Data Loss Prevention (DLP)
Install and configure the Network Monitor.
Enable the switch span port.
The Network Monitor then floods the network not connected to the span port with so many packets that the network goes down.
Disabling the span port brings the network back up.
There was a third network connection on the server.
That connection was, in some unknown way, causing a network loop when the span port on the switch was enabled.
Deleted the third network connection on the Monitor server.