Cisco Certified Network Associate (CCNA) 2025 – 400 Free Practice Questions to Pass the Exam

The PDU that Spanning Tree Protocol (STP) switches exchange to elect a root bridge is called the Bridge Protocol Data Unit (BPDU). BPDUs are crucial messages that carry information about the bridge's identity, path costs, and port states, which helps in the process of loop detection and network topology management.

In the context of STP, these BPDUs are used by switches to share their bridge identifier and path cost, enabling them to determine the best root bridge for the network. The switch with the lowest bridge ID becomes the root bridge, and this selection process is facilitated by the continuous exchange of BPDUs. This is essential for maintaining a loop-free topology in Ethernet networks.

The other options do not accurately represent the correct terminology used in the context of STP. For instance, Link Control Data Unit, Switch Protocol Data Unit, and Switch Management Data Unit are not recognized acronyms or terms that denote the messages exchanged in the Spanning Tree Protocol. Thus, BPDU is the established term that reflects the protocol's functionality in the election of the root bridge.