Allied Telesis AT-S62 - Manual (Page 520)

Chapter 23: Multiple Spanning Tree Protocol
520 Section IV: Spanning Tree Protocols
bridges in different regions. The result can be a physical loop, which
spanning tree disables by blocking ports.
This is illustrated in Figure 177. The example show two switches, each
residing in a different region. Port 5 in Switch A is a boundary port. It is an
untagged member of the Accounting VLAN, which has been associated
with MSTI 4. Port 15 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 177. 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.
AT-8524M AT-8524M
Region 1 Region 2
Switch A Switch B
Port 15
VLAN (untagged port): Sales
MSTI 12
VLAN (tagged port): Presales
VLAN (tagged port): Marketing
Port 5
MSTI 4
VLAN (untagged) port: Accounting