Saturday, May 16, 2009

Packet headers

There are a large number of protocols that can run over a switched network (the chart, from Agilent Technologies, shows the major protocol families). It is not reasonable to expect a layer 2 switch to be able to decode and report on all these protocols - the switch is there to forward packets and should only be concerned with the information it needs to make forwarding decisions. With sFlow, the switch simply forwards the Ethernet packet header and leaves it up to the traffic analyzer to decode the protocols.

This approach has a number of important advantages:
  1. Capturing packet headers simplifies the monitoring task on the switch, making it easy to implement in hardware.
  2. It is much easier to add new protocol decodes to a central traffic analyzer than it is to develop and deploy new switch firmware releases to add the new functionality. This is particularly true if you have a variety of switch models and vendors in your network.
  3. Packet headers are well standardized, they have to be, or you wouldn't be able to interconnect switches. If packets are decoded on the switches there can be differences in the way switches from different vendors decode the packets and report on the data, making it difficult to combine data to provide a network-wide view.
  4. Packet headers capture the complex layering (MAC, VLAN, MPLS, VPLS, IPv6 over IPv4 etc.) that is critical to understanding how traffic flows across the network.
In order to get the full benefit of sFlow monitoring, select an sFlow collector that decodes all the protocols that you use on your network.

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