H3C WA Series, Configuration Manual

Page: 37 / 76

7-11
z
They are physically linked with one another.
For example, all the bridges in region A0 in Figure 7-4 have the same MST region configuration:
z
The same region name,
z
The same VLAN-to-MSTI 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 bridges to the same MST region.
2) VLAN-to-MSTI mapping table
As an attribute of an MST region, the VLAN-to-MSTI mapping table describes the mapping relationships
between VLANs and MSTIs. In
Figure 7-4 , for example, the VLAN-to-MSTI 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-MSTI mapping table.
3) 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 a section of the CIST.
In Figure 7-4 , for example, the CIST has a section in each MST region, and this section is the IST in the
respective MST region.
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 “bridge”, the CST is a spanning tree calculated by these “bridges” through STP or
RSTP. For example, the red lines in
Figure 7-4 represent the CST.
5) CIST
Jointly constituted by ISTs and the CST, the CIST is a single spanning tree that connects all bridges in a
switched network.
In Figure 7-4 , for example, the ISTs in all MST regions plus the inter-region CST constitute the CIST of
the entire network.
6) 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 7-4 , 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.
7) 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 7-4 , the regional root of MSTI 1 is bridge B, while that of MSTI 2 is
bridge C.
8) Common root bridge
The common root bridge is the root bridge of the CIST.
In Figure 7-4 , for example, the common root bridge is a bridge in region A0.