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Foundry Configuration Guide for the FESX, FSX, and FWSX
B - 12
© Foundry Networks, Inc.
December 2005
Changing the Polling Interval
The polling interval defines how often sFlow byte and packet counter data for a port are sent to the sFlow
collector(s). If multiple ports are enabled for sFlow, the Foundry device staggers transmission of the counter data
to smooth performance. For example, if sFlow is enabled on two ports and the polling interval is 20 seconds, the
Foundry device sends counter data every ten seconds. The counter data for one of the ports are sent after ten
seconds, and counter data for the other port are sent after an additional ten seconds. Ten seconds later, new
counter data for the first port are sent. Similarly, if sFlow is enabled on five ports and the polling interval is 20
seconds, the Foundry device sends counter data every four seconds.
The default polling interval is 20 seconds. You can change the interval to a value from 1 to any higher value. The
interval value applies to all interfaces on which sFlow is enabled. If you set the polling interval to 0, counter data
sampling is disabled.
To change the polling interval, enter a command such as the following at the global CONFIG level of the CLI:
FESX424 Router(config)# sflow polling-interval 30
Syntax:
[no] sflow polling-interval <secs>
The <secs> parameter specifies the interval and can be from 1 to any higher value. The default is 20 seconds. If
you specify 0, counter data sampling is disabled.
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.
You can change the default (global) sampling rate. You also can change the rate on an individual port, overriding
the default sampling rate of 512. With a sampling rate of 512, on average, one in every 512 packets forwarded on
an interface is sampled.
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:
Foundry 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 1/1,
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.
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.
Summary of Contents for FastIron Edge Switch X424
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