...,Its not difficult , I have configured it previously in my company.................., to configure it u must of 2 NIC Cards,.......
One is for internel interface and other is for external.....
U have to just add default gateway as ur internal ip-adress provided by BSNL...
If u r having a problem .., then first try to configure it in WINDOWS ....
If u can't configure it in windows ...., then u must call Customer support of BSNL for this...................
After that replicate the same in Linux..............
Gaurav.