Global Catalog Connectivity

I have 3 domain controllers. One on an internal LAN, Private IP address. One on the DMZ of a 3Com Firewall, public IP. One on a remote site, behind an Earthlink firewall, private IP with a NAT to a public IP.

Right now, the DC on the DMZ is theonly Global Catalog. I need to move the GC to the server behind the 3Com firewall. What I need to know, is how to configure the firewall to allow connectivity to the GC. I have opened numerous ports, to no avail.

Any thoughts would be appreciated