ip phone qos markings query

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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Received on Tue Apr 27 2010 - 12:20:19 ART