I had a similar issue lately. It was caused by incompatible spanning tree protocols. The solution: make the cisco installation a separate spanning tree domain by disabling bpdu's on the link to the rest of the network. I used the following command:
description Uplink to Netgear
spanning-tree bpdufilter enable
It may not be possible to implement this if it is not acceptable to have two spantree domains. Still you can use this method for troubleshooting, and implement your customized solution afterwards.
The problem is most likely something known as the Dynamic trunking Protocol (DTP). It is a multicast message that is sent between Cisco switches to form a trunk. A non-Cisco device will forward this and it will seem to the Cisco switches plugged into this device that they are directly connected and they will form an ISL trunk by default. When only one switch is plugged up the problem should not occur, but once the second one is attached both ports will start sending and expect to receive frames with ISL headers.
To correct the problem, set the ports attached to the Netgear switch as access ports. This will turn off the DTP protocol on the ports.
We are pleased to announce availability of Beta software for 16.6.3.
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