Allied Telesis AT-S62 - Manual (Page 518)

Chapter 23: Multiple Spanning Tree Protocol
518 Section IV: Spanning Tree Protocols
ˆ The CIST must have a regional root for communicating with other
regions and single-instance spanning trees.
ˆ MSTP is compatible with STP and RSTP.
ˆ A port transmits CIST information even when it’s associated with
another MSTI ID. However, in determining network loops, MSTI takes
precedence over CIST. (This is explained more in “Associating VLANs
to MSTIs” on page 518.)
Note
The AT-S62 implementation of MSTP complies with the IEEE
802.1s standard and is compatible with versions from other vendors
that conform to the standard.
Associating VLANs to MSTIs
Allied Telesyn recommends that you assign all VLANs on a switch to an
MSTI. You should not leave a VLAN assigned to just the CIST, 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 here.
An MSTP BPDU contains the instance to which the port transmitting the
packet belongs. By default, all ports belong to the CIST instance. So CIST
would be included in the BPDU. If the port is a member of a VLAN that has
been assigned to another MSTI, that information is also included in the
BPDU.
This is illustrated in Figure 175. Port 8 in Switch A is a member of a VLAN
assigned to MSTI ID 7 while Port 1 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 175. CIST and VLAN Guideline - Example 1
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