At work my HP ZBook is fed through a nice thunderbolt docking station. I plugin one cable, and everything is connected: power, monitors, network, etc.

Then we started working on a C# project that uses multicast to communicate. I couldn’t receive any messages. I thought it was a VLAN issue first, so I took my notebook and plugged it in the wall, using the outlet a colleague uses who didn’t have these problems with the same software. Suddenly my communication was working as well. So I ‘proved’ that the problem was the network right?

Wrong.

After hours of switching cables in the patch panels, checking traffic in the logs of the switch, I suddenly realized that by taking my laptop to another location, I wasn’t using the docking station anymore. As it turns out the network port on the docking station seems to have problems with the multicast messages.

I don’t know why, but when I use the network interface on my laptop instead of the one on the docking station, the multicast communication is working fine.

So, save yourself hours of investigating this problem, and check your other ethernet port first ;-)