Tuesday, March 27, 2018

Stopping a loop behind an IP phone

Hi, All

I had an issue the other day where someone plugged in a small 5-port netgear switch behind a Mitel phone and caused a loop. It wasn’t enough traffic to prevent management access so I was able to locate and shut down the offending port ahead of the phone pretty easily.

The network is all Procurve switches, multiple VLAN’s and a layer 3 switch as the core. The port was a trunk and had the voice VLAN tagged and the data VLAN untagged. Spanning tree was on with bpdu-protection enabled to shut the port if it receives a BPDU since it is essentially an access port but the port stayed open. I am assuming that since the phone was between the small switch with the loop and my switch that BPDU’s don’t get beyond the phone to be included in the looped traffic. I could be wrong but that was my best guess.

Is there something more I should be setting with spanning tree that could have shut the port when this happens? I tried enabling the HP loop-protect feature with transmit-interval of 1 but that didn’t stop it when I introduced the same scenario on a lab switch.

What am I missing? TIA!



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