Types of Bridge Protocol Data Unit (BPDUs)

In this article, we will understand the types of Bridge Protocol Data Unit (BPDU). It is the process in which switches are presented in the STP domain that enables exchanging messages. The Spanning Tree Algorithm would use the transmission of BPDU to recognize the type of network. After the devices are attached to the ports, the data transmission does not start immediately rather they start moving from one state to another. This movement is used by the BPDU to determine the topology of the network. Hence, enabling the participation of the switches to gather information about one another.

Types of BPDU

There are mainly two types of BPDU −

  • Configuration BPDU – It is responsible for providing information to all the switches

  • Topology Change Notification BPDU – It is responsible for announcing changes in the topology in the form of notifications.

Configuration BPDU

Configuration BPDU which are generated by the network’s root switch acts as a firewall to the network to save it from external threats. It also controls and authenticates the flow of information going away from the root bridge. This type of BPDU is exchanged between the devices to elect a root node from them by comparing the lowest switch ID that is transmitted and the current root switch.


The configuration BPDU works in the following steps −

  • After the network starts every switch considers itself the root switch

  • The configuration BPDUs are generated by each switch and transmitted at regular intervals.

  • The switch port receives the configuration BPDU and compares its switch ID and the received BPDU root switch ID

  • If the root switch ID is greater −

    • It continues to work as a non-root switch

    • Doesn’t generate configuration BPDU

    • Updates some fields of the received BPDU

    • Sends this updated BPDU back to its destination port

  • If the root switch ID is lesser −

    • Responds with its configuration BPDU

    • If a path failure occurs, the root port does not receive the configuration BPDU. In this case, the switch port considers itself as the root and sends configuration BPDU and TCN BPDU to itself. Then, an STP calculation process is done to form a new path and network connectivity restoration is done.


  • The flow of data within the network is controlled and authenticated

  • Network protection from external threats

  • Root switch selection of the network

  • Loop detection in network topology

Topology Change Notification BPDU

The TCN BPDU is generated by the non-root switches and directed towards the root switch of the network. It is used to create a notification to the root node about the topological changes in the network along with the currently regulating network topology. The root switch after receiving this notification alarms all the other switches of the network about this topological change. Various reasons like the link or switch failure may be responsible for the topology change of the network.


The working process of TCN BPDU can be explained in the following steps.

  • As and when a switch encounters a change in the topology, it creates a TCN BPDU

  • The TCN BPDU contains all the information about the currently changed topology and transmits it to the root node

  • The root node after receiving the notification should send an acknowledgement to the switch sending the notification. This acknowledgement is known as Topology Change Acknowledgement (TCA) BPDU.

  • The TCN BPDU may not directly traverse to the root node of the network in the case of the multilevel structure of the network.

  • The intermediate switch which received the TCN BPDU from the initial switch again creates a TCN BPDU of its own and directs it upstream towards the root node.

  • The root node after finally receiving the TCN BPDU should notify the whole network about the change in the network.

  • The root node creates a configuration and directs it downstream to the whole network.


  • Helps to carry the notification about the topological changes in the network

  • Notifies the root node about changes.

Significance of BPDU

The important information like the switch ID or MAC address of the computer and the network it is connected to is stored by the Bridge Protocol Data Unit. The users connected to the network are also authenticated by the credentials by taking into account the switch port address and adjacent port address. It authenticates the messages that are being transferred both upstream and downstream in the network. It manages the layers of communication in an organization by applying the spanning tree protocol. It conditions when the server is under attack like Broadcast Storm it works efficiently.


Thus, BPDU is a fundamental tree traversing convention which transmits every message from all switches to the network along with various other characteristics of the network. A BPDU which is sent by the root bridge to the other switch nodes is called a configuration BPDU, the switches receive it, process it and advertise the data received by them with the help of a Hello BPDU.