Loop Guard handles the subtle failure: a blocked port stops receiving BPDUs (upstream STP process hung, CPU overload) — classic STP would age out and forward → loop. With Loop Guard the port goes loop-inconsistent and stays blocked until BPDUs return.

	Loop Guard	UDLD
Protects against	missing BPDUs (STP logic)	unidirectional links (physical)
Action	loop-inconsistent state	err-disable
Granularity	per VLAN	per port

Use on redundant uplinks / blocked alternate ports; combine with Root Guard; never on access ports.