1-11For example, all the devices in region A0 in Figure 1-4 have the same MST region configuration asfollows: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), andz The same MSTP revision level (not shown in the figure).Multiple MST regions can exist in a switched network. You can assign multiple devices to the same MSTregion.VLAN-to-MSTI mapping tableAs an attribute of an MST region, the VLAN-to-MSTI mapping table describes the mapping relationshipsbetween VLANs and MSTIs. In Figure 1-4, for example, the VLAN-to-MSTI mapping table of region A0is: VLAN 1 is mapped to MSTI 1, VLAN 2 to MSTI 2, and the rest to CIST. MSTP achieves loadbalancing by means of the VLAN-to-MSTI mapping table.ISTAn 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 internalspanning tree (CIST) of the entire network. An IST is a section of the CIST. An IST is a special MSTI.In Figure 1-4, for example, the CIST has a section in each MST region, and this section is the IST in therespective MST region.CSTThe CST is a single spanning tree that connects all MST regions in a switched network. If you regardeach MST region as a “device”, the CST is a spanning tree calculated by these devices through STP orRSTP. CSTs are indicated by red lines in Figure 1-4.CISTJointly constituted by ISTs and the CST, the CIST is a single spanning tree that connects all devices in aswitched network.In Figure 1-4, for example, the ISTs in all MST regions plus the inter-region CST constitute the CIST ofthe entire network.MSTIMultiple spanning trees can be generated in an MST region through MSTP, one spanning tree beingindependent of another. Each spanning tree is referred to as a multiple spanning tree instance (MSTI).In Figure 1-4, for example, multiple MSTIs can exist in each MST region, each MSTI corresponding tothe specified VLANs.Regional root bridgeThe root bridge of the IST or an MSTI within an MST region is the regional root bridge of the IST or theMSTI. Based on the topology, different spanning trees in an MST region may have different regionalroots.For example, in region D0 in Figure 1-4, the regional root of MSTI 1 is device B, while that of MSTI 2 isdevice C.
