Licensing Required

Why is it so hard...

What licenses are required for 3 Windows servers to backup to an EMC Data Domain?

I am getting so many different answers from my peers.

I think I need  

Count  Product

   1  Enterprise Server (Master)

   2  Media Server

   3  Open Drive Whatever It Is called

Note: It is a count, not a sequential list, that was a coincidence.

Thanks for any timely replies.