Re: QoS and DSCP Marking

Unanswered Question
Jun 29th, 2009


I would like to know if this can be done.

If my packets on R1 are marked, I do no marking on R2 and when it reaches R3, I match these DSCP markings via CBWFQ with bandwidth statements for classes that I have defined.

R1 -- R2 --R3.

Therefore, I am marking on R1 and doing nothing on R2 and on R3 I am matching the marked traffic and allocating bandwidth to them. Can this be done? If no, what do I need to do to make this work ?

Pls advice,


I have this problem too.
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Jon Marshall Mon, 06/29/2009 - 04:26

Assuming these are routers then yes, if you mark them on R1 you can still match on these markings on R3.

You need to be careful if any of these devices are switches however as switches will remark if qos is enabled on them.


InternetB Mon, 06/29/2009 - 04:33

Thanks Jon. I have a switch between R2 and R3. My scenario looks like this

R1 --> R2 --> SW1 --> R3.

Therefore, I would need to do an 'mls qos trust dscp' on the interface coming to SW1 only right to preserve its DSCP marking ? Don't think I need to do an 'mls qos trust dscp' for the outbound traffic to R3 right ?



Jon Marshall Mon, 06/29/2009 - 04:45

If you have QOS enabled on the switch ie. "mls qos" then yes you will need to use "mls qos trust dscp" on the incoming interface.

However if you haven't enabled QOS on the switch then you don't need to do anything as the switch will simply pass the packet through unchanged.


Joseph W. Doherty Mon, 06/29/2009 - 11:53

Personally I believe, network devices shouldn't be rewriting parts of a frame/packet unless they have to or have been explicitly configured to do so. However, as Jon notes, some network devices can do so, such as some Cisco switches with QoS support "enabled" in their "default" configuration. (Cisco, I guess, could argue enabling QoS support is enough of an "explict" configuration.)

Anyway, you really should carefully review device operation for the devices along the path so there are no unpleasant "surprises". Besides the option Jon mentions of trusting DSCP, some devices now offer a "no mls qos rewrite ip dscp" command.

An example of QoS switch configuration , and the command I just noted, can be found here:


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