This is a normal alert, if there is no follow up alert that shows Status:Cleared, then you need to check if your Veth0 or management port is online and can ping your DNS server from within the DD CLI. This alert is telling you because you restarted external network, DD can no longer see the DNS server, under normal circumstance this alert will auto clear itself once DNS is accepting requests  Hope this helps
Hello,  When I've just rebooted my network conectivity my DataDomain gave me a lot of erro's like:   Does it mean that DataDomain sends some data to emc.com? What kind of data and for what reason it sends?  DNSUnresponsive Message:Unable to communicate with configured DNS. Severity:CRITICAL Class:Network Object ID:  Event ID:EVT-NETM-00009 Additional Info:DNS-Servers = see output from net show dns Status:Cleared Description:  Action: 
Datadomain 3300 - Reboot network -