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AT-S63 Management Software Features Guide
Section V: Spanning Tree Protocols 243
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. 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 31. The example show two switches, each
residing in a different region. Port 1 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 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,
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 31. Spanning Regions - Example 1
There are several ways to address this issue. One is to have only one
MSTP region for each subnet in your network.
Region 1 Region 2
Switch A
Switch B
FAULT
RPS
MASTER
POWER
CLASS 1
LASER PRODUCT
STATUS
TERMINAL
PORT
1357911
24681012
13 15 17 19 21 23R
14 16 18 20 22 24R
AT-9424T/SP
Gigabit Ethernet Switch
1357911131517192123R
24681012141618202224R
2324
L/A
D/C
D/C
L/A
D/C
L/A
1000 LINK / ACT
HDX /
COL
FDX
10/100 LINK / ACT
PORT ACTIVITY
L/A
1000 LINK / ACT
SFP
SFP
24
SFP
23
FAULT
RPS
MASTER
POWER
GBIC
23
GBIC
24
CLASS 1
LASER PRODUCT
STATUS
TERMINAL
PORT
1357911
2 4 6 8 10 12
13 15 17 19 21 23R
14 16 18 20 22 24R
AT-9424T/GB
Gigabit Ethernet Switch
1357911131517192123R
24681012141618202224R
2324
L/A
D/C
D/C
L/A
D/C
L/A
1000 LINK / ACT
HDX /
COL
FDX
10/100 LINK / ACT
PORT ACTIVITY
L/A
1000 LINK / ACT
GBIC
Port 5
MSTI 4
VLAN (untagged) port: Accounting
Port 16
MSTI 12
VLAN (untagged port): Sales
VLAN (tagged port): Presales
VLAN (tagged port): Marketing