XServe G5 - hardware indicates both ethernet ports are working; software only sees one

XServe G5

System Profiler only sees one ethernet port: en0

Server Monitor (Tiger) also only sees en0

But when cables are plugged into both ports, both green link lights come on, and both show blue activity lights.

Whichever switch (Cisco catalyst 2960) is connected to en0 (lower port) works fine; whichever is connected to the second (upper) port does not actually deliver any data to computers attached to that switch, they cannot see the server even though the switch indicates that something (the server) is attached to it.

I know the bottom ethernet port has to be connected first, and then the upper, and I did that. I have restarted the server.

Only the bottom port has ever been used on this server and I'm trying to use the upper one for the first time, do I need to configure it somehow? The manual is extremely minimal and doesn't mention anything about needing to manually configure it.

Thanks.