CAS SERVER HA

Hi Team,

   I would like to know how to create High availability for Exchange 2013 cas cu 21. Currently i am having two cas server in our DC. Now i am building it for other site (DR). I want the cas server should come up seamless when my CAS server goes down .

Please help me how to configure.Is the cas array will help in case of disaster. 

Thanks 

Alex

Alexy