Operation Manual – MSTPH3C S3100 Series Ethernet Switches Chapter 1 MSTP Configuration1-13III. VLAN mapping tableA VLAN mapping table is a property of an MST region. It contains information abouthow VLANs are mapped to MSTIs. For example, in Figure 1-4, the VLAN mapping tableof region A0 is: VLAN 1 is mapped to MSTI 1; VLAN 2 is mapped to MSTI 2; and otherVLANs are mapped to CIST. In an MST region, load balancing is implementedaccording to the VLAN mapping table.IV. ISTAn internal spanning tree (IST) is a spanning tree in an MST region.ISTs together with the common spanning tree (CST) form the common and internalspanning tree (CIST) of the entire switched network. An IST is a special MSTI; it is abranch of CIST in the MST region.In Figure 1-4, each MST region has an IST, which is a branch of the CIST.V. CSTA CST is a single spanning tree in a switched network that connects all MST regions inthe network. If you regard each MST region in the network as a switch, then the CST isthe spanning tree generated by STP or RSTP running on the "switches".VI. CISTA CIST is the spanning tree in a switched network that connects all switches in thenetwork. It comprises the ISTs and the CST.In Figure 1-4, the ISTs in the MST regions and the CST connecting the MST regionsform the CIST.VII. Region rootA region root is the root of the IST or an MSTI in an MST region. Different spanningtrees in an MST region may have different topologies and thus have different regionroots.In region D0 shown in Figure 1-4, the region root of MSTI 1 is switch B, and the regionroot of MSTI 2 is switch C.VIII. Common root bridgeThe common root bridge is the root of the CIST. The common root bridge of the networkshown in Figure 1-4 is a switch in region A0.IX. Port roleDuring MSTP calculation, the following port roles exist: root port, designated port,master port, region boundary port, alternate port, and backup port.z A root port is used to forward packets to the root.
