3Com 2928 User Guide - Page 221

Table 1-6 Ports states supported by different port roles Port state Forwarding Learning Discarding Root Designated port/master port port √ √ √ √ √ √ Port role Boundary port √ √ √ Alternate port - - √ Backup port - - √ How MSTP Works MSTP divides an entire Layer 2 network into multiple MST regions, which are interconnected by a calculated CST. Inside an MST region, multiple spanning trees are calculated, each being called an MSTI (Among these MSTIs, MSTI 0 is called the CIST). Similar to RSTP, MSTP uses configuration BPDUs to calculate spanning trees. The only difference between the two protocols is that an MSTP BPDU carries the MSTP configuration on the device from which this BPDU is sent. CIST calculation The calculation of a CIST tree is also the process of configuration BPDU comparison. During this process, the device with the highest priority is elected as the root bridge of the CIST. MSTP generates an IST within each MST region through calculation, and, at the same time, MSTP regards each MST region as a single device and generates a CST among these MST regions through calculation. The CST and ISTs constitute the CIST of the entire network. MSTI calculation Within an MST region, MSTP generates different MSTIs for different VLANs based on the VLAN-to-MSTI mappings. MSTP performs a separate calculation process, which is similar to spanning tree 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 Devices MSTP is compatible with STP and RSTP. STP and RSTP protocol packets can be recognized by devices running MSTP and used for spanning tree calculation. In addition to basic MSTP functions, the device provides the following functions for ease of management: z Root bridge hold z Root bridge backup z Root guard z BPDU guard z Loop guard z TC-BPDU (a message that notifies the device of topology changes) guard 1-14