Connecting vlans across different regions – Allied Telesis AT-S63 User Manual
Page 291
AT-S63 Management Software Menus Interface User’s Guide
Section II: Advanced Features
291
To avoid this issue, always assign all VLANs on a switch, including the
Default_VLAN, to an MSTI. This guarantees that all ports on the switch
have an MSTI ID and that helps to ensure that loop detection is based on
MSTI, not CIST.
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 94. 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 94. Spanning Regions - Example 1
Region 1
Region 2
Switch A
Switch B
FAULT
RPS
MASTER
POWER
CLASS 1
LASER PRODUCT
STATUS
TERMINAL
PORT
1
3
5
7
9
11
2
4
6
8
10
12
13
15
17
19
21
23R
14
16
18
20
22
24R
AT-9424T/SP
Gigabit Ethernet Switch
1
3
5
7
9
11
13
15
17
19
21
23R
2
4
6
8
10
12
14
16
18
20
22
24R
23
24
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
1
3
5
7
9
11
2
4
6
8
10
12
13
15
17
19
21
23R
14
16
18
20
22
24R
AT-9424T/GB
Gigabit Ethernet Switch
1
3
5
7
9
11
13
15
17
19
21
23R
2
4
6
8
10
12
14
16
18
20
22
24R
23
24
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 (taggedport): Marketing