Figure 143. cist and vlan guideline - example 2 – Allied Telesis AT-GS950/16PS User Manual
Page 364
Appendix A: MSTP Overview
364
Figure 143. CIST and VLAN Guideline - Example 2
When port 3 on switch B receives a BPDU, the switch notes the port sending
the packet belongs only to CIST 0. Therefore, switch B uses
CIST 0 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 port 3 to cancel the loop.
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 helps to ensure that loop detection is based on MSTI, not CIST.
1
3
5
7
9
11
13
15
AT-GS950/16PS
Gigabit Ethernet PoE+ Switch
plus
1
3
5
7
9
11
13 15R
2
4
6
8
10
12
14 16R
15
16
1
3
5
7
2
4
6
8
9
11
13
15R
10
12
14
16R
15
16
100 LINK
ACT
1000 LINK
SFP
ACT
CLASS 1
LASER PRODUCT
SFP
PD ERR
PD ON
ACT
1000 LINK
ACT
10/100 LINK
PORT ACTIVITY
POE MAX
SYSTEM
POE
1
3
5
7
9
11
13
15
AT-GS950/16PS
Gigabit Ethernet PoE+ Switch
plus
1
3
5
7
9
11
13 15R
2
4
6
8
10
12
14 16R
15
16
1
3
5
7
2
4
6
8
9
11
13
15R
10
12
14
16R
15
16
100 LINK
ACT
1000 LINK
SFP
ACT
CLASS 1
LASER PRODUCT
SFP
PD ERR
PD ON
ACT
1000 LINK
ACT
10/100 LINK
PORT ACTIVITY
POE MAX
SYSTEM
POE
2719
Port 1
(Default VLAN)
Switch A
Switch B
Instances: CIST 0 and MSTI 15
Port 6
Port 8
(VLAN 3)
BPDU Packet
Instances: CIST 0
Port 3 (Blocked)
BPDU Packet