On Jul 9, 17:57, "Daniel W. McRobb" <dwm@ans.net> wrote:
The sampling Curtis mentioned on the NSS routers is a bit different. For one, it doesn't really impact forwarding performance (and hopefully, if/when implemented on the Bay, will not impact forwarding performance).
Not to pick nits, but what I quoted was a cache snapshot; caches don't impact performance under normal circumstances, though their construction may do so.
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).
But what we're specifically looking for in terms of continuous sampling (such as that we do on the NSS routers) is a net matrix... if you changed the Cisco flow switching stuff to use network numbers (and mask), you'd have something very much like what we're looking for in terms of continuous sampling. From there we build AS matrices, etc.
Yes ... but you shouldn't need anything special for that. We have been doing the same for a long time, using regular IP accounting on the edge routers, which is then summarised over a full routing table. The only discrepancies that occur are if changes in routing occur between the time of accounting and processing, but this tends not to be a problem.
'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 about access products either. '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. This is particularly true for busy end routers (like NAP peers) and core routers.
The other thing about the flow-switching data that's different than the NSS (and probably what we'll get from Bay) is that there aren't really any nice ways of retrieving/storing the data automatically.
This used to be true, but "probably" isn't true any longer.
Maybe, maybe not. I can't imagine trying to get flow stat data from a router w/ 40,000 routes and (probably) a monstrous flow table via UDP. A lot of this boils down to granularity... for continuous collection on busy routers for network architecture purposes, host-level granularity is frivolous and overly consumptive of many resources. 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? Daniel ~~~~~~
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.
'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.
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". -- ------ ___ --- Per G. Bilse, Mgr Network Operations Ctr ----- / / / __ ___ _/_ ---- EUnet Communications Services B.V. ---- /--- / / / / /__/ / ----- Singel 540, 1017 AZ Amsterdam, NL --- /___ /__/ / / /__ / ------ tel: +31 20 6233803, fax: +31 20 6224657 --- ------- 24hr emergency number: +31 20 421 0865 --- Connecting Europe since 1982 --- http://www.EU.net e-mail: bilse@EU.net
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