Allied Telesis AT-GS950/48, User Manual

Page: 384 / 410

Appendix A: MSTP Overview
384
Each MSTI functions as an independent spanning tree within a region.
Consequently, each MSTI must have a root bridge to locate physical loops
within the spanning tree instance. An MSTI’s root bridge is called a
regional
root . The MSTIs within a region may share the same regional root or they can
have different regional roots.
A regional root for an MSTI must be within the region where the MSTI is
located. An MSTI cannot have a regional root that is outside its region.
A regional root is selected by a combination of the MSTI Bridge Priority value
and the bridge’s MAC address. The MSTI priority is analogous to the RSTP
bridge priority value. Where they differ is that while the RSTP bridge priority is
used to determine the root bridge for an entire bridged network, the MSTI
priority is used only to determine the regional root for a particular MSTI.
The range for this parameter is the same as the RSTP bridge priority; from 0
to 61,440 in sixteen increments of 4,096. To set the parameter, you select the
increment that represents the desired MSTI priority value according to
Table 8.
MST Region
Guidelines
Following are several points to remember about regions.
 A network can contain any number of regions, and a
region can contain any number of switches.

The AT-GS950/48 switch can belong to only one region at
a time.
 A region can contain any number of VLANs.
 All of the bridges in a region must have the same
configuration name, revision level, VLANs, and VLAN to
Table 8. Regional Bridge Priority Value Increments
Bridge Priority
Selections
0 32768
4096 36864
8192 40960
12288 45056
16384 49152
20480 53248
24576 57344
28672 61440