BPDU stands for Bridge Protocol Data Unit, which is a type of message used by the Spanning Tree Protocol (STP) to exchange information between network switches. BPDU messages are sent out by switches to communicate their status and configuration information to other switches in the network.

BPDU messages contain information such as the identity of the switch that sent the message, the priority of the switch, the identity of the root bridge, and the cost of the path from the sending switch to the root bridge. By exchanging BPDU messages, switches can determine the topology of the network and identify the best path to the root bridge, which is the switch that serves as the central point of the STP.

BPDU messages are sent periodically by switches and are used to detect changes in the network topology, such as the addition or removal of switches or links. When a switch receives a BPDU message, it compares the information in the message to its own configuration and makes decisions about its role in the STP, such as whether to become the root bridge or to block certain ports to prevent loops.

BPDU messages play a critical role in the operation of STP and are used to ensure that the network topology remains loop-free and optimized for performance. In addition to STP, BPDU messages are also used in other Spanning Tree protocols such as Rapid Spanning Tree Protocol (RSTP) and Multiple Spanning Tree Protocol (MSTP).

Configuration BPDU Frame

A BPDU frame consists of a standard Ethernet header, which includes the source and destination MAC addresses, a type field set to 0x0000, and a payload containing the BPDU message. The BPDU message contains information such as the switch ID, the priority of the switch, the identity of the root bridge, and the cost of the path from the sending switch to the root bridge.

The payload of BPDU includes the folowing fields:

• Protocol ID - Fixed at 0x0000, which represents IEEE 802.1d.
• Protocol version ID - Spanning tree protocol version ID. The protocol version ID for STP is 0x00.
• BPDU type - Type of the BPDU. The value is 0x00 for a configuration BPDU.
• Flags - An 8-bit field indicates the purpose of the BPDU. The lowest bit is the Topology Change (TC) flag. The highest bit is the Topology Change Acknowledge (TCA) flag. All other bits are reserved.
• Root ID - Root bridge ID formed by the priority and MAC address of the root bridge.
• Root path cost - Cost of the path to the root bridge.
• Bridge ID - Designated bridge ID formed by the priority and MAC address of the designated bridge.
• Port ID - Designated port ID formed by the priority and global port number of the designated port.
• Message age - Age of the configuration BPDU while it propagates in the network.
• Max age - Maximum age of the configuration BPDU stored on the switch.
• Hello time - Configuration BPDU transmission interval.