Configuration BPDU

Configuration BPDUs are used most commonly and are sent to exchange topology information among STP devices.

Each bridge actively sends configuration BPDUs during initialization. After the network topology becomes stable, only the root bridge actively sends configuration BPDUs. Other bridges send configuration BPDUs only after receiving configuration BPDUs from upstream devices. A configuration BPDU is at least 35 bytes long, including the parameters such as the BID, root path cost, and PID. A bridge processes a received configuration BPDU only when it finds that at least one of the sender BID and PID is different from that on the local receive port. If both fields are the same as those on the receive port, the bridge drops the configuration BPDU. In this way, the bridge does not need to process BPDUs with the same information as the local port.

A configuration BPDU is sent in one of the following scenarios:
After STP is enabled on ports of a device, the designated port on the device sends configuration BPDUs at Hello intervals.
When the root port on a device receives a configuration BPDU, the device sends a copy of the configuration BPDU to each of its designated ports.
When a designated port receives an inferior configuration BPDU, the designated port immediately sends its own configuration BPDU to the downstream device.