if no qos enabled on 3550/3750 switch

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Jun 8th, 2007
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f a 3550/3750 switch is not qos enabled and IP phone connects to it. How does it happen when packets arrive on the outbound interface to wan(dot1q between them):

1. dscp reserved?

2. how about cos? since no qos enabled, switch won't look at the default dscp-cos map. Will the cos be reserved or 0? Thanks

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thisisshanky Fri, 06/08/2007 - 09:51
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since qos is not enabled, the dscp will be preserved when it passes through the switch. cos is sent across a link only if the link is a trunk. Now there is an internal DSCP/COS that is used by the switch when the packet traverses through the backplane of the switch. This DSCP is usually derived on ingress either using the trust state of the ingress port or using ACLs or policy maps that have been used to remark the packet on ingress. This DSCP is used in conjunction with DSCP-COS value and the cos value generated is usually used for outbound queueing on egress ports. All the above works only if QOS is enabled on the switch. So to answer your second question, if the outbound port is a trunk link cos is set to 0, otherwise cos is not sent.



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ciscoforum Fri, 06/08/2007 - 11:26
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Hi Sankar

Thanks for your response. One more question regarding this: if that's the case, is the voice packet will be residing in the non-priority queue before transmitted out the switch(you know what i am refering to is 1q4t this kind of switch queue) right?


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