Re: Problems migrating from netflow to sFlow: results are low

From: Christian Hammers <>
Date: 08/11/05
Message-ID: <>


Thanks for the quick answers. To Jay: No, we're using only Linux (with
nprobe) and Cisco to export netflow. The lack of proper netflow export
was one reason for migrating to sflow :-)

On Wed, Aug 10, 2005 at 10:05:15AM -0700, neil mckee wrote:
> 1. Most Foundry switches only sample inbound traffic on an
> interface, so if you only enabled sflow on the uplink then you are
> probably only seeing the traffic in one direction.
That is not the case with the FastIron Edge series, which I could verify.
    * FastIron Edge Switches. FES devices support sFlow packet sampling
      for inbound and outbound traffic on sFlow-enabled ports.
    On these devices, sample data is collected from inbound traffic on ports
    enabled for sFlow. Outbound traffic is sampled on the FastIron Edge
    Switches only. However, both traffic directions are counted for byte and
    packet counter statistics sent to the collector.

> 2. sflowtool does not aggregate samples together, even if they are
> from the same flow. I don't know what program you are using to
> populate the database, but you might want to check that if two flows
> with the same key are entered then they end up being added together
> (rather than the second one replacing the first).
No, we save everything as-is what sflowtool exports us via netflow. The
final data looks like this (currently very low polling intervals and
        | time | s | d | dOctets |
        | 2005-08-11 00:46:59 | | | 202752 |
        | 2005-08-11 00:47:34 | | | 67072 |
        | 2005-08-11 00:49:05 | | | 744448 |
        | 2005-08-11 00:49:27 | | | 744448 |
        | 2005-08-11 00:49:40 | | | 744448 |
        | 2005-08-11 00:50:05 | | | 744448 |

> 3. It's helpful to check the frames counter as well as the bytes
> counter. The frames estimates will converge faster for a given
> sampling rate, and it is not subject to any differences in the way
> that bytes are counted. For netflow export, sflowtool uses the ip-
> len from the ip header to say how many layer3 bytes there were. If
> your netflow source was reporting the bytes including layer2 headers
> then there would be a sizable discrepancy. (There are also
> different ways to interpret udp packets if they are larger than 1518
> bytes and being fragmented at the IP layer.)
> You might cross-check using another product (
> products/collectors.php). Some of them are free downloads (e.g.
> sFlowTrend, ntop, pmccact).
Although our netflow also only exports layer3 and there shouldn't be
that much large UDP packets, I will try to install another converter.


Christian Hammers             WESTEND GmbH  |  Internet-Business-Provider
Technik                       CISCO Systems Partner - Authorized Reseller
                              L|tticher Stra_e 10      Tel 0241/701333-11                D-52064 Aachen              Fax 0241/911879
Received on Thu Aug 11 01:16:07 2005

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