sys admin network monitor

[redhat]

I am looking for an open source piece of software that will monitor all
of our LAN traffic.  I have a Cisco 3550 that handles our LAN and can
turn on port mirroring to monitor everything going out our router - at
least that is what I was told I had to do.  I was looking at a
Windows-based piece from Computer Associates (E Trust Intrusion
Detection) that was really nice.  It monitored everything - and I mean
everything and gave me full feedback on where people were going, etc. 
Problem was that it was way to expensive for what I could rationalize. 
Is there anyone out there that is familiar with CA's E Trust Intrusion
Detection and a possible open source alternative.  I have run ethereal
and it was nice but did not give me "all" of the information that I
really wanted.
thanks,
DF