CoS and DSCP

Unanswered Question
Mar 17th, 2007
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VoIP Qos

I have to deploy Qos in my network to give priotity to voice traffic.

THe QoS SRND guide says you van trust the COS as well as the DSCP which has confused me about what to use when.

When a cisco IP phone generates a voice packet, does it mark the traffic with DSCP value or COS vlaue.

How should i decide whether to trust the COS or the DSCP?

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hoogen_82 Sat, 03/17/2007 - 11:46
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CoS value and you should trust the COS value. Over the WAN it becomes Dscp. I believe your looking for giving priority to traffic on the LAN. If yo uneed it on the WAN then go far LLQ.


Jon Marshall Sat, 03/17/2007 - 13:13
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The IP phone can mark packets using both COS and DSCP markings.

Generally speaking you would trust the markings set by the IP phone. Contrast this with a user's PC where you would not trust any markings in the packets as this could allow a user to give priority to his traffic over everybody else.



mtechnology Sun, 03/18/2007 - 00:47
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I want to do end to end QoS.

So does this mean i should trust the COS Value on the LAN switches and use the COS-DSCP mappings or should i match the Voice VLAN itslef and set the vocie packets to DSCP EF and pass it on to the edge router.

Topology looks something like this

LANSW--->CoreSW--->Wan router


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