bridge for these VLANs has the priority of 32769 (default value + sysid) so wecan set the priority of DSW1 to a specific lower value. But notice that the priority must be a multiple of
4096.