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.
The Monitor Server is a HP ProLiant DL36JG5 running Windows Server 2008 R2.
The server is connected to a Cisco Catalyst 3850 WS-C3850-48U.
The Cisco is running IOS CAT3k-CAA-Universal-K9M.
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.