Allied Telesis AT-9000 Series - Manual (Page 686)

Chapter 45: Multiple Spanning Tree Protocol
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There are several ways to address this issue. The first 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. In this case, 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:
The two regions share three VLANs: Technical Support, Product
Management, and Accounting. You can group these three 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 untagged/tagged ports. See Figure 128.
Figure 128. Spanning Regions without Blocking
Table 67. Two Region Examples
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