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Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

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Sub-Sampling

The sFlow sample rate is not the frequency of sampling, but the number of packets that are skipped before the next sample is taken.

Therefore, the sFlow agent uses sub-sampling to create multiple sampling rates per port-pipe. To achieve different sampling rates for different ports in a port-pipe, the sFlow agent takes the lowest numerical value of the sampling rate of all the ports within the port-pipe and configures all the ports to this value. The sFlow agent is then able to skip samples on ports where you require a larger sampling rate value.

Sampling rates are configurable in powers of two. This configuration allows the smallest sampling rate possible on the hardware and also allows all other sampling rates to be available through sub-sampling.

For example, if Tengig 1/0 and 1/1 are in a port-pipe, and they are configured with a sampling rate of 4096 on interface Tengig 1/0, and 8192 on Tengig 1/1, the sFlow agent does the following:
  1. Configures the hardware to a sampling rate of 4096 for all ports with sFlow enabled on that port-pipe.
  2. Configures interface Tengig 1/0 to a sub-sampling rate of 1 to achieve an actual rate of 4096.
  3. Configures interface Tengig 1/1 to a sub-sampling rate of 2 to achieve an actual rate of 8192.
  • NOTE: Sampling rate backoff can change the sampling rate value that is set in the hardware. This equation shows the relationship between actual sampling rate, sub-sampling rate, and the hardware sampling rate for an interface:

    Actual sampling rate = sub-sampling rate * hardware sampling rate

    Note the absence of a configured rate in the equation. That absence is because when the hardware sampling rate value on the port-pipe exceeds the configured sampling rate value for an interface, the actual rate changes to the hardware rate. The sub-sampling rate never goes below a value of one.

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