3Com 8807 Command Reference Manual page 211

Use the undo
vlan-mappin
g modulo command to disable the function.
By default, all the VLANs are mapped to CIST, namely Instance 0.
Related command: region-name, revision-level, check region-configuration,
active region-configuration
Example
# Map VLAN to MSTI based on modulo 16.
<SW8800>system-view
System View: return to User View with Ctrl+Z.
[SW8800]stp region-configuration
[3Com-mst-region] vlan-mapping modulo 16