Mstp region – Allied Telesis AT-8100 Series User Manual

Page 864

Chapter 58: Multiple Spanning Tree Protocol (MSTP)

864

Table 94 illustrates the concept of regions. It shows one MSTP region
consisting of two AT-8100 Switches. Each switch in the region has the
same configuration name and revision level. The switches also have the
same five VLANs, and the VLANs are associated with the same MSTIs.

The AT-8100 Switch determines regional boundaries by examining the
MSTP BPDUs received on the ports. A port that receives an 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 97 on page 877 lists the increments.

Table 94. MSTP Region

Configuration Name: Marketing Region, Revision Level 1

Switch 1

Switch 2

MSTI ID 1:
VLAN: Sales (VID 2)
VLAN: Presales (VID 3)

MSTI ID 2:
VLAN: Accounting (VID 4)