doubt Router

Hello good personal day
I have a doubt:
I have a TMG server with two network cards, one card turned in another swtich on Router GVT
if I open the software Router ISP, it is the configuration of the DNS IP public consultation, which is in this case the DNs of ISP. ok up there.
In my DC Server tab forwards (routers), I put the DNS of google and not the Dns ISP.
My question now is:
When you request to which DNS which will be used?
Google this in my DNS forwarders or setted in this software Router ISP?
thank you