Allied Telesis AlliedWare Plus Operating System Version 5.4.4C (x310-26FT,x310-26FP,x310-50FT,x310-50FP) User Manual
Page 482
Spanning Tree Introduction: STP, RSTP, and MSTP
Software Reference for x310 Series Switches
18.14
AlliedWare Plus
TM
Operating System - Version 5.4.4C
C613-50046-01 REV A
While each MSTI can have multiple VLANs, each VLAN can be associated with only one
MSTI. Once these associations have been made, the bridges in each region can transmit
their spanning tree BPDUs and advertise their MSTIs. This in turn establishes the active
data paths between the bridges for each group of VLANs (that is, for each MSTI) and block
any duplicate paths within each instance. A particular advantage of this enhancement
applies where a large number of VLANs share a few internetwork paths. In this situation
there need only be as many Multiple Spanning Tree Instances (MSTIs) as there are source
and destination bridge pairs, remembering that a pair of bridges probably has multiple
paths between them.
In order to ensure that each bridge within a region maintains the same configuration
information (particularly their VID to MSTI mappings) and to ensure each bridge’s
membership of a particular region, the bridges exchange configuration information in the
form of MST Configuration Identifiers.
provides a breakdown of an MST
Configuration Identifier. A detailed explanation of bridge configuration identifiers can be
found in Section 13.7 of the IEEE 802.1Q-2003 standard.
Table 18-6: MST Configuration Identifier
Field Name
Description
Format Selector
A single octet field whose value of 0 indicates MSTP operation
Region Name
A name (up to 32 characters long) that identifies a particular
MST region, defined using the
Revision Level
A number representing the region’s revision level, defined
using the
revision (MSTP) command on page 19.12
Configuration Digest
A 16 octet (HMAC-MD5 based) signature created from the MST
configuration table.