H3C Technologies H3C SecPath F1000-E - Manual (Page 196)

3
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:
• Root bridge ID: consisting of the priority and MAC address of the root bridge.
• Root path cost: the path cost to the root bridge.
• Designated bridge ID: consisting of the priority and MAC address of the designated bridge.
• Designated port ID: designated port priority plus port name.
• Message age: age of the configuration BPDU while it propagates in the network.
• Max age: maximum age of the configuration BPDU can be maintained on a device.
• Hello time: configuration BPDU interval.
• Forward delay: the delay used by STP bridges to transit the state of the root and designated ports
to forwarding.
NOTE:
For simplicity, the descriptions and examples below involve only four fields of configuration BPDUs:
• Root bridge ID (represented by device priority)
• Root path cost
• Designated bridge ID (represented by device priority)
• Designated port ID (represented by port name)
Calculation process of the STP algorithm
• 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.
• 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:
Table 2 Selection of the optimum configuration BPDU
Step Actions
1
Upon receiving a configuration BPDU on a port, the device performs the following:
• If the received configuration BPDU has a lower priority than that of the configuration BPDU
generated by the port, the device discards the received configuration BPDU and does not
process the configuration BPDU of this port.
• If the received configuration BPDU has a higher priority than that of the configuration BPDU
generated by the port, the device replaces the content of the configuration BPDU generated
by the port with the content of the received configuration BPDU.
2
The device compares the configuration BPDUs of all the ports and chooses the optimum
configuration BPDU.