Among the things that I'm a little bit embarrassed to admit: I'm a bit of a fan of Mikrotik low-end kit. They're often the right choice when you need something cheap and cheerful for a NAT firewall, IPv6 tunnel endpoint, VPN server, or similar.

The other day I was in the midst of configuring up an RB750GL and... it died. Hmm. Swapped power supplies with the other one (had two to configure). Nope, no joy. Oh well, infant mortalities happen.

I plugged in the other one - with the other power supply. Got a ways down in my config and... it died. OK, this is too much to be coincidence.

I tried powering up the old one and it booted up. I got the config changes I'd made out of it (whew!) and then plugged it back into the ethernet and... it died.

OK, now I'm starting to see what is going on here... oh look, port 1 on the Mikrotik (the uplink/NAT outside) is a POE port.

The Mikrotik's POE is a non-standard-standard - 12-24 volts on some of the pins on the Ethernet connector. It's not 802.3af or 802.3at, it's the low-end "standard" espoused by folks like Mikrotik and Ubiquiti.

The switch in the basement is a Juniper EX3200-48P. 48 ports of IEEE-standard-compliant 802.3af.

Apparently hooking these together was an unhappy thing to do and eventually caused the self-resetting thermal breaker in the Mikrotik to pop. It reset after things were unplugged and had a chance to cool down.

I turned off POE on the Juniper (set poe interface ge-0/0/47 disable) so it didn't try to start a POE handshake and left things running for a couple of days - no further issues or lasting ill effects.