Allied Telesis AT-8100 Series - Manual (Page 872)

Chapter 58: Multiple Spanning Tree Protocol (MSTP)
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Connecting VLANs Across Different Regions
Special consideration needs to be taken into account when you connect
different MSTP regions or an MSTP region and a single-instance STP or
RSTP region. Unless planned properly, VLAN fragmentation can occur
between the VLANS of your network.
As mentioned previously, only the CIST can span regions. An MSTI
cannot. Consequently, you may run into a problem if you use more than
one physical data link to connect together various parts of VLANs that
reside in bridges in different regions. The result can be a physical loop,
which spanning tree disables by blocking ports.
This is illustrated in Figure 167. 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 16 is a tagged and untagged member of three different
VLANs, all associated with MSTI 12.
If both switches were a part of the same region, there would be no
problem because 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 is that spanning tree would
determine that a loop exists between the regions, and Switch B would
block a port.
Figure 167. Spanning Regions - Example 1