Qtech QSW-2800v2 [186/230] Mstp introduction

Qtech QSW-2800v2 [186/230] Mstp introduction
Chapter8 Reliability Configuration
8-4
When one link is blocked, it will not carry any flow and will be unable to achieve
load balance. This causes enormous waste of bandwidth.
All these defects of single spanning tree cannot be overcome. So the MSTP
supporting VLAN appears.
8.2.3 MSTP Introduction
MSTP Advantage
The MSTP is a new spanning tree protocol defined in IEEE 802.1s. Compared with
the STP and RSTP, its advantage is very obvious. Its features are as the following.
The MSTP introduces the concept of “domain” to divide one switching network
into several domains. There are multiple trees in every domain. These spanning
tress are independent of each other. Among these domains, the MSTP uses
CIST to ensure that there is no loop existed in the network.
The MSTP introduces the concept of “Instance” to map multiple VLAN into one
instance. This can save communication cost and resource occupancy rate.
Each instance topology calculation of the MSTP is independent (each instance
corresponding to a single spanning tree). In these instances, VLAN data sharing
can be loaded.
The MSTP can achieve rapid migration mechanism of port state similar to the
RSTP.
The MSTP can be compatible with the STP and RSTP.
MSTP Algorithm
1. Initial State
Each port of each device generates one configuration message that makes it to
be the root bridge at the initial time. The total root and domain root are its bridge
ID. The outer root path cost and inner root path cost are all zero. The designated
bridge ID is this bridge ID itself. The designated port is the bridge port itself. The
port receiving BPDU message is zero.
2. Interface Role Selection Principle
The selection principle of port role is shown as the Table 8-1.

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