Configuration manual

To change the sampling rate either globally or on an interface, use the following command.
• Change the global or interface sampling rate.
CONFIGURATION mode or INTERFACE mode
[no] sflow sample-rate sample-rate
sample-rate: The range is from 256 to 8388608 for the C-Series and S-Series. The range is from 2
to 8388608 for the E-Series.
The rate must be entered in factors of 2 (for example, 4096 or 8192).
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.
Back-Off Mechanism
If the sampling rate for an interface is set to a very low value, the CPU can get overloaded with flow
samples under high-traffic conditions.
In such a scenario, a binary back-off mechanism gets triggered, which doubles the sampling-rate (halves
the number of samples per second) for all interfaces. The backoff mechanism continues to double the
sampling-rate until the CPU condition is cleared. This is as per sFlow version 5 draft. After the back-off
changes the sample-rate, you must manually change the sampling rate to the desired value.
sFlow
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