Allied Telesis AT-S60 - Manual (Page 272)

Chapter 16: Multiple Spanning Tree
Section II: Advanced Features 272
This is illustrated in Figure 90. The example shows two switches, each
residing in a different region. Port 1 on a line card in Switch A is a
boundary port. It is an untagged member of the Accounting VLAN,
which has been associated with MSTI 4. Port 8 on another line card is a
tagged and untagged member of three different VLANs, all associated to
MSTI 12.
If both switches were a part of the same region, there would be no
problem since the ports reside in different spanning tree instances.
However, the switches are part of different regions and MSTIs do not
cross regions. Consequently, the result would be that spanning tree
would determine that a loop exists between the regions, and Switch B
would block a port.
Figure 90 Spanning Regions - Example 1
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.
Region 1 Region 2
Switch A Switch B
Port 8
VLAN (untagged port): Sales
MSTI 12
VLAN (tagged port): Presales
VLAN (tagged port): Marketing
Port 1
MSTI 4
VLAN (untagged) port: Accounting