Booting the installation cd with pxe and grub2

[gatinois]

Hi, please help,

I've set up a pxe server (dhcp & tftp) on my lan. That part works ok. When I boot the client computer it downloads and executes grub.pxe. I then get a grub2 prompt:
grub>
Next I set let up a loopback device:
grub>loopback loop (pxe)/install-x86-minimal-20140415.iso
grub>ls (loop)/ shows the iso file properly mounted on device loop.
Then I:
grub>linux (loop)/isolinux/gentoo
grub>initrd (loop)/isolinux/gentoo.igz
grub>boot
and the boot process ends with:
!! Could not find the root block device in .

If I enter /dev/sda1 it boots the debian system that is installed there. Anything else just gives me a similar error:
!! Block device XXX is not a valid root device...
!! Could not find the root block device in .

I've tried (pxe), (loop), etc... for a block device, but nothing works. What should I use, what is that block device for?
How do I get install-x86-minimal-20140415.iso to boot as if it was burned on a cd?

Thank you for your help.