I'm designing a business continuity plan. The company's website uses two internally-hosted servers at two different datacenters to provide name resolution. I've been asked to add a third WHOIS record hosted at UltraDNS or DynDNS, for "DR purposes". I.e.
ns1.company.com ns2.company.com NS1.ultradns.com
My question: What will happen if both of my internally-hosted DNS servers fail, and clients attempt to look up "company.com"? Will the client hang, time out, and then move on to the next? Is it entirely round-robin? Haven't been able to determine this from Googling, and wanted some backup.