Allied Telesis AT-S63 User Manual
Page 290
Chapter 17: MSTP
290
Section II: Advanced Features
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 92. 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 93. 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.
Figure 93. CIST and VLAN Guideline - Example 2
When port 4 on switch B receives a BPDU, the switch notes the port
sending the packet belongs only to CIST. Therefore, switch B uses CIST in
determining whether a loop exists. The result would be that the switch
detects a loop because the other port is also receiving BPDU packets
from CIST 0. Switch B would block a port to cancel the loop.
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
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
Switch A
Switch B
Port 1
Port 8
BPDU Packet
BPDU Packet
Instances: CIST 0 and MSTI 10
Instances: CIST 0 and MSTI 7
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
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
Switch A
Switch B
Port 1
Port 8
BPDU Packet
BPDU Packet
Instances: CIST 0 and MSTI 10
Instances: CIST 0
Port 15
Port 4