A Bridge Protocol Data Unit (BPDU) is a data message transmitted across a local area network to detect loops in network topologies. BPDUs are frames that contain information about the spanning tree protocol (STP) . A switch sends BPDUs using a unique source MAC address from its origin port to a multicast address with destination MAC. There are two kinds of BPDUs for 802.1D Spanning Tree: Configuration BPDU, sent by root bridges to provide information to all switches, and Topology Change Notification (TCN), sent by bridges towards the root bridge to notify changes in the topology, such as port up or port down. By default, the BPDUs are sent every 2 seconds. BPDU Guard is a security feature found in multiple networking devices that helps to prevent attacks on a network by blocking BPDUs that are sent from unauthorized devices. When BPDU Guard is enabled, it will immediately disable any port that receives a BPDU, reducing the risk of attacks on the network. It is recommended to enable BPDU Guard as an added layer of security for your network.