Allied Telesis AT-S62 - Manual (Page 492)

Chapter 24: Multiple Spanning Tree Protocol
Section IV: Spanning Tree Protocols 492
When port 3 on Switch B receives a BPDU, the switch notes the port
sending the packet belongs only to CIST. Consequently, Switch B uses
CIST in determining whether a loop exists. The result would be that the
switch would determine that a loop exists 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.
Connecting VLANs Across Different Regions
Special consideration needs to be taken into account when connecting
different MSTP regions or an MSTP region and a single-instance STP or
RSTP region. Unless planned properly, VLAN fragmentation can occur
between the VLANS of your network.
As mentioned previously, only the CIST can span regions. A MSTI cannot.
Consequently, you may run into a problem if you use more than one
physical data link to connect together various parts of VLANs that reside
in bridges in different regions. The result can be a physical loop, which
spanning tree disables by blocking ports.
This is illustrated in Figure 171. The example show two switches, each
residing in a different region. Port 5 in Switch A is a boundary port. It is
an untagged member of the Accounting VLAN, which has been
associated with MSTI 4. Port 15 is a tagged and untagged member of
three different VLANs, all associated to MSTI 12.
If both switches were a part of the same region, there would be no
problem since the ports reside in different spanning tree instances.
However, the switches are part of different regions and MSTIs do not
cross regions. Consequently, the result would be that spanning tree
would determine that a loop exists between the regions, and Switch B
would block a port.