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MSTP Overview 247
Figure 4 MSTP Concepts
MST Region
A multiple spanning tree region contains several physically and directly connected
MSTP-capable switches sharing the same region name, VLAN-spanning tree
mapping configuration and MSTP revision level configuration, and the network
segments between them. There can be several MST regions on a switching
network. You can group several switches into a MST region, using MSTP
configuration commands. For example, in
Figure 4, in MST region A0, the 4
switches are configured with the same region name, vlan mapping table (VLAN1
map to instance 1, VLAN 2 map to instance 2, other VLAN map to instance 0), and
revision level (not indicated in
Figure 4).
VLAN Mapping Table
A VLAN mapping table is an attribute of an MST region and is used for describing
the mapping relationship of VLAN and STI. For example, the VLAN mapping table
of MST region A0 in
Figure 4 is VLAN1 map to instance 1, VLAN 2 map to instance
2, other VLAN map to instance 0.
Internal Spanning Tree (IST)
The entire switching network has a Common and Internal Spanning Tree (CIST).
An MSTP region has an Internal Spanning Tree (IST), which is a fragment of CIST.
For example, every MST region in
Figure 4 has an IST.
Common Spanning Tree (CST)
CST connects the spanning trees of the MST region. Taking every MST region as a
“switch”, the CST can be regarded as their spanning tree generated with
STP/RSTP. For example, the red line indicates the CST in
Figure 4.
Common and Internal Spanning Tree (CIST)
A single spanning tree made of IST and CST. The CIST in Figure 4 is composed of
each IST in every MST region and the CST.
CIST: Common and Internal Spanning Tree
MSTI: Multiple SpanningTree Instance
Region A0
vlan 1 mapped to Instance 1
vlan 2 mapped to Instance 2
Other vlans mapped to CIST
Region B0
vlan 1 mapped to Instance 1
vlan 2 mapped to Instance 2
Other vlans mapped to CIST
Region C0
vlan 1 mapped to Instance 1
vlan 2 and 3 mapped to Instance 2
Other vlans mapped to CIST
Region A0
vlan 1 mapped to Instance 1,
region root B
vlan 2 and 3 mapped to
Instance 2, region root C
Other vlans mapped to CIST
BPDU
BPDU
BPDU
CST: Common
Spanning Tree
A
C
B
D