Specifications

Switching 8-41
Software Release 2.7.3
C613-03098-00 REV A
Multiple Spanning Tree Protocol (MSTP)
The multiple spanning tree protocol (MSTP) was developed to address the
limitations in the existing spanning tree protocols, STP and RSTP. These
limitations apply mainly to networks that use multiple VLANs with topologies
employing alternative physical links. MSTP is defined in IEEE Standard 802.1Q
2003. The protocol builds on, and remains compatible with, the following
previous standards:
■ IEEE Standard 802.1w 2001, which defines the rapid spanning tree protocol
(RSTP)
■ IEEE Standard 802.1D/D4 2003, which defines a draft standard for local
and metropolitan area networks
Multiple Spanning Tree Regions
Conceptually, MSTP views the total bridged network as one that comprises a
number of Multiple Spanning Tree Regions (MSTRs), where each region can
contain up to 64 spanning trees that operate locally, called Multiple Spanning
Tree Instances (MSTIs). The task of assigning each bridge to a particular region
is achieved by the member bridges each comparing their MST configuration
identifiers. More information on configuration identifiers is provided in
Table 8-12 on page 8-42, but for the moment an MST configuration identifier can
simply be thought of as an identifier that represents the mapping of VLANs to
MSTIs within each bridge. Therefore, bridges with identical MST configuration
identifiers, must have identical MSTI mapping tables.
While each MSTI can contain up to 4094 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 algorithms and advertise their
MSTIs. This in turn establishes the active data paths between the bridges for
each group of VLANs (i.e. for each MSTI) and block any duplicate paths. 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.
Table 8-12 on page 8-42 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