ntop vs nagios to Monitoring Networking traffic
Most solutions for bandwidth monitoring are going to Poll the SNMP (
IF-MIB) interface counters to figure out bandwidth.
Nagios monitoring is really going to functional for bandwidth only, not analysis of the traffic, also you will need some sort of graphing plugin.
If you want actual analysis and not just bandwidth, ntop is a good choice. Best practice would be to run ntop on a machine that doesn't also run things that effect production.
If you just want bandwidth graphs you might look at Cacti.