Les Mikesell wrote:
There are several tools that will collect interface traffic data via SNMP and record it so you can graph, show high/low/average values over a time span, etc. Cacti (in the epel repo) is probably the easiest to set up, OpenNMS (http://www.opennms.org) probably the most complete. These could also get their data from a port on a managed switch or router if that makes it easier to show the connections you need to split out.
One thing to note for billing, often times bandwidth is billed on a 95th percentile level, and cacti is not good for that if you want accuracy.
We use RTG(in my research last year it seemed RTG was the most frequently mentioned tool that was best for this purpose) to measure our main pipes for billing comparison purposes, matches much closer to what the ISPs say, and cacti is quite a bit off. I wouldn't rely on RTG for normal network monitoring(UI isn't that good etc), but for links where billing information is important at least for 95th percentile, don't rely on cacti alone.
Note RTG is not the same as MRTG, though I think I recall seeing RTG was inspired by MRTG.
Not sure how OpenNMS handles that sort of thing.
Not to knock cacti, I use it extensively, currently have a server collecting more than 20 million points of data a day.
nate