Quick Reference Guide

1084 PowerConnect B-Series TI24X Configuration Guide
53-1002269-02
sFlow
A
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
Dell 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. The software rounds the
value you enter to the next higher odd power of 2 to obtain the actual rate. This value becomes the
actual sampling rate. For example, if the configured sampling rate is 1000, then the actual rate is
2048 and 1 in 2048 packets are sampled by the hardware.
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, 2, and 3. If you configure the sampling rate on port 1 but leave the
other two ports using the default rate, then a change to the global sampling rate applies to ports 2
and 3 but not port 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
sFlow enabled on ports 2 and 8, and port 2 is using the default sampling rate of 512, and port 8 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 2 will be 1,
meaning that every sample taken by the hardware will be exported, while the subsampling factor
for port 8 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. In addition, the software will round
the value you enter up to the nearest value listed. You can display the rates you entered (the
configured rates) as well as the rates rounded up to by the software (the actual rates) 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 1087.