Ports necessary for Oracle 11g Replication across a firewall?
I don't think any other ports than the listener ports are required.
I suspect that the ports need to be open on your source AND target database/firewall, allowing both incoming and outgoing connections. Also, the host running RMAN has to be able to reach the listener ports, but that's probably default.

If tnsping works to both servers from the host you are running RMAN from, i think you should be all set.