As I suspected, the customer had disabled spanning tree due to concerns about the speed of failover. They had also managed to patch a layer 2 loop into their network during a minor change, causing an unchecked loop to circulate frames out of control, bringing down their entire cell site. I explained to them the value of STP, and why any outage caused by it would be better than the out of control loop they had. I was told to mind my own business. They didn’t want to enable spanning tree because it was slow. Yes, I said, but only when there is a loop! And in that case, a short outage is better than a meltdown. Then I realized the customer and I were in a loop, which I could break by closing the case. Newer technologies (such as SD-Access) obviate the need for STP, but if you’re doing classic Layer 2, please, use it.
Read the article over at SubnetZero here.