MSTP Overview 151Figure 50 Port rolesPort stateIn MSTP, a port can be in one of the following three states:■ Forwarding state. Ports in this state can forward user packets and receive/sendBPDU packets.■ Learning state. Ports in this state can receive/send BPDU packets.■ Discarding state. Ports in this state can only receive BPDU packets.Port roles and port states are not mutually dependent. Table 100 lists possiblecombinations of port states and port roles.Principle of MSTP MSTP divides a Layer 2 network into multiple MST regions. The CSTs are generatedbetween these MST regions, and multiple spanning trees (also called MSTIs) canbe generated in each MST region. As well as RSTP, MSTP uses configuration BPDUsfor spanning tree calculation. The only difference is that the configuration BPDUsfor MSTP carry the MSTP configuration information on the switches.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 calculatedby MSTP. At the same time, MSTP regards each MST region as a switch to calculatethe CSTs of the network. The CSTs, together with the ISTs, form the CIST of thenetwork.Table 100 Combinations of port states and port rolesPort role/Port stateRoot/port/MasterportDesignatedportRegion edgeportAlternateport Backup portForwarding ‚X ‚X ‚X - -Learning ‚X ‚X ‚X - -Discarding ‚X ‚X ‚X ‚X ‚XMST regionCABDPort 4Port 1 Port 2Connected to thecommon rootEdge portMaster port Alternate portDesignatedportBackup portPort 3Port 5 Port 6MST regionCABDPort 4Port 1 Port 2Connected to thecommon rootEdge portMaster port Alternate portDesignatedportBackup portPort 3Port 5 Port 6MST regionCABDPort 4Port 1 Port 2Connected to thecommon rootEdge portMaster port Alternate portDesignatedportBackup portPort 3Port 5 Port 6MST regionCMST regionCABDPort 4Port 1 Port 2Connected to thecommon rootEdge portMaster port Alternate portDesignatedportBackup portPort 3Port 5 Port 6
