Installation Of BIND As A Secondary (Slave) DNS Server On CentOS
After we have installed BIND as a master DNS server (NS1), we can now try to set up a secondary DNS server (NS2) with BIND on CentOS. NS2 acts as a backup if there are problems with NS1.
Read more: http://howtoforge.com/installation-of-bind-as-a-secondary-slave-dns-server-on-centos