If an interface with BPDU Guard enabled receives a BPDU from another switch, what happens?

When BPDU Guard is enabled on a switch interface, it is designed to enhance the security of the Spanning Tree Protocol (STP) by preventing potential loops that could occur when unexpected BPDUs (Bridge Protocol Data Units) are received on ports that are not supposed to receive them. BPDU Guard is often enabled on ports configured as edge ports, which are expected to connect only to end devices rather than other switches.

If an interface configured with BPDU Guard receives a BPDU from another switch, the proper response is for that interface to go into an err-disabled state, effectively shutting down the port. This action protects the network from possible loops and misconfigurations that could arise from connecting an unauthorized switch to the network. The err-disabled state means that the port is administratively disabled and must be manually re-enabled or be configured to recover automatically after a specific time.

In summary, the correct action taken when an interface with BPDU Guard receives a BPDU is to shut down that interface to prevent potential network issues, which aligns with the correct answer.