For server admin-types: bandwidth capping
 

We have a server with 30Mbps, and we recently switched from a provider who capped the bandwidth at 30Mbps to one that gives us 30Mbps on 95th percentile.

Most of the time, the box isn't using that much bandwidth, but it makes me nervous to think that we could have a sudden burst (this box serves some of our gallery traffic) that would last long enough to blow past 5%, and kill our bandwidth bill.

The ISP claims that they can't (or don't) cap bandwidth, but my limited research indicates it's quite possible to do it using IPTables, TC and/or IPFW. However, I know next to nothing about how to actually do this.

Anyone know if it's possible? And if so, how difficult it is to configure? Is there any downside, other than limiting the ability to burst past our 30Mbps cap?