Allied Telesis AT-9000 Series - Manual (Page 684)

Chapter 45: Multiple Spanning Tree Protocol
656
Figure 126. 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.