JMicron JMC250 Gigabit Ethernet card doesn't work with MikroTik hEx (RB750Gr3)

I would’ve bet it’s impossible, but: two otherwise healthy Ethernet devices can refuse to talk to each other!

TL;DR: if one device supports Green Ethernet and the other one doesn’t (e.g. because it was made in 2008 when Green Ethernet didn’t exist), they may fail to establish a link.

Recently I was looking for some light distraction from grown-up responsibilities, and so decided to start a homelab: a little environment where I could play with networking, virtualisation, and maybe even home automation. The hypervisor would run on the first laptop I ever bought, the Asus P52F from 2011. Its Intel Core i3-380M is pitifully slow by modern standards, but it will do for a start.

I wanted to put the hypervisor behind the TV, where my “NAS”¹ is already located, but I didn’t want to run yet another Ethernet cable to there. So I picked up a MikroTik hEx that was left over from earlier networking experiments, and configured it to properly pass all the relevant VLANs.

After plugging everything in, the NAS and the TV worked fine, but the hypervisor simply wouldn’t get an IP. In fact, it wouldn’t even bring up the Ethernet link; the port was dead as if nothing was plugged into it. Some messing with cables and ports showed that:

• the hEx and its ports are fine, because they work with other devices
• the laptop is fine too, because its Ethernet works with MikroTik CRS112-8G-4S-IN and some unmanaged Mercusys 5-port Gigabit switch I have
• there must be some incompatibility between the hEx and the laptop.

This was so against my experience with Ethernet so far that I literally spent like 10 minutes pacing the room and thinking what I could have missed.

After finally accepting the reality, I turned to web search and quickly found Ubuntu bug #1497005. There is a technology called Green Ethernet which lets the devices negotiate the transmission power; this saves energy because most links don’t span the 100 metres that Ethernet supports. Apparently my laptop is so old that it doesn’t support Green Ethernet, and that probably breaks the negotiation with this particular MikroTik model.

The bug suggests to run the port at 100 Mbit/s, but that’s not acceptable for me, so I didn’t even try. Instead, I put the laptop on a separate Ethernet cable, and connected everything else through the hEx.

There’s the first time for everything.