H3C S5100-SI Operation Manual page 195

6) Port ID
A port ID used on an H3C device consists of two bytes, that is, 16 bits, where the first six bits represent
the port priority, and the latter ten bits represent the port number.
The default priority of all Ethernet ports on H3C devices is 128. You can use commands to configure
port priorities. For details, see
How STP works
STP identifies the network topology by transmitting configuration BPDUs between network devices.
Configuration BPDUs contain sufficient information for network devices to complete the spanning tree
calculation. Important fields in a configuration BPDU include:
Root bridge ID, consisting of root bridge priority and MAC address.
Root path cost, the cost of the shortest path to the root bridge.
Designated bridge ID, designated bridge priority plus MAC address.
Designated port ID, designated port priority plus port number.
Message age: lifetime for the configuration BPDUs to be propagated within the network.
Max age, lifetime for the configuration BPDUs to be kept in a switch.
Hello time, configuration BPDU interval.
Forward delay, forward delay of the port.
The implementation of the STP algorithm involves only the following four parts of a configuration BPDU:
Root bridge ID
Root path cost
Designated bridge ID
Designated port ID
1) Detailed calculation process of the STP algorithm
Initial state
Upon initialization of a device, each device 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.
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:
Configuring Port Priority.
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