96
Figure 31 Network diagram and topology of MST region 3
MST region
A multiple spanning tree region (MST region) consists of multiple devices in a switched network and
the network segments among them. All these devices have the following characteristics:
• A spanning tree protocol enabled
• Same region name
• Same VLAN-to-instance mapping configuration
• Same MSTP revision level
• Physically linked together
Multiple MST regions can exist in a switched network. You can assign multiple devices to the same
MST region, as shown in Figure 30.
• The switch
ed network contains four MST regions, MST region 1 through MST region 4.
• All devices in each MST region have the same MST region configuration.
MSTI
MSTP can generate multiple independent spanning trees in an MST region, and each spanning tree
is mapped to the specific VLANs. Each spanning tree is referred to as a multiple spanning tree
instance (MSTI).
In Figure 31, MST
region 3 contains three MSTIs, MSTI 1, MSTI 2, and MSTI 0.
VLAN-to-instance mapping table
As an attribute of an MST region, the VLAN-to-instance mapping table describes the mapping
relationships between VLANs and MSTIs.
In Figure 31, the VLAN-to
-instance mapping table of MST region 3 is as follows:
• VLAN 1 to MSTI 1.
• VLAN 2 and VLAN 3 to MSTI 2.
• Other VLANs to MSTI 0.
MSTP achieves load balancing by means of the VLAN-to-instance mapping table.
CST
The common spanning tree (CST) is a single spanning tree that connects all MST regions in a
switched network. If you regard each MST region as a device, the CST is a spanning tree calculated
by these devices through STP or RSTP.
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