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Sampling rates are configurable in powers of two. This configuration 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 configured with a sampling rate of 4096 on interface Tengig
1/0, and 8192 on Tengig 1/1, the sFlow agent does the following:
1. Configures the hardware to a sampling rate of 4096 for all ports with sFlow enabled on that port-pipe.
2. Configures interface Tengig 1/0 to a sub-sampling rate of 1 to achieve an actual rate of 4096.
3. Configures interface Tengig 1/1 to a sub-sampling rate of 2 to achieve an actual rate of 8192.
NOTE: Sampling rate backoff 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 configured rate in the equation. That absence is because when the hardware sampling rate value on
the port-pipe exceeds the configured 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-Off Mechanism
If the sampling rate for an interface is set to a very low value, the CPU can get overloaded with flow samples under high-traffic
conditions.
In such a scenario, a binary back-off mechanism gets triggered, which doubles the sampling-rate (halves the number of samples
per second) for all interfaces. The backoff mechanism continues to double the sampling-rate until the CPU condition is cleared.
This is as per sFlow version 5 draft. After the back-off changes the sample-rate, you must manually change the sampling rate to
the desired value.
As a result of back-off, the actual sampling-rate of an interface may differ from its configured sampling rate. You can view the
actual sampling-rate of the interface and the configured 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 configuration from the LAG port.
Enabling Extended sFlow
Dell Networking OS supports 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
By default packing of any of the extended information in the datagram is disabled.
● Confirm that extended information packing is enabled.
show sflow
The bold line shows that extended sFlow settings are enabled on all three types.
Dell#show sflow
sFlow services are enabled
Global default sampling rate: 4096
Global default counter polling interval: 15
Global extended information enabled: switch
1 collectors configured
Collector IP addr: 10.10.10.3, Agent IP addr: 10.10.0.0, UDP port: 6343
77 UDP packets exported
sFlow
725