A broadcast storm happens when a network loop exists without protocols like Spanning Tree to block it. A single broadcast frame gets forwarded by one switch to another, which forwards it back, multiplying exponentially until the link capacity is saturated. This traffic overwhelms the processors of all connected devices, effectively taking down the entire network segment.
Heard on the job
Someone plugged both ends of a patch cable into the wall jack and caused a broadcast storm that took down the entire floor.
Insider tip
The fastest way to stop a broadcast storm in progress is to physically unplug the uplink cables between switches until the loop is broken.