Figure 125. Spanning Regions Without Blocking - Allied Telesis AT-GS950/8 User Manual

Appendix A: MSTP Overview
Switch A
Port 6
MSTI 12
VLAN (untagged) port: Accounting
VLAN (tagged port): Sales
VLAN (tagged port): Pre-Sales
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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
Accounting
Sales
Pre-Sales
Marketing
Product Management
Project Management
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.
Region 1

Figure 125. Spanning Regions without Blocking

Region 2 VLANs
Accounting
Sales
Pre-Sales
Technical Support
Software Engineering
Hardware Engineering
Region 2
Port 1
MSTI 16
VLAN (untagged) port: Accounting
VLAN (tagged port): Sales
VLAN (tagged port): Pre-Sales
Switch B