alert tool for out-of-norm bandwidth?

I'm looking for some sort of tool that tracks bandwidth on a switch port and when the bit rate "suddenly" changes from the baseline alerts me. I'm playing with Team Cymru's darknet project and basically need an automated way of noting darknet traffic increase and then trigger a nagios alert. If there isn't something like this, I'll have to write it I guess. thanks. -- matthew zeier - "Curiosity is a willing, a proud, an eager confession of ignorance." - Leonard Rubenstein