Filter
Top Products
AT-S63 Management Software Features Guide
Section V: Spanning Tree Protocols 241
Associating VLANs to MSTIs
Allied Telesis recommends that you assign all VLANs on a switch to an
MSTI. You should not leave a VLAN assigned to just the CIST, including
the Default_VLAN. This is to prevent the blocking of a port that should be
in the forwarding state. The reason for this guideline is explained below.
An MSTP BPDU contains the instance to which the port transmitting the
packet belongs. By default, all ports belong to the CIST instance. So CIST
is included in the BPDU. If the port is a member of a VLAN that has been
assigned to another MSTI, that information is also included in the BPDU.
This is illustrated in Figure 29. Port 8 in switch A is a member of a VLAN
assigned to MSTI ID 7 while port 1 is a member of a VLAN assigned to
MSTI ID 10. The BPDUs transmitted by port 8 to switch B would indicate
that the port is a member of both CIST and MSTI 7, while the BPDUs from
port 1 would indicate the port is a member of the CIST and MSTI 10.
Figure 29. CIST and VLAN Guideline - Example 1
At first glance, it might appear that because both ports belong to CIST, a
loop would exist between the switches and that MSTP would block a port
to stop the loop. However, within a region, MSTI takes precedence over
CIST. When switch B receives a packet from switch A, it uses MSTI, not
CIST, to determine whether a loop exists. And because both ports on
switch A belong to different MSTIs, switch B determines that no loop
exists.
A problem can arise if you assign some VLANs to MSTIs while leaving
others just to CIST. The problem is illustrated in Figure 30. The network is
the same as the previous example. The only difference is that the VLAN
containing port 8 on Switch A has not been assigned to an MSTI, and
belongs only to CIST with its MSTI ID 0.
FAULT
RPS
MASTER
POWER
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/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
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/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
Switch A
Switch B
Port 1
Port 8
BPDU Packet
BPDU Packet
Instances: CIST 0 and MSTI 10
Instances: CIST 0 and MSTI 7