Allied Telesis AT-S62 - Manual (Page 361)

AT-S62 User’s Guide
Section IV: Spanning Tree Protocols 361
The AT-8524M switch determines regional boundaries by examining the
MSTP BPDUs received on the ports. A port that receives a MSTP BPDU
from another bridge with regional information different from its own is
considered to be a boundary port and the bridge connected to the port
as belonging to another region.
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 331 lists the increments.
Region Guidelines
Here are several points to remember about regions.
❑ A network can contain any number of regions and a region can
contain any number of switches that support MSTP.
❑ An AT-8524M 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.
❑ 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.