CRM and ERP Integration

Dear friends,

The guy in functional team ask me for a solution to integrate CRM system and ERP system. As I know there are three choice:

- ERP system there is additional CRM module

- CRM system there is additonal ERP module

- two independent ERP,CRM system integrate together by some interface.

Is there interface to connect CRM and ERP as Core Interface which integrate ERP and SCM system.

Could you please tell me the big picture, because I could not found the exact solution and I have a little knowledge about functional of these system.

I am looking forward from you.

Thanks and Best Regards.