I have a L009 on RouterOS and a CSS326 on SwOS, and I experienced a (not easily reproducible) strange issue, where plugging in an ethernet lead for a trunk port I was setting up, caused all lights on all ports (including unused ports) to blink on the Switch, then on the Router (only the connected ports on router).

I can't find any documentation or previous threads to explain this - what would cause all LEDs on a device to blink like that? Could it be a short or something?

I checked the ethernet lead with a cable tester and there don't appear to be any issues with it. I have since plugged the cable back into the original Switch + Router port that caused this (after trying many other ports), and the issue isn't occurring anymore.

Both devices are powered by their original power blocks, and there is no PoE in use.

EDIT: Reproduced it now, and it's primarily the Switch blinking the ports (and causing Router ports connected to the Switch to blink) - an independently connected port on the Router just stays lit as normal.

Broadcast storm perhaps? Ethernet loop or faulty network device?

Thanks, it looks to be that - still learning a few basics.