Operation Manual – MSTPH3C S7500 Series Ethernet Switches Chapter 1 MSTP Configuration1-61.1.3 Implementation of MSTPMSTP divides a network into multiple MST regions at Layer 2. The CST is generatedbetween these MST regions, and MSTIs can be generated in each MST region. As wellas RSTP, MSTP uses configuration BPDUs to generate spanning trees. The onlydifference is that the configuration BPDUs for MSTP carry the MSTP configurationinformation on the switches.I. Generating the CISTThrough configuration BPDU comparing, the switch that is of the highest priority in thenetwork is chosen as the root of the CIST. In each MST region, an IST is generated byMSTP. At the same time, MSTP regards each MST region as a switch to figure out theCST of the network. The CST, together with the ISTs, forms the CIST of the network.II. Generating an MSTIIn an MST region, different MSTIs are generated for different VLANs depending on theVLAN-to-MSTI mappings. Each MSTI is figured out independently, in the same way asSTP/RSTP.III. Implementation of 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 comparison is based on thepath costs. Suppose S is the sum of the root path cost and the corresponding pathcost of the port. The less the S value is, the higher the priority of the configurationBPDU 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.
