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Questions about QoS Mapping

I am changing the QoS mapping from voice signaling 26 to 24.

Please correct me if I am wrong the following statement.

1. Since the Call Signaling change from 26 to 24, I have to change the mls qos map cos-dscp 0 8 16 26 32 46 48 56 to mls qos map cos-dscp 0 8 16 24 32 46 48 56, Right???

2. At the switch port level (IP Phone), e.g

Interface fa 0/1

mls qos trust dscp

3. I make sure Callmanager setting at enterprise parameter and service parameter is CS3.

4. At the LLQ, I match CS3 intead af31.

After all these step, I dont see any match if I show "show policy-map interface". If I add af31 back... i saw packet hit, weird.

Anyone knows why??



Re: Questions about QoS Mapping

1. i usually use, 24 for cs3, 34 for af41, 46 for ef. Note that 34 is for video.

mls qos map cos-dscp 0 8 16 24 34 46 48 56

2. since you are doing a cos to dscp mapping global to the switch, i would do a trust-cos setting rather than trust dscp setting. i would also add the commmand,

int fa0/1

mls qos trust cos

mls qos trust device cisco-phone

3. This is the default setting in 4.1 and up.

4. i would do a match-any class-map and match both just in case a voice control packet is tagged af31 instead of cs3. (a misconfigured switch or router)

class-map match-any Control

match ip dscp cs3 af31




Re: Questions about QoS Mapping


I will try your recommedation. The CCM cluster was upgraded from 3.2 to 3.3 then 4.0 then 4.1, so the parameter is still AF31.

One question, what do you mean cos to dscp mapping global to the switch, then I have to use cos instead of dscp.

So when I should use dscp, uplink to another switch or router???


Re: Questions about QoS Mapping

If you did the upgrade and af31 carried over in the service parameters, make sure the settings were changed and also try restarting the CCM service and use a sniffer to make sure that in the lan side, the phones are definitely marking the traffic properly.

The mls qos map command is global to the switch, which means on any port, if a packet comes with COS 5, its converted to a internal DSCP of 46 for forwarding with in the switch backplane. Same applies for Cos 3 maps to DSCP of 24. Since you are using this mapping, the port trust state has to be set to mls qos trust cos, so that the switch acknowledges the cos setting sent by the phone. If you use mls qos trust dscp as the trust setting then you dont need to use the global mls qos cos map.

I typically use mls qos trust dscp command on uplinks to a router or even another switch if the upstream router/ switch is sending dscp which is usually the case of a remote site where the router receives packets marked with DSCP and sends the packet on to the switch.



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Hall of Fame Super Silver

Re: Questions about QoS Mapping


You should only trust dscp on the uplinks to the router, everything else should trust cos including phones and trunks to other switches.


New Member

Re: Questions about QoS Mapping

Is this true for all switches? I was told you should trust DSCP on all uplinks, not just those to the router.