Chapter 21: Multiple Spanning Tree Protocol
242 Section V: Spanning Tree Protocols
Figure 30. CIST and VLAN Guideline - Example 2When 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.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.
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
24 6 8 10 12 14 16 18 20 22 24R
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
24 6 8 10 12 14 16 18 20 22 24R
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

Port 15

Port 4