I’m about to take in my first couple co-location customers and I’m wondering what would be the best way to audit their bandwidth? Should I bother with writing something, or is MRTG (or RRDtool) good enough?

It is very easy to write something using SNMP to track bandwidth usage. I use a home-grown script, and also monitor with MRTG. I use an old version of MRTG, however, and it only does peak, current, and average. What percentile do you plan to bill on? If it is the 95th, at least my version won't work, not sure about the latest version of MRTG.

I plan to bill at 95% also, but as I understand it, MRTG probes for current util. every n minutes, right?

There's a link to an outside site at the MRTG site that explains 95th percentile billing using MRTG.

http://www.seanadams.com/95/

Hope you find it useful.

Nice patch, thanks for the tip. I'll install it and see how it looks on my test servers. Does anyone use it in a production environment?

Yes, nice page -- thanks.