All systems are shut down for patching and then restarted.
After restart, the Data Collector (DC) does not start/reconnect to the Data Aggregator (DA).
All supported DX NetOps Performance Management releases
The DA must be running before the DC’s start because the DC will make a registration call to the DA and if there is no answer it cannot start.
Make sure the DA is fully started before starting the DC’s
If the DA is already running, a simple restart of the DC dcmd service should resolve the problem.