This is known issue caused by UBNT. They block frames with unusual ethernet frame type, which are being used by ROMON: https://community.ubnt.com/t5/UniFi-Feature-Requests/Passing-RoMON-packets/idi-p/1976558 . There are reports that EdgeSwitch can be configured to accept and pass these, but also people are saying that on UniFi switches, it did not work. Personally, I was not able to set it up to allow romon pass through these.and I ended up with creating overlay management network with MPLS/VPLS: http://forum.mikrotik.com/t/issue-with-romon-over-eoip/95957/1