Allied Telesis AT-GS950/48 - Manual (Page 388)

Appendix A: MSTP Overview
388
Figure 158. 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.