Recently, I customer was saying they were having issues on their new Mist WLAN. Marvis is a great tool but it failed me on this one occasion.
I looked at the Service Levels for the network and then notice in Insights / Events all sorts of Events for AP Reflection. I asked several Juniper folks and they all said, “that’s a loop.” I also looked at Switch Inights and STP Topology Changed“
Finally, a colleague said “check the logs.” He always says that!
Then I open Remote Shell on one of the switches and use show log messages | match TOPO_CH. At this point, just use any switch if you arent sure where the loop may be. I did and sure enough I see a lot of messages with “TOPO_CH” in the output.
I see the port as xe-0/1/3. Knowing that’s the uplink port, I look to see which switch that is so I can investigate it.
Switch ending with a MAC of 66:5b is my next target. Open that switch and go to Remote Shell and repeat the above steps.
You’re looking for a port that is not an uplink. You may have to look at several switches. In this case, the port reported back with the loop is ge-0/0/39. Not an uplink so what is on that port?
On that port was a Ubiquiti switch. This is not part of the expected network. When I disabled the port, all the messages stopped. I reported it to the customer to see why the loop is happening on that device.