DNS issue on DR site

Hi ,

We are facing the following issue

Customer has 2 sites : Primary & DR 

Primary:  When primary DNS server is down the client is able to query the alternate DNS in the TCPIP settings. The client able to do the resolution from secondary dns 

DR : The clients are not able to query the 2ndary dns if the primary is down. If the 2ndary dns IP is moved to primary dns entry, it is fine. 

Regards,

Ashok