Allied Telesis AT-GS950/24 - Manual (Page 383)

AT-GS950/24 Web Interface User Guide
383
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 159. The example shows 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 159. 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.
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.