STP BPDU Types

In conventional IEEE 802.1D STP, there are two types of Bridge Protocol Data Units (BPDUs):

  • Configuration BPDU, sent by root bridges to provide information to all switches.
  • TCN (Topology Change Notification), sent by bridges towards the root bridge to notify changes in the topology.

Both of these play a vital role in the STP topology change process which takes place in the event that an STP topology changes, such as when a port goes up down.

https://networklessons.com/spanning-tree/spanning-tree-topology-change-notification-tcn/