Please find attached one of our Switch configuration. this switch has been interconnected another switches in the network.
Today morning one of the user brought a Dlink switch and connected two port to the attached switch. there was two connection were dropped in cabin from the switch. suddenly all our other switch including the attached switch were started blinking . network is tottaly collapsed...no one was able to work.
atlast we idenfied the culprit and switched of the Dlink switch.
As per my understading Spanning -tree is been configured in the swiches. why it was not blocking the other link from the Dlink switch?
Could you please check the attached configuration a and guide me to prevent such incidents in future.
If you say a user brought it to work, I assume it's one of those cheap-o, 3 for a buck, home-user switches. Those don't have spanning tree. All the switches in the topology need to have spanning-tree enabled. I believe what happened was the dumb D-Link switch, not knowing spanning tree, doesn't participate in the whole BPDU, this port is forwarding, this port is blocking message exchange, so both links on both switched were in FWD mode. It probably only took a couple of broadcasts to take your network down.
I encountered this scenario about a year back, and what I did was enable Broadcast Storm Control on the "Smart Switch" (P.S: It wasn't CISCO)
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