User's Manual

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Chapter 36 Configuring MSTP
36.1 MSTP Introduction
36.1.1 Overview
Multiple Spanning Tree Protocol (MSTP) is used to establish a simple and complete topology in the bridge
LAN. MSTP is compatible with STP (Spanning Tree Protocol) and RSTP (Rapid Spanning Tree Protocol).
Both STP and RSTP only construct a single spanning tree topology in a network and the packets of all VLANs
are forwarded along with this unique topology. STP converges too slowly, while RSTP guarantees the a rapid
and stable network topology through handshake.
MSTP keeps the fast handshake of RSTP to guarantee fast topology establishment, and at the same time
MSTP allows different VLANs to be classified into different spanning trees to establish multiple tree topologies
in the network. In a MSTP-constructing network, frames that belong to different VLANs can be forwarded on
different paths to realize the load balance of VLAN data.
Different from PVST (per-VLAN Spanning Tree), MSTP permits multiple VLANs to be classified into the same
spanning tree topology, effectively reducing spanning trees that are used to support VLANs.
36.1.2 MST Region
In MSTP, the relationship of VLAN and spanning tree is described through a MSTP. The MST configuration
table, along with a configuration name and a configuration edit number, makes up of a MST configuration
identifier.
In a network, the bridges that interconnect with others and possess the same MST configuration identifier are
regarded that they are in the same MST region. The bridges in the same MST region generally have the same
VLAN settings so that the frames of these VLANs can only be running at the inside of this MST region.
36.1.3 IST, CST, CIST and MSTI
Figure 2.1 shows an MSTP network, which consists of 3 MST regions and a switch running 802.1D STP
protocol.