Allied Telesis AT-S62 - Manual (Page 491)

AT-S62 Menus Interface User’s Guide
Section IV: Spanning Tree Protocols 491
Figure 169 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.
A problem can arise if you assign some VLANs to MSTIs while leaving
others just to CIST. The problem is illustrated in Figure 170. The network
is the same as the previous example. The only difference is that the VLAN
containing Port 8 on Switch A has not been assigned to an MSTI, and
belongs only to CIST with its MSTI ID 0.
Figure 170 CIST and VLAN Guideline - Example 2
Instance: CIST 0 and MSTI 10
Instances: CIST 0 and MSTI 7
Port 8 Switch A Switch B
BPDU Packet
Port 1
BPDU Packet
AT-8524M AT-8524M
Instance: CIST 0 and MSTI 10
Instances: CIST 0
Port 8 Switch A Switch B
BPDU Packet
Port 1
BPDU Packet
AT-8524M AT-8524M
Port 3
Port 15