Wednesday, May 27, 2009

Measurement traffic

The charts, based on measurements from switches in a production environment, compare NetFlow and sFlow in terms of the load that they generate on the network. The following observations can be made based on this data:
  • NetFlow monitoring generates periodic bursts of traffic; the periodicity is confirmed by the sharp spikes in the frequency chart. This behavior is typical of flow-based traffic monitoring protocols (see Exporting IP flows using IPFIX) since flow generation involves maintaining a cache of active flows on the switch and the use timers to trigger flow export.
  • sFlow monitoring generates a random pattern of traffic with no periodicity and no bursts. The randomness is confirmed by the flat frequency chart.
Network-wide visibility involves collecting traffic data from large numbers of switches and routers. The bursts of traffic generated by flow monitoring can cause problems with delay, packet loss and jitter that will effect other traffic on the network. The periodicity observed in flow monitoring creates the risk that the different streams of monitoring traffic will synchronize and reinforce each other as large numbers of devices are monitored.

It is essential that the technology used to manage network traffic does not itself cause traffic problems. The random, low-level, background traffic that sFlow generates ensures that large networks can be safely monitored without any adverse effects. This behavior is no accident, sFlow was designed to be scalable and the random packet sampling mechanism in sFlow is one of the reasons that its traffic is well behaved.

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