Unanswered Question
Apr 2nd, 2007
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Hi all, when tagging the traffic using cos at the layer 2 level, how do we tell the switch etc what kind of traffic we want to give a priority too, ie if we want to tell it to look for voice then apply the relevant policy, how would we do this ?


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m-haddad Mon, 04/02/2007 - 11:48
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Under the switchport you issue the command

"mls qos trust Cos". The switch will then start looking at the COS tag and according to the tag it will prioritize the traffic.

Hope this helps,

Appreciate your rating,


carl_townshend Mon, 04/02/2007 - 14:09
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thanks for that, so how do we tell cos that the paticular traffic is voice ?

jcovarru Mon, 04/02/2007 - 15:07
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Normaly, VoIP phones, mark the CoS value in their L2 frame.

For example you might have the topology

Phone --- L2switch --- Router1

If the phone is setting the CoS value you just need to trusted on the L2 Switch.

Then on the Inbound Interface between L2 switch and the Router you need to convert the CoS value to a DSCP value.

This dscp value later can be matched on oubound policy on LLQ.


Service-policy inbound (between switch and Router)

Class CONE

match cos 5

policy-map PONE

class CONE

set dscp 40

Later on the Router outbound serial interface

service-policy outbound PTWO

Class CTWO

match dscp 40

policy-map PTWO

class CTWO

priority 64


Hope this helps!

carl_townshend Tue, 04/03/2007 - 00:19
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thanks for that, so are we saying here that the phone or whatever device you plug in has its own set cos value, and all you do is tell the switch to trust it ? can you set the qos on the switch ?


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