Operation Manual – MSTPH3C S5600 Series Ethernet Switches Chapter 1 MSTP Configuration1-171.2.3 Principle of MSTPMSTP divides a Layer 2 network into multiple MST regions. The CSTs are generatedbetween these MST regions, and multiple spanning trees (also called MSTIs) can begenerated in each MST region. As well as RSTP, MSTP uses configuration BPDUs forspanning tree calculation. The only difference is that the configuration BPDUs forMSTP carry the MSTP configuration information on the switches.I. Calculate the CISTThrough comparing configuration BPDUs, the switch of the highest priority in thenetwork is selected as the root of the CIST. In each MST region, an IST is calculated byMSTP. At the same time, MSTP regards each MST region as a switch to calculate theCSTs of the network. The CSTs, together with the ISTs, form the CIST of the network.II. Calculate an MSTIIn an MST region, different MSTIs are generated for different VLANs based on theVLAN-to-MSTI mappings. Each spanning tree is calculated independently, in the sameway as how STP/RSTP is calculated.III. Implement STP algorithmIn the beginning, each switch regards itself as the root, and generates a configurationBPDU for each port on it as a root, with the root path cost being 0, the ID of thedesignated bridge being that of the switch, and the designated port being itself.1) Each switch sends out its configuration BPDUs and operates in the following waywhen receiving a configuration BPDU on one of its ports from another switch:z If the priority of the configuration BPDU is lower than that of the configurationBPDU of the port itself, the switch discards the BPDU and does not change theconfiguration BPDU of the port.z If the priority of the configuration BPDU is higher than that of the configurationBPDU of the port itself, the switch replaces the configuration BPDU of the port withthe received one and compares it with those of other ports on the switch to obtainthe one with the highest priority.2) Configuration BPDUs are compared as follows:z The smaller the root ID of the configuration BPDU is, the higher the priority of theconfiguration BPDU is.z For configuration BPDUs with the same root IDs, the path costs are compared.Suppose S is the sum of the root path costs and the corresponding path cost of theport. The less the S value is, the higher the priority of the configuration BPDU is.z For configuration BPDUs with both the same root ID and the same root path cost,the designated bridge ID, designated port ID, the ID of the receiving port arecompared in turn.3) A spanning tree is calculated as follows:
