Re: ip phone qos markings query

Hi,
what you call "uplinks" in your change is, I suspect, at the
distribution switches, right ? I mean, QoS trusting is only meaningfull
in the incoming direction, sounds like you know that.

Anyways, couple of points:
-you are missing "trust cos" in your access port.
 "trust device" only *disables* the trust if the device is not a phone.
-I would say that yes, 5->46 (EF) is best practice nowadays
-Yes, the phone does not touch the PC DSCP value, but it does generate
traffic with EF. Setting trust cos will eventually wipe the PC's DSCP
marking on access switch exit (if you don't enable transparent DSCP).
-COS on output comes always from internal DSCP (i.e. it's not mantained
from input but recreated) so changing your trust should not cause
trouble, given that your new trusted QoS source is ok.

I would say that using DSCP instead of COS should give you a more robust
QoS implementation, even in presence of a misconfigured switch...

-Carlos

Derek Hendridge @ 27/04/2010 8:20 -0300 dixit:
> Hi,
>
> I have a question regarding using IP phones (7941) and Qos.. There is
> existing qos configuration whereby a switch is using the 'switchport
> voice vlan 20' and 'switchport access vlan 10'. On the switchport we
> are trusting the phone using 'mls qos trust device cisco-phone'. On
> the uplinks between our switches we are trusting cos using 'mls qos
> trust cos'. I want to change this to trust dscp on the uplinks
> instead.
>
> mls qos
> !
> interface FastEthernet0/14
> switchport access vlan 10
> switchport mode access
> switchport voice vlan 20
> mls qos trust device cisco-phone
> !
> interface gi0/1
> mls qos trust cos <- this will change to 'mls qos trust dscp'
> !
>
> My understanding is that by default the IP Phone will mark voice
> traffic with CoS value of 5 and data with CoS value of 0. I don't
> think that this marks the ip dscp value as well, only the cos value.
> My understanding is also that because you are trusting the ip phone
> (essentially trusting the cos value), that the switch will use the
> cos-to-dscp mapping table to modify the dscp value of the packet
> according to the cos value. So by default the voice traffic will map
> cos 5 -> dscp 40. I want to ensure that by changing the trust value on
> the switch uplinks that i will not somehow remove the cos value
> assigned to the voice packet and that internally within the switch,
> that i will maintain priority for the voice traffic in terms of the
> hardware queue that the packet is put in.
>
> In addition, is it best practive to modify the default cos-to-dscp
> mapping such that cos 5 is market dscp 46?
>
> Switch12(config)#mls qos map cos-dscp 0 8 16 24 32 46 48 56
>
> I don't have access to any equipment to check this. This assumes that
> qos is enabled on the switch and you have disabled dscp transparency
> (as is the default). fyi i am using 3560s and 3750s.
>
> Derek.
>
>
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-- 
Carlos G Mendioroz  <tron_at_huapi.ba.ar>  LW7 EQI  Argentina
Blogs and organic groups at http://www.ccie.net