sFlow
18-12 Configuring Network Monitoring
2. When a Packet Flow Sample is generated, the sFlow Agent examines the list of counter
sources and adds counters to the sample datagram, least recently sampled first.
Counters are only added to the datagram if the sources are within a short period, 5 seconds
say, of failing to meet the required sampling interval.
3. Periodically, say every second, the sFlow Agent examines the list of counter sources and sends
any counters that need to be sent to meet the sampling interval requirement.
The set of counters is a fixed set defined in Section 5 of the document entitled “sFlow Version 5”
available from sFlow.org (http://www.sflow.org).
Sampling Implementation Notes
Although the switch hardware has the capability to sample packets on any port, to ensure that
CPU utilization is not compromised, the number of sFlow samplers that can be configured per
switch or stack of switches is limited to a maximum of 32. There is no limitation on the number of
pollers that can be configured.
Under certain circumstances, the switch will drop packet samples that the sFlow implementation
is not able to count and therefore cannot correctly report sample_pool and drops fields of flow
samples sent to the sFlow Collector. Under heavy load, this sample loss could be significant and
could therefore affect the accuracy of the sampling analysis.
Configuring sFlow
Overview
sFlow is disabled by default, and therefore must be manually enabled.
Configuring Collectors
In order for an sFlow Collector to be assigned to receive sample datagrams from the sFlow Agent
on the switch, an entry for that Collector must be configured in the switch’s sFlow Receivers Table.
An entry must contain an owner identity string, a non-zero timeout value, and the IP address of
the Collector. Configure the identity string and timeout value with the set sflow receiver owner
command and the IP address with the set sflow receiver ip command.
An entry without an owner identity string is considered unclaimed and cannot be assigned as a
receiver to sampler or poller instances.
Once the timer set by the set sflow receiver owner command expires, the receiver/Collector and
all the samplers and pollers associated with this Collector expire and are removed from the
switch’s configuration. In order to start sending sample data to the Collector again, the Collector
must be reconfigured with a new timeout value and samplers and pollers must be configured
again. Therefore, you should consider setting the timeout value to the largest value that is
reasonable for your environment.
You can clear the IP address, maximum datagram size, or UDP port without deleting an entry
from the sFlow Receivers Table with the clear sflow receiver command. If you clear the owner or
timeout, the entire entry is cleared. If you enter only an entry index and none of the optional
parameters, the entire entry is cleared.
Once an entry is cleared, all pollers and samplers associated with that receiver are also removed
from the switch configuration.
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