Issues with Time Machine over Firewall

I am having issues with time machine over a firewall. I am at a large university and my XServe that i am trying to serve as the time machine server is located in a Colocation Server room across campus. It is on a different subnet and is has a cisco firewall between the computers i want to backup. What i have tried: • Open all ports of firewall - no luck • manage the computer via OD and set the time machine to an AFP share that is set a time machine drive. - this fails when backups are attempted any idea how to solve it?