Can't explain QoS behaviour..

[marl.beynon]

I have a 3750G configured with autoQoS. i have then set up a number of ports for the requirments of the project. Of concern are two ports, both configured not to trust and to override the cos value with the default port cos value that i have set at 5. These ports will only see voice traffic.

i then send 5000 pings between the devices on the end of these two ports. if we start at port A (the origionator of the ping), i see 5000 incomming packets with a DSCP of 0 and a cos of 5, as i would expect. Following the path these packets would then exit port B, where i see 5000 packets with DSCP 46 and cos 5, again as i would expect, (the switch has assigned the DSCP value).

Here is what i cant explain. The 5000 reply packets recived at port B have a cos of 5 (as this is the overide port cos value) but also a DSCP of 46. The router that is acting as device B is not configured to assign a DSCP value so where does it come from.

Is this just normal behaviour? i cant check on the other port i have configured (port C), as the router attached to that is configured to assign an IP-prec value, so the reply packets would recive a DSCP value there (mapped from the IP-Prec).

As you can see i am new to QoS, and there is probably a simple explanation to this behaviour. I just cant see it yet.

Thanks in advance

[rfroom]

Most Catalyst switches, including the 3750, use the concept of internal DSCP for classification as a packet "travels" the internals of the switch. Since you are setting the ingress CoS to 5, the internal DSCP is derived from the CoS-to-DSCP mapping table. The default mapping table maps CoS of 5 to DSCP 40. I believe with auto QoS enabled, it changes the default mapping table to map CoS 5 to DSCP 46.

On egress, the switch sets the CoS value based on the internal DSCP value and the default DSCP-CoS Mapping table. The switch does not rewrite the DSCP back to 0 despite classifying traffic solely based on CoS.