HP 3100 Series Configuration Manual page 66

Figure 17 Network diagram and topology of MST region 3
To MST region 4
Device A
Device C
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. In
MST region 4, and 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 17, MST region 3 comprises 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 17, the VLAN-to-instance mapping table of MST region 3 is: VLAN 1 to MSTI 1, VLAN 2
and VLAN 3 to MSTI 2, and 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.
The blue lines in
MST region 3
VLAN 1 MSTI 1
VLAN 2&3 MSTI 2
Other VLANs MSTI 0
Figure 16, the switched network comprises four MST regions, MST region 1 through
Figure 16 represent the CST.
A
Device B
C
A
C
Device D
57
B A
D C
MSTI 1
B
D
MSTI 0
Topology of MSTIs in MST region 3
B
D
MSTI 2
Regional root
MSTI