3Com S7906E Command Reference Manual page 449

You cannot map the same VLAN to different MSTIs. If you map a VLAN that has been mapped to an
MSTI to a new MSTI, the old mapping will be automatically removed.
By using the vlan-mapping modulo command, you can quickly specify a VLAN for each MSTI. This
command maps each VLAN to the MSTI whose ID is (VLAN ID–1) %modulo + 1, where (VLAN ID-1)
%modulo is the modulo operation for (VLAN ID–1). If the modulo value is 15, for example, then VLAN 1
will be mapped to MSTI 1, VLAN 2 to MSTI 2, VLAN 15 to MSTI 15, VLAN 16 to MSTI 1, and so on.
Related commands: region-name, revision-level, check region-configuration, active
region-configuration.
Examples
# Map VLANs to MSTIs as per modulo 8.
<Sysname> system-view
[Sysname] stp region-configuration
[Sysname-mst-region] vlan-mapping modulo 8
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