Vlans Across Different Regions; Figure 124. Spanning Regions - Example 1 - Allied Telesis AT-GS950/24 User Manual

AT-GS950/24 Web Interface User Guide

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. A 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 124. The example show two switches, each
residing in a different region. Port 7 in switch A is a boundary port. It is an
untagged member of the Accounting VLAN, which has been associated with
MSTI 4. Port 6 is a tagged and untagged member of two different VLANs, both
associated to 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 in this
example, 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, Switch B would block a port and the
Accounting VLAN would be disabled between the two regions.

Figure 124. Spanning Regions - Example 1

There are several ways to address this issue. One is to configure only one
MSTP region for each subnet in your network. This will eliminate the potential
situation of a loop and blocked port(s) between multiple regions.
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