Allied Telesis AT-S63 - Manual (Page 286)

Chapter 17: MSTP
286 Section II: Advanced Features
The same is true for any ports connected to bridges running the single-
instance spanning tree STP or RSTP. Those ports are also considered as
part of another region.
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 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, 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 specify the increment that represents the desired MSTI priority
value. Table 12 on page 251 lists the increments.
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 AT-9400 Series switches.
❑ An AT-9400 Series 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 MSTI associations.
❑ An MSTI cannot span multiple regions.