Monitoring – Network Traffic Monitoring Techniques

I'm going to advocate Pfsense again. PFsense can by default graph outgoing / ingoing bandwidth. If you add the Bandwithd plugin, you can graph the traffic coming out of each node of your network on a per-IP basis. It even displays the type of traffic.

It's really simple to setup and works very well. You can put in transparently on your network by putting it in network bridge.

Just found the following. Observium has support for nfsen, which could collect flows: http://www.observium.org/wiki/Configuration_Options#NfSen

Reference: http://postman.memetic.org/pipermail/observium/2013-December/004931.html