sFlow Fundamentals

sFlow monitors traffic in a data network. Use sFlow to monitor routers and switches in the network, and capture traffic statistics about those devices. sFlow uses sampling to provide scalability for network-wide monitoring, and therefore applies to high speed networks. The switch sends the sampled data as a User Datagram Protocol (UDP) packet to the specified host and port.

Note

Note

sFlow and Application Telemetry send mirrored packets from a common source to a common destination. sFlow sends samples directly to the destination, while Application Telemetry sends mirrored packets through a GRE tunnel, to the same destination. For more information, see Common Elements Between sFlow and Application Telemetry.

sFlow consists of the following:

Operational Considerations

The following section describes operational considerations for deploying sFlow.

Configuration Considerations

The following section describes configuration considerations for sFlow.

Example

After you configure the sFlow agent on the network device that you want to monitor, the system collects flow samples or counter samples, and exports these traffic statistics as sFlow datagrams to the sFlow collector on a server or appliance.

For example, after the buffers reach capacity or a timeout is triggered, an sFlow datagram, which is a UDP packet, sends the measurement information to the sFlow collector buffers. The UDP payload contains the sFlow datagram.

The following figure shows the sFlow agent on various routers and switches with sFlow datagrams being sent to the sFlow collector.

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Three sFlow agents send sFlow datagrams to one collector.
Table 1. sFlow legend

Number

Description

1

sFlow collector

2

sFlow datagrams

3

sFlow agents

As a general rule, drop action occurs after sampling completes. However, in situations related to Layer 1 errors such as, MTU exceeded packets, the drop action occurs before sampling begins. For errors such as, frame too long, packets are dropped due to the size of the frame being greater than the interface MTU. In this situation, the packets are dropped before sampling begins so only counter polling occurs. To enable trace, use line-card 1 trace level 232 <0–4>.

Important

Important

The defined sampling rate, an average of 1 out of n packets/operations does not provide a 100% accurate result, but it does provide a result with quantifiable accuracy.