With ASUS the iGPU such as the excellent Ivy Bridge 2500 or 4000 will only work out-of-the-box if you have no other GPU installed in your system.
In case you have any other Graphics Card installed, you will have to do the following steps in order to get the iGPU running:
- Boot with your external Graphics card into UEFI/BIOS and select:
Advanced Config -> Advanced Tab -> System Agent Config -> Graphics -> GPU Multi Monitor -> Enable
It is always a good idea to get familiar with the other options in the UEFI Bios as well. Now you boot into your Operating System with the DVI or Display Port Cable attached to your iGPU. It worked if you can see your OS Welcome screen. I tested it with Microsoft Windows, but this behaviour should be the same for any Operating System.