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Recently I managed to trigger storm control the university end-point I was using (they told me it triggers at 10% bandwidth broadcast traffic). The end-point had a switch plugged in, the switch had a PC and DHCP router plugged in (both switch and router are home category). I'm not sure what happened, by I may have accidentally plugged a port of the switch into another port of the switch (all my cables were blue ...), creating a loop for a few minutes, while I was moving everything around. Could this generate a broadcast storm? It might be important: the PC runs nmap and arp-scan every 15 minutes on our local network.

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    Short answer: yes it might be. Because you accidentally plugged a port of the switch into antoher switch you created a loop. If spanning tree wouldn't be active, this loop could be the cause for a network shutdown. If spanning tree was active, it could take some minutes (depends on the used spanning tree) for the layer 2 devices to get back to normal. – Sascha R. Nov 10 '17 at 14:03
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A bridge loop is practically guaranteed to cause a broadcast storm. A broadcast frame entering any switch port will get forwarded to both looped ports, both copies consequently returning back to the switch, both being forwarded out the original source port and also sent into the loop again, circling until you break the loop.

Essentially, for each broadcast frame entering the switch, you get two frames exiting the switch again on each port per forwarding cycle. With a switching latency of 20 μs, that's 100,000 frames/s.

For a simple ARP request, each frame runs 528 bits over the wire, so 100K frames/s are already more than 50 Mbit/s.

Now, a second broadcast frame comes along and enters the switch, producing another 100,000 frames/s...

  • Wow, I'll look twice next time before I plug in anything :D – fbence Nov 10 '17 at 19:44

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