On Jul 9, 20:31, "Daniel W. McRobb" <dwm@ans.net> wrote:
When I asked Cisco about this (a while ago), they said flow-switching incurred a 20% overhead (which someone there called "minimal", which I didn't agree with).
I guess this has changed; we have a clear performance improvement.
'tis very unwise to run IP accounting on a very busy router. We wouldn't dare turn it on for a core router w/ 40,000 routes. Some of our customers who have done so on border routers immediately turn arond and complain about performance problems. :-( And we're not talking
Yes, if the box doesn't have the CPU to do it, don't. But I said 'edge routers'. Core routers are the wrong places to do it anyway, depending on paradigm; but generally, traffic will be collected from and broken down into fairly small streams. It's only when it passes through core routers the proportions are too large; a 'border router' for a large network/customer is in this context a core router.
"fairly small" streams is only true if you don't have bandwidth-heavy customers. We have several customers w/ lots of bandwidth, who use it.
'tis also very important to know the route taken for a net when analyzing the data. Discrepancies here can be huge and completely invalidate any conclusions you might make about how much traffic is traversing a given path.
For this we like combined interface stats; AS-based traffic matrices are (from our point of view) mostly useful for end-to-end measurements.
traversing a given path. This is particularly true for busy end routers (like NAP peers) and core routers.
One wouldn't do it here anyway, as noted.
Then you potentially miss huge contributions to your network load (NAP routers <-> T3 customers, for example).
I also prefer measurements that won't potentially get dropped in transit (throwing a big unknown into confidence level of data). Is there an efficient means of retrieving flow data from a Cisco and not potentially dropping a bunch of it on the floor?
You may drop something from time to time, but this simply turns the data collection into sampling. With a sufficient sample size, this is as good as anything; you still want to correlate your information with other sources, such as interface stats, and adjust accordingly. But please, let us not turn this into "my box is better than your box".
I wasn't doing that. I asked an honest question. Dropping data at indeterminate points, unknown to the measurement tool, is not turning collection into sampling (well, at least not useful sampling). If you have no means of determining how many were dropped, and under what conditions, you have essentially useless data. Interface stats won't get you that information. If you drop traffic at times of heavy bursts you lose some of the most interesting data (that which may be causing your network real pain). Is there an efficient means of retrieving flow data from a Cisco and not potentially dropping a bunch of it on the floor? Will there be at some point in the future? We'd use it if it was available. We'd even pay for it, I imagine. :-) Daniel ~~~~~~