Bridge Protocol Data Unit (BPDU) Frame

The information about the spanning tree protocol is stored in frames called Bridge Protocol Data Unit, referred by using the term BPDUs. The following pieces of information are used to transmit the BPDUs by the switches:

  • A unique MAC address from its origin
  • Uses a multicast address as destination MAC address.

The switches should share all the information about the switches itself and also the information about their connections to work STP algorithms properly. And the information which is shared by the switches is BPDUs(Bridge Protocol Data Unit). Other layer2 switches or the bridges which are listen to accept this information can accept it only and which are transmitted in the form of multicast frames. The switches will take care of disabling the ports if there is any loop is presented in the network topology from one device to another. There should be the only path for the transmission.

There may many situations that cause a change in the network including,

  • Addition of new link
  • Link gets to go down
  • Addition of new switch
  • Failure of switch

In such a situation, a new loop-free topology can be created by executing the STP algorithm with all this information along with the BPDUs by the switches. The main advantage of using his BPDU is it speeds up the convergence. Where convergence is the total amount of time that is taken to handle all the changes occurred and getting the network back up and running. All the information about the switch id will be contained in the BPDU frame.

We can say that the default advertisement time of BPDU is 2 seconds. This amount of time will reduce the amount of downtime and all the changes made have been shared with all other switches in that network very quickly.

Following are the different kinds of BPDU:

  1. Configuration BPDU: This type of Bridge Protocol Data Unit provides information to the switches and is used by the Spanning tree protocol.
  2. Topology Change Notification(also referred to as TCN): This type of Bridge Protocol Data Unit tells about all the changes that are happened in the network topology.
  3. Topology Change Acknowledgment(also referred to as TCA): This type of Bridge Protocol Data Unit is used for the confirmation of the reception of the above type, TCN.

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Purpose of using BPDU:

  • To avoid Broadcast storms
  • To avoid layer 2 switching loops