Operation Manual – MSTPH3C S3100 Series Ethernet Switches Chapter 1 MSTP Configuration1-5Table 1-3 Selection of the root port and designated portsStep Description1 A non-root-bridge device takes the port on which the optimum configurationBPDU was received as the root port.2Based on the configuration BPDU and the path cost of the root port, thedevice calculates a designated port configuration BPDU for each of the restports.z The root bridge ID is replaced with that of the configuration BPDU of theroot port.z The root path cost is replaced with that of the configuration BPDU of theroot port plus the path cost corresponding to the root port.z The designated bridge ID is replaced with the ID of this device.z The designated port ID is replaced with the ID of this port.3The device compares the calculated configuration BPDU with theconfiguration BPDU on the port whose role is to be determined, and acts asfollows based on the comparison result:z If the calculated configuration BPDU is superior, this port will serve asthe designated port, and the configuration BPDU on the port will bereplaced with the calculated configuration BPDU, which will be sent outperiodically.z If the configuration BPDU on the port is superior, the device stopsupdating the configuration BPDUs of the port and blocks the port, sothat the port only receives configuration BPDUs, but does not forwarddata or send configuration BPDUs.Note:When the network topology is stable, only the root port and designated ports forwardtraffic, while other ports are all in the blocked state – they only receive STP packets butdo not forward user traffic.Once the root bridge, the root port on each non-root bridge and designated ports havebeen successfully elected, the entire tree-shaped topology has been constructed.The following is an example of how the STP algorithm works. The specific networkdiagram is shown in Figure 1-2. The priority of Device A is 0, the priority of Device B is 1,the priority of Device C is 2, and the path costs of these links are 5, 10 and 4respectively.
