Friday, June 26, 2009

Sampling rates

A previous posting discussed the scalability and accuracy of packet sampling and the advantages of packet sampling for network-wide visibility.

Selecting a suitable packet sampling rate is an important part of configuring sFlow on a switch. The table gives suggested values that should work well for general traffic monitoring in most networks. However, if traffic levels are unusually high the sampling rate may be decreased (e.g. use 1 in 5000 instead of 1 in 2000 for 10Gb/s links).

Configure sFlow monitoring on all interfaces on the switch for full visibility. Packet sampling is implemented in hardware so all the interfaces can be monitored with very little overhead.

Finally, select a suitable counter polling interval so that link utilizations can be accurately tracked. Generally the polling interval should be set to export counters at least twice as often as the data will be reported (see Nyquist-Shannon sampling theory for an explanation). For example, to trend utilization with minute granularity, select a polling interval of between 20 and 30 seconds. Don't be concerned about setting relatively short polling intervals; counter polling with sFlow is very efficient, allowing more frequent polling with less overhead than is possible with SNMP.

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