Dell PowerConnect B-RX Configuration Manual page 1096

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Configuring sFlow
Changing the sampling rate
The sampling rate is the average ratio of the number of packets incoming on an sFlow-enabled
port, to the number of flow samples taken from those packets. By default, all sFlow-enabled ports
use the default sampling rate, which is 2048. With a sampling rate of 2048, on average, one in
every 2048 packets forwarded on an interface is sampled.
You can change the default (global) sampling rate. You also can change the rate on an individual
port, overriding the default sampling rate.
NOTE
sFlow uses CPU resources to send sFlow samples to the collector. If you set a low sampling rate, CPU
utilization can become high.
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 2,000 to 512, 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.
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, 1/2, and 5/1. If you configure the sampling rate on port 1/1 but
leave the other two ports using the default rate, then a change to the global sampling rate applies
to ports 1/2 and 5/1 but not port 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.
Sampling rate for new ports
When you enable sFlow on a port, the port's sampling rate is set to the global default sampling rate.
This also applies to ports on which you disable and then re-enable sFlow. The port does not retain
the sampling rate it had when you disabled sFlow on the port, even if you had explicitly set the
sampling rate on the port.
Changing the default sampling rate
To change the default (global) sampling rate, enter a command such as the following at the global
CONFIG level of the CLI.
BigIron RX(config)# sflow sample 2048
Syntax: [no] sflow sample <num>
1024 BigIron RX Series Configuration Guide
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