Switch – STP not preventing switching loop

You should not be using "on" for link aggregation as it can lead to problems. On the side with aggregation statically on, it will use the interfaces in the etherchannel, no matter what the configuration is on the other side.

While storm control (from comments) can be very helpful with some of the problems that result, it does not resolve all of them. For instance if one of the remote side links is an access port on a different VLAN, all traffic that goes down that port will likely never reach its destination. Depending on how constant the traffic and the load balancing across the etherchannel, this may result in a complete "outage" for some hosts.

I always recommend the use of LACP over PAgP, so instead of desirable/auto or on, use either active on both sides or active on one and passive on the other. The reason for this is that LACP is standards based while PAgP is Cisco proprietary.

Of course this is in part dependent on the hardware platform, so check the appropriate documentation for your platform.

Agree with both Ron and Mike Pennington here. The entire point of Spanning Tree is to prevent broadcast storms from blowing up your network and you have just recently observed a practical lesson in how that happens.

Knowing the version of your Netgear switch would be helpful. I'm looking at an older version of the ProSafe switch user manual for the GS748TS and the Spanning-Tree settings are quite granular, so you should be able to configure the switch to prevent such things from happening in the future.

On the older NetGear ProSafe switches I have used the default configuration is that STP is enabled in RSTP (rapid spanning tree) with BPDU flooded to all ports on the switch. This is a good configuration for everyday use. In order to help you more, we would need to know how many VLANs are on your network and so forth.

But at the end of the day this is a hard lesson in the need for company policies that govern what can and cannot be connected to a production network. At my employer (a large Fortune 500 manufacturing enterprise) our network policy is that all access ports on switches are configured with BPDUguard enabled such that when any unknown switching device is plugged into it the port becomes error disabled. If you have SNMP monitoring on your switches, then you will get an alert.

There is a small price to be paid in user frustration but this is more than made up for in the fact that you will automatically have visibility and hopefully control over what gets connected and you won't have to deal with the network outages in the future.