Dell Force10 E600i FTOS Version 8.4.1.5 Configuration Guide for ExaScale - Page 917

Sub-sampling e Sub-sampling is available only on platform: t The sFlow sample rate is not the frequency of sampling, but the number of packets that are skipped before the next sample is taken. Although a sampling rate can be configured for each port, TeraScale line cards can support only a single sampling rate per port-pipe. Therefore, 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, sFlow Agent takes the lowest numerical value of the sampling rate of all the ports within the port-pipe, and configures all ports to this value. 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 allows the smallest sampling rate possible to be configured on the hardware, and also allows all other sampling rates to be available through sub-sampling. For example, if Gig 1/0 and 1/1 are in a port-pipe, and they are configured with a sampling rate of 4096 on interface Gig 1/0, and 8192 on Gig 1/1, 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. Configure interface Gig 1/0 to a sub-sampling rate of 1 to achieve an actual rate of 4096. 3. Configure interface Gig 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 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 CPU condition is cleared. This is as per sFlow version 5 draft. Once the back-off changes the sample-rate, users must manually change the sampling rate to the desired value. As a result of back-off, the actual sampling-rate of an interface may differ from its configured sampling rate. The actual sampling-rate of the interface and the configured sample-rate can be viewed by using the show sflow command. sFlow | 917