DNS Flood: NXDOMAIN Flood 

A DNS flood is a form denial of service (DoS) attack at the DNS resolution level. It entails flooding a DNS server, which clients consult for server name resolution, with DNS queries for nonexistent (NXDOMAIN) domains.
This ties up the DNS server with trying to resolve and return results for invalid requests and can flood the server's cache thereby prohibiting legitimate server name resolution requests from being serviced.

Persisting DNS entries of valid and operationally important domain and servers, from an organizational pespective, in the DNS server's cache will allow DNS queries from legitimate users to still get served even when an NXDOMAIN flood attack is on-going.