Brocade FastIron Ethernet Switch Administration Guide User Manual
Page 250
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 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. 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
on page 256.
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.
device(config)#sflow sample 2048
Syntax: [no] sflow samplenum
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
• 4096
• 8192
• 32768
• 131072
• 524288
• 2097152
• 8388608
• 33554432
Network Monitoring
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FastIron Ethernet Switch Administration Guide
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