4-11
Basic Concepts in MSTP
Figure 4-4 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
Figure 4-4 are running MSTP. This section explains some basic concepts of
MSTP.
MST region
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
Figure 4-4 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.
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