Administrator Guide

Sub-Sampling
The sFlow sample rate is not the frequency of sampling, but the number of packets that are skipped before the next sample is taken.
Therefore, the sFlow agent uses sub-sampling to create multiple sampling rates per port-pipe. To achieve dierent sampling rates for
dierent ports in a port-pipe, the sFlow agent takes the lowest numerical value of the sampling rate of all the ports within the port-
pipe and congures all the ports to this value. The sFlow agent is then able to skip samples on ports where you require a larger
sampling rate value.
Sampling rates are congurable in powers of two. This conguration allows the smallest sampling rate possible on the hardware and
also allows all other sampling rates to be available through sub-sampling.
For example, if Tengig 1/0 and 1/1 are in a port-pipe, and they are congured with a sampling rate of 4096 on interface Tengig 1/0,
and 8192 on Tengig 1/1, the sFlow agent does the following:
1. Congures the hardware to a sampling rate of 4096 for all ports with sFlow enabled on that port-pipe.
2. Congures interface Tengig 1/0 to a sub-sampling rate of 1 to achieve an actual rate of 4096.
3. Congures interface Tengig 1/1 to a sub-sampling rate of 2 to achieve an actual rate of 8192.
NOTE: Sampling rate backo can change the sampling rate value that is set in the hardware. This equation shows the
relationship between actual sampling rate, sub-sampling rate, and the hardware sampling rate for an interface:
Actual sampling rate = sub-sampling rate * hardware sampling rate
Note the absence of a congured rate in the equation. That absence is because when the hardware sampling rate value on the
port-pipe exceeds the congured sampling rate value for an interface, the actual rate changes to the hardware rate. The sub-
sampling rate never goes below a value of one.
Back-O Mechanism
If the sampling rate for an interface is set to a very low value, the CPU can get overloaded with ow samples under high-trac
conditions.
In such a scenario, a binary back-o mechanism gets triggered, which doubles the sampling-rate (halves the number of samples per
second) for all interfaces. The backo mechanism continues to double the sampling-rate until the CPU condition is cleared. This is as
per sFlow version 5 draft. After the back-o changes the sample-rate, you must manually change the sampling rate to the desired
value.
As a result of back-o, the actual sampling-rate of an interface may dier from its congured sampling rate. You can view the actual
sampling-rate of the interface and the congured sample-rate by using the show sflow command.
sFlow on LAG ports
When a physical port becomes a member of a LAG, it inherits the sFlow conguration from the LAG port.
Enabling Extended sFlow
The MXL switch support extended-switch information processing only.
Extended sFlow packs additional information in the sFlow datagram depending on the type of sampled packet. You can enable the
following options:
• extended-switch — 802.1Q VLAN ID and 802.1p priority information.
• extended-router — Next-hop and source and destination mask length.
• extended-gateway — Source and destination AS number and the BGP next-hop.
• Enable extended sFlow.
sflow [extended-switch] [extended-router] [extended-gateway] enable
734
sFlow