Allied Telesis AT-GS950/8 - Manual (Page 344)

Appendix A: MSTP Overview
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Another approach is to configure multiple regions in a subnet and group the
VLANs that need to span two or more regions into the same MSTI. If other
VLANs also exist that do not span multiple regions, they can be assigned to
other MSTIs within their respective region.
Here is an example. Assume that you have two regions that contain the
following VLANS:
Region 1 VLANs Region 2 VLANs
Accounting Accounting
Sales Sales
Pre-Sales Pre-Sales
Marketing Technical Support
Product Management Software Engineering
Project Management Hardware Engineering
The two regions share three VLANs: Accounting, Sales, and Presales. You
could group these three VLANs into the same MSTI in each region. For
instance, for Region 1 you might group the three VLANs in MSTI 12 and in
Region 2 you could group them into MSTI 16. After they are grouped, you can
connect the VLANs across the regions using a link of
untagged/tagged ports a shown in Figure 125.
Figure 125. Spanning Regions without Blocking
Switch A Switch B
Region 1 Region 2
Port 6
MSTI 12
VLAN (untagged) port: Accounting
VLAN (tagged port): Sales
VLAN (tagged port): Pre-Sales Port 1
MSTI 16
VLAN (untagged) port: Accounting
VLAN (tagged port): Sales
VLAN (tagged port): Pre-Sales