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1-11 

Basic concepts in MSTP  

CST

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 D0
VLAN 1 mapped to instance 1, 
B as regional root bridge
VLAN 2 mapped to instance 2, 
C as regional root bridge
Other VLANs mapped to CIST

BPDU

BPDU

BPDU

C

D

B

A

 

 

Assume that all devices in 

 are running MSTP. This section explains some basic concepts of 

MSTP.  

A multiple spanning tree region (MST region) consists of multiple devices in a switched network and the 
network segments among them. These devices have the following characteristics: 

z

 

All are MSTP-enabled, 

z

 

They have the same region name, 

z

 

They have the same VLAN-to-instance mapping configuration, 

z

 

They have the same MSTP revision level configuration, and 

z

 

They are physically linked with one another. 

For example, all the devices in region A0 in 

 have the same MST region configuration: 

z

 

The same region name, 

z

 

The same VLAN-to-instance mapping configuration (VLAN 1 is mapped to MSTI 1, VLAN 2 to 
MSTI 2, and the rest to the common and internal spanning tree (CIST, that is, MSTI 0), and 

z

 

The same MSTP revision level (not shown in the figure). 

Multiple MST regions can exist in a switched network. You can use an MSTP command to assign 
multiple devices to the same MST region.