Dell PowerEdge M420 Reference Manual page 681

All VLANs that are not explicitly mapped to an MST instance are mapped to
the common and internal spanning tree (CIST) instance (instance 0) and
cannot be unmapped from the CIST.
For two or more switches to be in the same MST region, they must have the
same VLAN mapping, the same configuration revision number, and the same
name.
PowerConnect MSTP supports mapping of VLANs to MST instances, even
though the underlying VLAN may not be defined on the switch. Traffic
received on VLANs not defined on the port received is dropped.
Example
The following example maps the entire range of VLANs to MST instances
(MST instance 0 is mapped to VLAN 1 by default). Additionally, two 10G
ports have some, but not all, of the VLANs mapped to MST instances.
console(config)#spanning-tree mode mst
console(config)#spanning-tree mst 1 priority 8192
console(config)#spanning-tree mst 2 priority 28672
console(config)#spanning-tree mst configuration
console(config-mst)#instance 1 add vlan 2-199
console(config-mst)#instance 1 add vlan 350
console(config-mst)#instance 1 add vlan 400-449
console(config-mst)#instance 1 add vlan 500-1999
console(config-mst)#instance 1 add vlan 2200-2499
console(config-mst)#instance 1 add vlan 2600-2799
console(config-mst)#instance 1 add vlan 3000-4093
console(config-mst)#instance 2 add vlan 200-349
console(config-mst)#instance 2 add vlan 351-399
console(config-mst)#instance 2 add vlan 450-499
console(config-mst)#instance 2 add vlan 2000-2199
console(config-mst)#instance 2 add vlan 2500-2599
console(config-mst)#instance 2 add vlan 2800-2999
console(config-mst)#exit
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Spanning Tree Commands