Testing Quality of Service (QoS) in IP Networks

Various tools exist to test you QoS configurations across your network, but most of these are costly and often tricky to set up.

If you have  a Cisco network there is a simple way to ensure your packet DSCP markings are being carried throughout the environment, just using your Cisco CLI.

The basic set up is this:

Create IP SLA probes destined towards an edge IP address with the IP packet TOS field set from a edge WAN router or L3 switch in the path you wish to measure QoS across.

This will send ICMP packets marked with DSCP across your network. Then you have  a few options as to how you validate the packets markings are carried through the network.

The simplest way to do this is to check the outbound queues on each device in path to see if the QoS queue counters are incrementing at the same rate you are sending your probes, but this can be a problem in a noisy environment.

Another way is to use the inbuilt packet capture feature built into Cisco IOS 12.4 and up. By setting a simple filter you can only capture the IP SLA probe packets, and then export these to Wireshark to check the packet DSCP is intact (or view the packets in the IOS CLI)

An example IP SLA TOS probe is shown below

  • rtr 1
  • type echo protocol ipIcmpEcho source-ipaddr
  • request-data-size 200
  • tos 96
  • timeout 1000
  • frequency 10
  • hours-of-statistics-kept 12
  • rtr schedule 1 life 604800 start-time now




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