1-11MSTIA multiple spanning tree instance (MSTI) refers to a spanning tree in an MST region.Multiple spanning trees can be established in one MST region. These spanning trees are independentof each other. For example, each region in Figure 1-4 contains multiple spanning trees known as MSTIs.Each of these spanning trees corresponds to a VLAN.VLAN mapping tableA VLAN mapping table is a property of an MST region. It contains information about how VLANs aremapped to MSTIs. For example, in Figure 1-4, the VLAN mapping table of region A0 is: VLAN 1 ismapped to MSTI 1; VLAN 2 is mapped to MSTI 2; and other VLANs are mapped to CIST. In an MSTregion, load balancing is implemented according to the VLAN mapping table.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 internal spanning tree (CIST)of the entire switched network. An IST is a special MSTI; it is a branch of CIST in the MST region.In Figure 1-4, each MST region has an IST, which is a branch of the CIST.CSTA CST is a single spanning tree in a switched network that connects all MST regions in the network. Ifyou regard each MST region in the network as a device, then the CST is the spanning tree generated bySTP or RSTP running on the "devices".CISTA CIST is the spanning tree in a switched network that connects all devices in the network. It comprisesthe ISTs and the CST.In Figure 1-4, the ISTs in the MST regions and the CST connecting the MST regions form the CIST.Region rootA region root is the root of the IST or an MSTI in an MST region. Different spanning trees in an MSTregion may have different topologies and thus have different region roots.In region D0 shown in Figure 1-4, the region root of MSTI 1 is Device B, and the region root of MSTI 2 isDevice C.Common root bridgeThe common root bridge is the root of the CIST. The common root bridge of the network shown in Figure1-4 is a device in region A0.Port roleDuring MSTP calculation, the following port roles exist: root port, designated port, master port, regionedge port, alternate port, and backup port.z A root port is used to forward packets to the root.z A designated port is used to forward packets to a downstream network segment or device.z A master port connects an MST region to the common root. The path from the master port to thecommon root is the shortest path between the MST region and the common root.
