1-14Table 1-6 Ports states supported by different port rolesPort rolePort state Rootport/master portDesignatedportBoundaryportAlternateport Backup portForwarding √ √ √ — —Learning √ √ √ — —Discarding √ √ √ √ √How MSTP WorksMSTP divides an entire Layer 2 network into multiple MST regions, which are interconnected by acalculated CST. Inside an MST region, multiple spanning trees are calculated, each being called anMSTI (Among these MSTIs, MSTI 0 is called the CIST). Similar to RSTP, MSTP uses configurationBPDUs to calculate spanning trees. The only difference between the two protocols is that an MSTPBPDU carries the MSTP configuration on the device from which this BPDU is sent.CIST calculationThe calculation of a CIST tree is also the process of configuration BPDU comparison. During thisprocess, the device with the highest priority is elected as the root bridge of the CIST. MSTP generatesan IST within each MST region through calculation, and, at the same time, MSTP regards each MSTregion as a single device and generates a CST among these MST regions through calculation. The CSTand ISTs constitute the CIST of the entire network.MSTI calculationWithin an MST region, MSTP generates different MSTIs for different VLANs based on theVLAN-to-MSTI mappings. MSTP performs a separate calculation process, which is similar to spanningtree calculation in STP/RSTP, 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.Implementation of MSTP on DevicesMSTP is compatible with STP and RSTP. STP and RSTP protocol packets can be recognized bydevices running MSTP and used for spanning tree calculation.In addition to basic MSTP functions, the device provides the following functions for ease ofmanagement:z Root bridge holdz Root bridge backupz Root guardz BPDU guardz Loop guardz TC-BPDU (a message that notifies the device of topology changes) guard
