Operation Manual – MSTPH3C S3610&S5510 Series Ethernet Switches Chapter 1 MSTP Configuration1-142) VLAN-to-instance mapping tableAs an attribute of an MST region, the VLAN-to-instance mapping table describes themapping relationships between VLANs and MST instances. In Figure 1-4, 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 toMST instance 2, and the rest to CIST). MSTP achieves load balancing by means of theVLAN-to-instance mapping table.3) ISTInternal spanning tree (IST) is a spanning tree that runs in an MST regionISTs in all MST regions and the common spanning tree (CST) jointly constitute thecommon and internal spanning tree (CIST) of the entire network. An IST is a section ofthe CIST in the given MST region.In Figure 1-4, for example, the CIST has a section in each MST region, and this sectionis the IST in the respective MST region.4) CSTThe 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 bythese devices through STP or RSTP. For example, the red lines in Figure 1-4 describethe CST.5) CISTJointly constituted by ISTs and the CST, the CIST is a single spanning tree thatconnects all devices in a switched network.In Figure 1-4, for example, the ISTs in all MST regions plus the inter-region CSTconstitute the CIST of the entire network.6) MSTIMultiple spanning trees can be generated in an MST region through MSTP, onespanning tree being independent of another. Each spanning tree is referred to as amultiple spanning tree instance (MSTI). In Figure 1-4, for example, multiple spanningtree can exist in each MST region, each spanning tree corresponding to a VLAN. Thesespanning trees are called MSTIs.7) Regional root bridgeThe root bridge of the IST or an MSTI within an MST region is the regional root bridge ofthe MST or that MSTI. Based on the topology, different spanning trees in an MST regionmay have different regional roots.For example, in region D0 in Figure 1-4, the regional root of instance 1 is device B,while that of instance 2 is device C.8) Common root bridgeThe common root bridge is the root bridge of the CIST.
