sFlow (ver. 5) is a procedure for statistical monitoring of traffic in networks.NVIDIA Onyx supports an sFlow sampling mechanism (agent), which includes collecting traffic samples and data from counters. The sFlow datagrams are then sent to a central collector.

The sampling mechanism must ensure that any packet going into the system has an equal chance of being sampled, irrespective of the flow to which it belongs. The sampling mechanism provides the collector with periodical information on the amount (and load) of traffic per interface by loading the counter samples into sFlow datagrams.

The sFlow packets are encapsulated and sent in UDP over IP. The UDP port number that is used is the standard 6343 by default.

Flow Samples 

The sFlow agent samples the data path based on packets. 

Truncation and sampling rate are the two parameters that influence the flow samples. In case of congestion the flow samples can be truncated to a predefined size before it is assigned to the CPU. The truncation can be set to any value between 64 to 256 bytes with the default being 128 bytes.

The sampling rate can be adjusted by setting an average rate. The system assures that a random number of packets is sampled, however, the sample rate on average converges to the configured rate. Valid values range between 4000 to 16777215 packets.

The sFlow sends flow samples using an expanded flow sample format.

Statistical Samples 

The sFlow agent samples interface counters time based. Polling interval is configurable to any value between 5-3600 seconds with the default being 20 seconds.

The sFlow sends statistical samples using an expanded counter sample format.

The following statistics are gathered by the CPU:

sFlow Datagrams

The sFlow datagrams contain flow samples and statistical samples.

The sFlow mechanism uses IP protocol, therefore if the packet length is more than the interface MTU, it becomes fragmented by the IP stack. The MTU may also be set manually to anything in the range of 200-9216 bytes. The default is 1400 bytes.

Sampled Interfaces

sFlow must be enabled on physical or LAG interfaces that require sampling. When adding a port to a LAG, sFlow must be disabled on the port. If a port with enabled sFlow is configured to be added to a LAG, the configuration is rejected. Removing a port from a LAG disables sFlow on the port regardless of the LAG’s sFlow status.

Configuring sFlow

1. Unlock the sFlow commands.

   switch (config) # protocol sflow

2. Enable sFlow on the system.

   switch (config) # sflow enable

3. Enter sFlow configuration mode.

   switch (config) # sflow
   switch (config sflow) # 

4. Set the central collector’s IP.

   switch (config sflow) # collector-ip 10.10.10.10

5. Set the agent-ip used in the sFlow header.

   switch (config sflow) # agent-ip 20.20.20.20

6. (Optional) Set the sampling rate of the mechanism.

   switch (config sflow) # sampling-rate 16000

   This means that one every 16000 packet gets collected for sampling.

7. (Optional) Set the maximum size of the data path sample.

   switch (config sflow) # max-sample-size 156

8. (Optional) Set the frequency in which counters are polled.

   switch (config sflow) # counter-poll-interval 19

9. (Optional) Set the maximum size of the datagrams sent to the central collector.

   switch (config sflow) # max-datagram-size 1500

10. Enable the sFlow agent on the desired interfaces. 

    switch (config interface ethernet 1/1)# sflow enable
    switch (config interface port-channel 1)# sflow enable

Verifying sFlow

To verify the attributes of the sFlow agent: 

switch (config)# show sflow
sflow protocol: enabled
sflow: enabled
sampling-rate: 16000
max-sampled-size: 156
counter-poll-interval: 19
max-datagram-size: 1500
collector-ip: 10.10.10.10
collector-port: 6343
agent-ip: 20.20.20.20

ingress ports:
Interfaces:
Ethernet: eth1/1
Port-channel: po1

Statistics:
Total Samples: 2000
Number of flow samples: 1200
Estimated Number of flow discarded: 0
Number of statistic samples: 800
Number of datagrams: 300

Additional Reading and Use Cases

For more information about this feature and its potential applications, please refer to the following community post:

• HowTo Configure sFlow on NVIDIA Switches

sFlow Commands

protocol sflow

switch (config) # protocol sflow

sflow enable (global)

switch (config) # sflow enable

sflow

switch (config) # sflow
switch (config sflow) #

sampling-rate

switch (config) # protocol sflow

max-sample-size

switch (config sflow) # max-sample-size 165

counter-poll-interval

switch (config sflow) # counter-poll-interval 30

max-datagram-size

switch (config sflow) # max-datagram-size 9216

collector-ip

switch (config sflow) # collector-ip 10.10.10.10

agent-ip

switch (config sflow) # agent-ip 20.20.20.20

clear counters

switch (config sflow) # clear counters

sflow enable (interface)

switch (config interface ethernet 1/1)# sflow enable

show sflow 

switch (config)# show sflow
sflow protocol: enabled
sflow: enabled
VRF name: mgmt
sampling-rate: 16000
max-sample-size: 128
counter-poll-interval: 20
max-datagram-size: 1400
ip-agent: 0.0.0.0

ingress ports:
Interfaces:
Ethernet eth1/2 eth1/1

Statistics:
Total Samples: 0
Number of flow samples: 0
Estimated Number of flow discarded: 0
Number of flow statistics samples: 0
Number of datagrams: 0