> > But when ns1.example.com dns was down, actually some people can't > > lookup the domain.
What Chris said. If a user's system (stub resolver) is only configured to know about ns1.example.com, when ns1 becomes non-responsive, the net breaks for said user. The user should have both nameservers configured for their stub resolver (e.g. /etc/resolv.conf should contain addresses of both name servers).
