Allied Telesis AT-S60 - Manual (Page 270)

Chapter 16: Multiple Spanning Tree
Section II: Advanced Features 270
This is shown in Figure 88. Port 8 on a line card in Switch A is a member
of a VLAN assigned to MSTI ID 7. Port 1 on another line card in the same
switch is a member of a VLAN assigned to MSTI ID 10. The BPDUs
transmitted by port 8 to Switch B would indicate that the port is a
member of both CIST and MSTI 7, while the BPDUs from Port 1 would
indicate the port is a member of the CIST and MSTI 10.
Figure 88 CIST and VLAN Guideline - Example 1
At first glance, it might appear that since both ports belong to CIST, a
loop would exist between the switches and that MSTP would block a
port to stop the loop. However, within a region, MSTI takes precedence
over CIST. When Switch B receives a packet from Switch A, it uses MSTI,
not CIST, to determine whether a loop exists. And since both ports on
Switch A belong to different MSTIs, Switch B determines that no loop
exists.
Instance: CIST 0 and MSTI 10
Instances: CIST 0 and MSTI 7
Port 8
Switch A Switch B
BPDU Packet
Port 1
BPDU Packet