H3C Technologies H3C S7500E Series Switches - Manual (Page 164)

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They have the same region name,
They have the same VLAN-to-instance mapping configuration,
They have the same MSTP revision level configuration, and
They are physically linked with one another.
For example, all the devices in region A0 in Figure 13-4 have the same MST region configuration:
The same region name,
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
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.
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 13-4 , for example, the VLAN-to-instance mapping
table of region A0 is as follows: VLAN 1 is mapped to MSTI 1, VLAN 2 to MSTI 2, and the rest to CIST.
MSTP achieves load balancing by means of the VLAN-to-instance mapping table.
IST
An internal spanning tree (IST) is a spanning tree that runs in an MST region.
ISTs in all MST regions and the common spanning tree (CST) jointly constitute the common and
internal spanning tree (CIST) of the entire network. An IST is the section of the CIST in an MST region,
as shown in
Figure 13-4 .
CST
The 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. For example, the red lines in
Figure 13-4 represent the CST.
CIST
Jointly constituted by ISTs and the CST, the CIST is a single spanning tree that connects all devices in
a switched network.
In Figure 13-4 , for example, the ISTs in all MST regions plus the inter-region CST constitute the CIST
of the entire network.
MSTI
Multiple spanning trees can be generated in an MST region through MSTP, one spanning tree being
independent of another. Each spanning tree is referred to as a multiple spanning tree instance (MSTI).
In
Figure 13-4 , for example, multiple spanning trees can exist in each MST region, each spanning tree
corresponding to the specific VLAN(s). These spanning trees are called MSTIs.
Regional root bridge
The root bridge of the IST or an MSTI within an MST region is the regional root bridge of the IST or the
MSTI. Based on the topology, different spanning trees in an MST region may have different regional
roots.
For example, in region D0 in Figure 13-4 , the regional root of MSTI 1 is device B, while that of MSTI 2
is device C.