Networking
Loopback Detection
Also known as: Loop Guard, Loop Protection
A switch feature that detects and blocks Layer 2 loops on access ports — where STP isn't running or a user accidentally connected two ports.
Loopback detection is a switch feature that identifies and responds to Layer 2 loops — specifically on access ports where STP may not be running or where a user has accidentally connected two ports together.
STP handles loops between switches. Loopback detection handles a different problem: a user accidentally plugging both ends of a cable into ports on the same switch, or a cheap unmanaged switch connected downstream that creates a loop STP can't see.
When loopback detection fires, the switch can shut down the offending port, send an SNMP alert, or both. The loop is broken before it becomes a broadcast storm.
This matters most on access-layer ports — the ports that end users and devices plug into. Most access switches let you enable loopback detection on all access ports as a default policy, providing a safety net for the most common causes of unintentional loops.
It's a simple feature to enable and cheap insurance against a class of incidents that can bring down an entire network segment in seconds.