Remote Network Monitoring
December 2005 © Foundry Networks, Inc. B - 13
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 4 has sFlow enabled on ports
4/2 and 4/8, and port 4/2 is using the default sampling rate of 512, and port 4/8 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 4/2 will be 1, meaning that every sample taken by the hardware will be exported,
while the subsampling factor for port 4/8 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. See “Displaying sFlow Information” on page B-15.
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:
FESX424 Router(config)# sflow sample 2048
Syntax: [no] sflow sample <num>
The <num> parameter specifies the average number of packets from which each sample will be taken. The
software rounds the value you enter to the next higher odd power of 2. This value becomes the actual default
sampling rate and is one of the following.
•2
•8
•32
• 128
• 512
• 2048
• 8192
• 32768
• 131072
• 524288
• 2097152
• 8388608
• 33554432
• 134217728
• 536870912
• 2147483648
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.
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