Which mechanism should be added to a network design to identify unidirectional Spanning Tree Protocol failures through BPDU loss?

UDLD detects physical unidirectional link conditions by exchanging protocol PDUs, but it operates independently of STP and does not specifically react to BPDU loss within the spanning tree topology the way loop guard does.

Loop guard monitors non-designated and root ports for continuous BPDU reception; if a port stops receiving BPDUs - the characteristic symptom of a unidirectional link where transmission works but reception fails - it moves that port to a loop-inconsistent blocking state rather than transitioning it to forwarding. This directly prevents the Layer 2 loop that would otherwise form because the switch incorrectly assumes no designated bridge exists on that segment, making loop guard the correct mechanism for identifying STP failures caused by BPDU loss.

BPDU guard disables a PortFast-enabled access port upon receiving any BPDU, protecting against unauthorized switch connections, but it does not monitor for the absence of BPDUs or detect unidirectional STP failures.

Root guard prevents a port from accepting superior BPDUs that would change the root bridge position, protecting the STP topology from rogue root bridges, but it does not monitor BPDU reception or respond to unidirectional link failures.