260 Brocade ICX 6650 Administration Guide
53-1002600-01
sFlow
11
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 page267.
Sampling rate for new ports