Allied Telesis AT-S62 - Manual (Page 490)

Chapter 24: Multiple Spanning Tree Protocol
Section IV: Spanning Tree Protocols 490
❑ Each MSTI must have a regional root for locating loops in the
instance. MSTIs can share the same regional root or have different
roots. A regional root is determined by the MSTI priority value and
a bridge’s MAC address.
❑ The regional root of a MSTI must be in the same region as the
MSTI.
❑ The CIST must have a regional root for communicating with other
regions and single-instance spanning trees.
❑ MSTP is compatible with STP and RSTP.
❑ 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 490.)
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 169. 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.