Allied Telesis AT-S63 - Manual (Page 289)

AT-S63 Management Software Menus Interface User’s Guide
Section II: Advanced Features 289
❑ All of the bridges in a region must have the same configuration
name, revision level, VLANs, and VLAN to MSTI associations.
❑ An MSTI cannot span multiple regions.
❑ 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 is 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 289.
Note
The AT-S63 MSTP implementation complies fully with the new IEEE
802.1s standard. Any other vendor’s fully compliant 802.1s
implementation is interoperable with the AT-S63 implementation.
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
below.
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
is 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 92. 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