RPC uses port 135 to connect to the port mapper, then assigns a random high port for further communication to the server. This essentially means you need to open 135 and pretty much all ports 1000 and up.
However, in practice, what I see is that it connects to TCP 135, then pretty much sticks to TCP 445 every time it communicates. I had my firewall admin verify this is what he saw too. TCP 445 is a well-known MS file sharing port.
What I'd try is opening TCP 135 and 445 on your firewall going to your NetApp controllers and see if that works.