Allied Telesis AT-S63 - Manual (Page 292)

Chapter 17: MSTP
292 Section II: Advanced Features
There are several ways to address this issue. One is to have only one
MSTP region for each subnet in your network.
Another approach is to group those VLANs that need to span regions
into the same MSTI. Those VLANs that do not span regions can be
assigned to other MSTIs.
Here is an example. Assume that you have two regions that contain the
following VLANS:
Region 1 VLANs Region 2 VLANs
Sales Hardware Engineering
Presales Software Engineering
Marketing Technical Support
Advertising Product Management
Technical Support CAD Development
Product Management Accounting
Project Management
Accounting
The two regions share three VLANs: Technical Support, Product
Management, and Accounting. You could group those VLANs into the
same MSTI in each region. For instance, for Region 1 you might group
the three VLANs in MSTI 11 and in Region 2 you could group them into
MSTI 6. After they are grouped, you can connect the VLANs across the
regions using a link of tagged ports.