3Com 2928 User Guide - Page 210

All the ports on the root bridge are designated ports. How STP Works The devices on a network exchange BPDUs to identify the network topology. Configuration BPDUs contain sufficient information for the network devices to complete spanning tree calculation. Important fields in a configuration BPDU include: z Root bridge ID: consisting of the priority and MAC address of the root bridge. z Root path cost: the cost of the path to the root bridge. z Designated bridge ID: consisting of the priority and MAC address of the designated bridge. z Designated port ID: designated port priority plus port name. z Message age: age of the configuration BPDU while it propagates in the network. z Max age: maximum age of the configuration BPDU can be maintained on a device. z Hello time: configuration BPDU interval. z Forward delay: the delay used by STP bridges to transit the state of the root and designated ports to forwarding. For simplicity, the descriptions and examples below involve only four fields in the configuration BPDUs: z Root bridge ID (represented by device priority) z Root path cost z Designated bridge ID (represented by device priority) z Designated port ID (represented by port name) Calculation process of the STP algorithm z Initial state Upon initialization of a device, each port generates a BPDU with itself as the root bridge, in which the root path cost is 0, designated bridge ID is the device ID, and the designated port is the local port. z Selection of the optimum configuration BPDU Each device sends out its configuration BPDU and receives configuration BPDUs from other devices. The process of selecting the optimum configuration BPDU is as follows: 1-3