H3C Technologies H3C S12500-X Series Switches - Manual (Page 81)

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• 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 Figure 18 , the switched network comprises four MST regions, MST region 1 through 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 19 , 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 19 , 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
Figure 18 represent the CST.
IST
An internal spanning tree (IST) is a spanning tree that runs in an MST region. It is also called MSTI 0, a
special MSTI to which all VLANs are mapped by default.
In
Figure 18 , MSTI 0 is the IST in MST region 3.
CIST
The common and internal spanning tree (CIST) is a single spanning tree that connects all devices in a
switched network. It consists of the ISTs in all MST regions and the CST.
In
Figure 18 , the ISTs (MSTI 0) in all MST regions plus the inter-region CST constitute the CIST of the entire
network.
Regional root
The root bridge of the IST or an MSTI within an MST region is the regional root of the IST or MSTI. Based
on the topology, different spanning trees in an MST region might have different regional roots.
In MST region 3 in
Figure 19 , the regional root of MSTI 1 is Device B, the regional root of MSTI 2 is Device
C, and the regional root of MSTI 0 (also known as the IST) is Device A.
Common root bridge
The common root bridge is the root bridge of the CIST.
In
Figure 18 , the common root bridge is a device in MST region 1.