Thursday, July 25, 2019

Is this switch setup creating a network loop?

One of the sites we manage has a network that was setup by a contractor some time before I started here, and we haven't made a whole lot of changes to the network itself. I've been tasked with troubleshooting an issue that effects primarily the WiFi.

Our documentation for this site was very incomplete, and its a remote site that we don't get to often. What we had done was upgrade the SonicWall with a more-or-less 1-to-1 swap on the configuration. After that switch over we started to get more complaints about the WiFi - I don't know if thats because for a while they saw more of us or if they had an uptick in incidents, but I was going off of the idea that there must be a problem with the SonicWall.

Until today. I've realized that there seems to be a loop of some sort and my background knowledge with this setup and different switchport modes is causing me to use extra caution. Here's the basics:

There's 3 Cisco switches, two (Switch1 and Switch2) are set up as Access switches with STP running and have interconnected DAC cables between them, the last one has every port set to Trunk. Switch 2 and 3 are connected together via an ethernet cable. Switch2 is supposed to have the only uplink to the WiFi APs. We have a Ubiquiti Toughswitch that has all the WiFi APs plugged into it. I can see the MAC addresses for the WiFi APs off of separate ports on Switch 2 and Switch 3, neither port being the uplink port between the two of them.

So, it appears that we have a access port with STP facing the Touchswitch, and a trunk port facing the touchswitch, on two different switches that are otherwise themselves connected.

Outages last 1-2 minutes on average, come seeming at random, and only seem to effect WiFi users. Are these the types of conditions that would create a loop?



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