Round robin is the default behavior of DNS servers. You must have two records for name server of your domain. One is DC DNS, the other is DR DNS. All requester receive these two records but the order changes every request. For example, in the first response, let DC DNS be the first record, and DR DNS be the second record, as for the second request DC DNS will be the first record, and DR DNS will be the second record.
If DR DNS is down, this means no response, then client tries the other record(DNS), in our example: DR DNS.
That is it.