This is an incomplete bug report. I write it down the same for future reference or just for someone to pick it up.

Case: RB2011 with latest version (6.32.2) with no configuration applied. Connect one port to a live LAN on any of RB ethernet port. After a few seconds the CPU goes straight to 100%, profiler says it's all "management". The only solution is power-cycling the device.
After disabling the discovery interfaces on IP neighbor the problem disappear.

The LAN is very low traffic, however I see that there are some Ubiquiti UniFi connected, on which I bet for the cause of the problem. With a torch running I see some multicast packets coming from them right before che CPU spike.

The scary part is that the problem seems to "pass" from one router to another, as the problem happened also on another RB2011 connected through a wireless link a few seconds after the first one.