Allied Telesis AT-GS950/24 - Manual (Page 381)

AT-GS950/24 Web Interface User Guide
381
Associating VLANs to MSTIs
When you are using Multiple Spanning Tree, Allied Telesis recommends that
you assign each of the VLANs to one of the existing MSTIs on a switch. You
should not leave any VLAN unassigned, including the Default VLAN. This is to
prevent the blocking of a port that should be in the forwarding state. The
reason for this guideline is explained below.
An MSTP BPDU contains information identifying the Multiple Spanning Tree
instance that is associated with the port transmitting the BPDU packet. By
default, all ports of the AT-GS950/24 switch belong to the CIST instance. So
the CIST identification is always included in the BPDU. If the port is also a
member of a VLAN that has been assigned to an MSTI, that information is
included in the BPDU too.
This is illustrated in Figure 157. Port 1 in switch A is a member of the Default
VLAN and has been assigned to MSTI ID 10, and port 8 is a member of VLAN
3 assigned to MSTI ID 10. The BPDUs transmitted by port 8 to switch B
indicate that the port is a member of both CIST 0 and MSTI 15, while the
BPDUs from port 1 indicate the port is a member of the CIST 0 and MSTI 10.
Figure 157. CIST and VLAN Guideline - Example 1
At first glance, it might appear that because 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 because both ports on switch A belong to different
MSTIs, switch B determines that no loop exists.
A problem can arise, however, if you assign certain VLANs to MSTIs while
leaving others assigned only to CIST. Figure 158 on page 382 illustrates the
issue. The network is similar to the previous example. The primary difference
is that the VLAN 2 containing port 1 on Switch A has not been assigned to an
MSTI and only belongs to CIST (MSTI ID 0).