setting up ports to trust dscp but remark everything else back to 0

Unanswered Question
Aug 19th, 2009

Hi all

can anyone tell me the easiest way to trust dscp values on my switchports from my siemens phones, but anything else that comes in ie from pc's servers etc remark back to 0.


I have this problem too.
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Peter Paluch Wed, 08/19/2009 - 00:45


We need more information.

1.) How are the phones connected - are they connected to dedicated ports or do they share uplinks with PCs? (PC -> Phone -> Switch)

2.) Is the voice traffic sent from the phones 802.1Q tagged or not?

Best regards,


carl_townshend Wed, 08/19/2009 - 23:51

Hi there

the pc's connect to the phones, and the phones connect to the switch, the phones mark DSCP EF46 in the packet and it is tagged with a vlan 128, the pc packets are untagged.

The phones and pc's are in different vlan/subnets.



Peter Paluch Thu, 08/20/2009 - 02:00

Hi Carl,

Thanks for the additional information. I suppose that you have configured your switchport as access ports with an auxiliary voice VLAN 128.

We have a problem here. The switchport can be configured to trust either DSCP or CoS marking in general. As far as I know, there is no way to configure the type of trust on per-VLAN basis. Therefore we can't configure the port to trust the DSCP, rather, we can configure it to trust CoS markings and for untagged frames we will assume the CoS=0. According to the CoS value, the switch will rewrite the DSCP value in the IP header according to a CoS-DSCP map. This approach requires that the phone also uses 802.1p CoS priority markings inside the 802.1Q tag but I assume that it already does.

The configuration would be as follows:

mls qos ! Not supported on older Catalysts

mls qos map cos-dscp 0 8 16 26 32 46 48 56

interface Fa0/1

mls qos trust cos ! Trust CoS markings

mls qos cos 0 ! Untagged frames will be assigned CoS=0

The switch will automatically rewrite the DSCP value according to the CoS-DSCP map.

Best regards,


carl_townshend Thu, 08/20/2009 - 04:46

hi there

thanks for that,

1 last question,

whats does the map mean

mls qos map cos-dscp 0 8 16 26 32 46 48 56


Peter Paluch Thu, 08/20/2009 - 05:16

Hi Carl,

If a CoS is trusted in an incoming frame, then the DSCP value of the packet inside that frame is considered untrusted and the switch will rewrite that value. The CoS-to-DSCP map defines how to rewrite it.

The CoS-to-DSCP map contains 8 values that represent the DSCP value that should be written into the packet header. It is sorted from CoS 0 to CoS 7, so the rewriting will be:

If CoS=0, then DSCP=0

If CoS=1, then DSCP=8

If CoS=2, then DSCP=16

If CoS=3, then DSCP=26

If CoS=4, then DSCP=32

If CoS=5, then DSCP=46

If CoS=6, then DSCP=48

If CoS=7, then DSCP=56

You may want to sniff the voice packets sent by your IP phone and check whether the CoS value in the frame is as you expected, and optionally modify this map in such a way that the CoS value of your voice packets gets rewritten to DSCP EF=46.

Best regards,



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