246 CHAPTER 30: MSTP C ONFIGURATIONFor each switch in a network, the port through which the configuration BPDUwith the highest priority is received is chosen as the root port of the switch.■ Determining the designated portFirst, the switch generates a designated port configuration BPDU for each of itsport using the root port configuration BPDU and the root port path cost, with theroot ID being replaced with that of the root port configuration BPDU, root pathcost being replaced with the sum of the path cost of the root port configurationBPDU and the path cost of the root port, the ID of the designated bridge beingreplaced with that of the switch, and the ID of the designated port being replacedwith that of the port.The switch then compares the resulting configuration BPDU with the originalconfiguration BPDU received from the corresponding port on another switch. Ifthe latter takes precedence over the former, the switch blocks the local port andremains the port’s configuration BPDU unchanged, so that the port can onlyreceive configuration messages and cannot forward packets. Otherwise, theswitch sets the local port to the designated port, replaces the originalconfiguration BPDU of the port with the resulting one and releases it regularly.MSTP Implementationon SwitchesMSTP is compatible with both STP and RSTP. That is, switches with MSTPemployed can recognize the protocol packets of STP and RSTP and use them togenerate spanning trees. In addition to the basic MSTP functions, 3Com seriesswitches also provide the following other functions for the convenience of users tomanage their switches.■ Root bridge retaining■ Root bridge backup■ Root protection■ BPDU protection■ Loop guardRoot BridgeConfigurationTable 171 lists MSTP-related configurations about root bridges.Table 171 Root bridge configurationOperation Remarks Related sectionMSTP configuration RequiredTo prevent network topology jittercaused by other relatedconfigurations, you arerecommended to enable MSTP afterother related configurations areperformed.“MSTP Configuration” onpage 258MST regionconfigurationRequired “MST Region Configuration”on page 247Root bridge/secondaryroot bridgeconfigurationRequired “Root Bridge/Secondary RootBridge Configuration” onpage 249
