DNS gets reset each time i connect to pppoe

[Guest]

i use rp-pppoe to connect to my ISP.
i have manually configured the ip and sub-net mask, and also applied service name, Login Name, password and DNS as provided by my ISP.

Now the problem is each time i connect using sudo pppoe-start, i have to again configure my DNS. i can't get how the DNS disappear with each login?

Can anybody help me with this?