Allied Telesis AT-9000 Series - Figure 133: CIST and VLAN Guideline - Example 2

Chapter 47: Multiple Spanning Tree Protocol (MSTP)
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Figure 133. 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.
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.