IP SLA & DSCP testing

I had an occasion to do some IP SLA testing. We needed simple icmp-echo probes that would measure delay & loss. We also wanted to configure icmp-echo probes with DSCP=EF in order to measure delay & loss of our priority traffic.

Interestingly enough, the DSCP values are copied in the reply packets – at least from a Cisco router. IP SLA responder is not necessary for this to work.

The following configuration worked for us:

ip sla 1
icmp-echo 172.16.0.4
source-ip 172.16.0.2
frequency 5
history distributions-of-statistics-kept 10
history statistics-distribution-interval 100
history lives-kept 2

history buckets-kept 60

history filter all
ip sla 2
icmp-echo 172.16.0.4

source-ip 172.16.0.2
tos 184

frequency 5

history distributions-of-statistics-kept 10
history statistics-distribution-interval 100
history lives-kept 2

history buckets-kept 60

history filter all

You can then use the following command to schedule both probes:

ip sla group schedule 1 1-2 schedule-period 5 start-time now

You can see that in probe 2 I added a TOS value of 184, which is equivalent to DSCP=EF. I used the probe to generate traffic and captured the resultant packets to verify they were indeed DSCP=EF.

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