How Stp Works - H3C LS-3100-52P-OVS-H3 Operation Manual

All the ports on the root bridge are designated ports.
Path cost
Path cost is a reference value used for link selection in STP. By calculating path costs, STP selects
relatively robust links and blocks redundant links, and finally prunes the network into a loop-free tree.

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 cost of the path 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.
Hello time: configuration BPDU transmission interval.
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 of configuration BPDUs:
Root bridge ID (represented by device priority)
Root path cost (related to the rate of the link connected to the port)
Designated bridge ID (represented by device priority)
Designated port ID (represented by port name)
Calculation process of the STP algorithm
1) 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.
2) 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