Problem with a loopback

Greetings Everybody!
I have a strange issue I’m hoping someone can help me with.
I put together a utility power monitoring device as described here http://gregsowell.com/?p=2093.
From what I understand (and have tested) it’s as simple as twisting the send and receive pairs together.
I plugged it into port 5 on my RB750 (in bridge mode) and within 30 seconds the Access Point at that site and both ends of the Back Haul from that site to another were down. Completely unresponsive to ping and/or power cycling.

I can go on explaining what it did to the rest of my network, but at this point I think its irrelevant.
I disabled port 5 on the RB750 that the loopback with relay was plugged into and the problem went away.
I have recreated this issue at a different site with a different loopback-relay and RB750.

I welcome any and all thoughts..

Sorry, I’m running v4.16 due to the rebooting and ntp time sync issues.

Did you remove port 5 from the switch before doing this? I have not tried that setup, but if it is still on the switch, I can’t see that being a good thing.

that worked!
thanks!