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Qos using dscp remarking.

Occasional Collector

Qos using dscp remarking.

Hello folks. I am testing some options of qos. Actually dscp remarking.

But it is not working.  I am remarking  the dscp field to EF(46). It has no effect.

On the other hand when I remark  dot1p field to 5, the tests works fine.


Why remarking dot1p works fine and remarking dscp does not ?







Honored Contributor

Re: Qos using dscp remarking.



can you pls provide your config example ?

Also note that simply remarking the dscp value does not mean the traffic will be handled by a dedicated queue, so 2 actions may be required if that is what you want,


best regards,Peter

Occasional Collector

Re: Qos using dscp remarking.

I Peter, thanks for your reply.


I am using two switches (5500) with a just 10 mbps uplink between them to use 100% of its bandwidth.

There are two computer connected to one of the switches sending traffic to a third computer

connected to the second notebook.  All three computers witch Gpbs  network cards.


With the uplink at 100% of utilization I tried the test described above to verify if

the Qos works as expected priorizing the traffic of one of the computers


More details after the configuration commands.


The configuration I am using is a very simple one.:


#simple acl to aply de Qos only do ip

acl number 3100

rule 5 permit ip source


# traffic classifier for the Acl above

traffic classifier testeQOS operator and

if-match acl 3100


# Behavior marking dscp as ef (46)

traffic behavior testeQOS. When I use: remark dot1p 5 or 6 ou 7 it works fine.

remark dscp ef


# Policy binding the classifier to the behavior

qos policy testeQOS

classifier testeQOS behavior testeQOS


#appling the qos policy to vlan 2 inbound, where is the computer with ip

qos vlan-policy testeQOS vlan 2 inbound


To test this policy I generate traffic from computer and from computer with a software

called JPerf. Before applying the Qos both computer transmit data at the same rate.

After the Qos policy is applied, the computer has its transmission rate increased and the  computer has its transmission rate decrased, but only if my behavior use the command "remark dot1p 5".


Using  only the command "remark dscp ef" there is no effect with the transmission rates. They keep

exactly the same trasmission rate proving that there is no priority being applied.


Thanks for your interest.




Honored Contributor

Re: Qos using dscp remarking.

This is normal behavior.

Remarking dscp is just what it is: remark dscp, but it will not process the packet in a dedicated queue.

Assigning the packet to a specific queue is done with


remark local-precedence x


This combination will have the desired effect.


Remark dot1p will also modify the internal local-precedence, which is why you see the different behavior.


Hope this helps,Peter