sFlow
Fixed Switch Configuration Guide 18-11
sFlow Agent Functionality
Packet flow sampling and counter sampling are performed by sFlow Instances associated with
individual Data Sources within the sFlow Agent. Packet flow sampling and counter sampling are
designed as part of an integrated system. Both types of samples are combined in sFlow datagrams.
Packet flow sampling will cause a steady, but random, stream of sFlow datagrams to be sent to the
sFlow Collector. Counter samples may be taken opportunistically in order to fill these datagrams.
In order to perform packet flow sampling, an sFlow Sampler Instance is configured with a
sampling rate. The packet flow sampling process results in the generation of packet flow records.
In order to perform counter sampling, an sFlow Poller Instance is configured with a polling
interval. The counter sampling process results in the generation of counter records. The sFlow
Agent collects counter records and packet flow records and sends them in the form of sFlow
datagrams to sFlow Collectors.
Sampling Mechanisms
Two forms of sampling are performed by the sFlow Agent: statistical packet-based sampling of
switched or routed packet flows, and time-based sampling of counters.
Packet Flow Sampling
The packet flow sampling mechanism carried out by each sFlow Instance ensures that any packet
observed at a Data Source has an equal chance of being sampled, irrespective of the packet flow(s)
to which it belongs.
Packet flow sampling is accomplished as follows:
1. When a packet arrives on an interface, the Network Device makes a filtering decision to
determine whether the packet should be dropped.
2. If the packet is not filtered (dropped), a destination interface is assigned by the switching/
routing function.
3. At this point, a decision is made on whether or not to sample the packet. The mechanism
involves a counter that is decremented with each packet. When the counter reaches zero a
sample is taken.
4. When a sample is taken, the counter indicating how many packets to skip before taking the
next sample is reset. The value of the counter is set to a random integer where the sequence of
random integers used over time is the Sampling Rate.
Packet flow sampling results in the generation of Packet Flow Records. A Packet Flow Record
contains information about the attributes of a packet flow, including:
• Information on the packet itself — a packet header, packet length, and packet encapsulation.
• Information about the path the packet took through the device, including information relating
to the selection of the forwarding path.
Counter Sampling
The primary objective of the counter sampling is to, in an efficient way, periodically export
counters associated with Data Sources. A maximum sampling interval is assigned to each sFlow
Instance associated with a Data Source.
Counter sampling is accomplished as follows:
1. The sFlow Agent keep a list of counter sources being sampled.
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