Brocade Communications Systems 6650 Configuration considerations, Configured rate and actual rate

11 sFlow

Configuration considerations

The sampling rate is a fraction in the form 1/N, meaning that, on average, one out of every N packets will be sampled. The sflow sample command at the global level or port level specifies N, the denominator of the fraction. Thus a higher number for the denominator means a lower sampling rate since fewer packets are sampled. Likewise, a lower number for the denominator means a higher sampling rate because more packets are sampled. For example, if you change the denominator from 512 to 128, the sampling rate increases because four times as many packets will be sampled.

NOTE

Brocade recommends that you do not change the denominator to a value lower than the default. Sampling requires CPU resources. Using a low denominator for the sampling rate can cause high CPU utilization.

Configured rate and actual rate

When you enter a sampling rate value, this value is the configured rate as well as the actual sampling rate.

Change to global rate

If you change the global sampling rate, the change is applied to all sFlow-enabled ports except those ports on which you have already explicitly set the sampling rate. For example, suppose that sFlow is enabled on ports 1/1/1, 1/1/2, and 1/1/3. If you configure the sampling rate on port 1/1/1 but leave the other two ports using the default rate, then a change to the global sampling rate applies to ports 1/1/2 and 1/1/3 but not port 1/1/1. sFlow assumes that you want to continue using the sampling rate you explicitly configured on an individual port even if you globally change the sampling rate for the other ports.

Module rate

While different ports on a module may be configured to have different sampling rates, the hardware for the module will be programmed to take samples at a single rate (the module sampling rate). The module sampling rate will be the highest sampling rate (i.e. lowest number) configured for any of the ports on the module.

When ports on a given module are configured with different sampling rates, the CPU discards some of the samples supplied by the hardware for ports with configured sampling rates which are lower than the module sampling rate. This is referred to as subsampling, and the ratio between the port sampling rate and the module sampling rate is known as the subsampling factor. For example, if the module in slot 1has sFlow enabled on ports 1/1/1 and 1/1/3, and port 1/1/1 is using the default sampling rate of 512, and port 1/1/3 is configured explicitly for a rate of 2048, then the module sampling rate will be 512 because this is this highest port sampling rate (lowest number). The subsampling factor for port 1/1/1 will be 1, meaning that every sample taken by the hardware will be exported, while the subsampling factor for port 1/1/3 will be 4, meaning that one out of every four samples taken by the hardware will be exported. Whether a port's sampling rate is configured explicitly, or whether it uses the global default setting, has no effect on the calculations.

You do not need to perform any of these calculations to change a sampling rate. For simplicity, the syntax information in this section lists the valid sampling rates. You can display the rates you entered for the default sampling rate, module rates, and all sFlow-enabled ports by entering the show sflow command. Refer to “Displaying sFlow information” on page 267.

Sampling rate for new ports

260	Brocade ICX 6650 Administration Guide
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