Connecting to two ISP in local network using two different network

I have a single network interface eth0, We have two internet providers one ISP using modem IP 192.168.1.1 (BSNL India) Other ISP modem IP was 192.168.2.1 (Airtel) now when ever my internet goes down in any one of the ISP it want to access other one how can i configure ? DId i want to configure interface settings ? I'am pretty sure we need a route. Any help will be most appreciated.