H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 169)

20-11
z
They have the same VLAN-to-instance mapping configuration,
z
They have the same MSTP revision level configuration, and
z
They are physically linked with one another.
For example, all the devices in region A0 in Figure 20-4 have the same MST region configuration:
z
The same region name,
z
The same VLAN-to-instance mapping (VLAN 1 is mapped to MST instance 1, VLAN 2 to MST
instance 2, and the rest to the command and internal spanning tree (CIST). CIST refers to MST
instance 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 group
multiple devices to the same MST region.
2) 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 MST instances. In
Figure 20-4 , for example, the VLAN-to-instance
mapping table of region A0 describes that the same region name, the same VLAN-to-instance mapping
(VLAN 1 is mapped to MST instance 1, VLAN 2 to MST instance 2, and the rest to CIST). MSTP
achieves load balancing by means of the VLAN-to-instance mapping table.
3) IST
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 the given MST region.
In Figure 20-4 , 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 “device”, the CST is a spanning tree calculated by these devices through STP or
RSTP. For example, the red lines in
Figure 20-4 describe the CST.
5) 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 20-4 , 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 20-4 , for example, multiple spanning tree can exist in each MST region, each spanning tree
corresponding to a VLAN. 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 MST or that
MSTI. Based on the topology, different spanning trees in an MST region may have different regional
roots.
For example, in region D0 in Figure 20-4 , the regional root of instance 1 is device B, while that of
instance 2 is device C.