Allied Telesis AT-S63 - Manual (Page 562)

Chapter 24: Multiple Spanning Tree Protocol
562 Section V: Spanning Tree Protocols
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 15 on page 529 lists the increments.
Region Guidelines
Following are several points to remember about regions.
ˆ A network can contain any number of regions and a region can contain
any number of AT-9400 Series switches.
ˆ An AT-9400 Series switch can belong to only one region at a time.
ˆ A region can contain any number of VLANs.
ˆ All of the bridges in a region must have the same configuration name,
revision level, VLANs, and VLAN to MSTI associations.
ˆ An MSTI cannot span multiple regions.
ˆ 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.
ˆ The regional root of a MSTI must be in the same region as the MSTI.