To fix the defect of STP and RSTP, the IEEE released 802.1s in 2002, defining the Multiple
Spanning Tree Protocol (MSTP). MSTP implements fast convergence and provides multiple
paths to load balance VLAN traffic.
MSTP divides a switching network into multiple regions, each of which has multiple spanning
trees that are independent of each other. Each spanning tree is called a Multiple Spanning Tree
Instance (MSTI) and each region is call a Multiple Spanning Tree (MST) region.
NOTE
An instance is a collection of VLANs. Binding multiple VLANs to an instance saves communication costs
and reduces resource usage. The topology of each MSTI is calculated independent of one another, and
traffic can be balanced among MSTIs. Multiple VLANs that have the same topology can be mapped to
one instance. The forwarding status of the VLANs for a port is determined by the port status in the MSTI.
Figure 7-2
Multiple spanning trees in an MST region
S1
S2
S3
S4
S5
S6
VLAN3
VLAN2
VLAN3
VLAN2
VLAN3
VLAN2
VLAN2
VLAN2
VLAN3
VLAN3
HostC
(
VLAN3
)
HostB
(
VLAN2
)
HostA
(
VLAN2
)
HostD
(
VLAN3
)
spanning tree(root bridge:S6)
spanning tree(root bridge:S4)
VLAN2
VLAN2
VLAN3
VLAN3
, MSTP maps VLANs to MSTIs in the VLAN mapping table. Each
VLAN can be mapped to only one MSTI. This means that traffic of a VLAN can be transmitted
in only one MSTI. An MSTI, however, can correspond to multiple VLANs.
Two spanning trees are calculated:
l
MSTI 1 uses S4 as the root switching device to forward packets of VLAN 2.
l
MSTI 2 uses S6 as the root switching device to forward packets of VLAN 3.
In this manner, devices within the same VLAN can communicate with each other; packets of
different VLANs are load balanced along different paths.
Huawei AR530&AR550 Series Industrial Switch Routers
Configuration Guide - Ethernet Switching
7 MSTP Configuration
Issue 01 (2014-11-30)
Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
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