1-14z Forwarding state. Ports in this state can forward user packets and receive/send BPDUs.z Learning state. Ports in this state can receive/send BPDUs but do not forward user packets.z Discarding state. Ports in this state can only receive BPDUs.Port roles and port states are not mutually dependent. Table 1-6 lists possible combinations of portstates and port roles.Table 1-6 Combinations of port states and port rolesPort rolePort stateRoot/masterportDesignatedportRegionBoundaryportAlternateportBackupportForwarding √ √ √ — —Learning √ √ √ — —Discarding √ √ √ √ √Principle of MSTPMSTP divides a Layer 2 network into multiple MST regions. The CSTs are generated between theseMST regions, and multiple spanning trees (also called MSTIs) can be generated in each MST region. Aswell as RSTP, MSTP uses configuration BPDUs for spanning tree calculation. The only difference is thatthe configuration BPDUs for MSTP carry the MSTP configuration information on the switches.1) Calculate the CISTThrough comparing configuration BPDUs, the switch of the highest priority in the network is selected asthe root of the CIST. In each MST region, an IST is calculated by MSTP. At the same time, MSTPregards each MST region as a switch to calculate the CSTs of the network. The CSTs, together with theISTs, form the CIST of the network.2) Calculate an MSTIWithin an MST region, MSTP generates different MSTIs for different VLANs based on theVLAN-to-instance mappings. MSTP performs a separate calculation process, which is similar tospanning tree calculation in STP, for each spanning tree. For details, refer to How STP works.In MSTP, a VLAN packet is forwarded along the following paths:z Within an MST region, the packet is forwarded along the corresponding MSTI.z Between two MST regions, the packet is forwarded along the CST.MSTP Implementation on SwitchesMSTP is compatible with both STP and RSTP. That is, MSTP-enabled switches can recognize theprotocol packets of STP and RSTP and use them for their respective spanning tree calculation.The 3Com switches 4210 support MSTP. After MSTP is enabled, the switch operates in MSTP mode bydefault. If the network contains switches that run the STP/RSTP protocol, you can use commands toconfigure the switch to operate in STP-compatible mode or RSTP-compatible mode (see Configuringthe MSTP Operation Mode for more information):z In STP-compatible mode, all ports of the 3Com switch 4210 send out STP BPDUsz In RSTP mode, all ports of the 3Com switch 4210 send out RSTP BPDUs.
