QoS clarification

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Mar 10th, 2009
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My QoS is a bit rusty, was looking for some help. My understanding is a switch classifies traffic w/ the dscp map. Then the router 'marks or tags' based on the info given from the switch. The router does not change the classification. So if, my dscp map is bad on my switch, my tags are probably bad on the router. Is this correct?

Thanks in advance,


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Nicholas Matthews Tue, 03/10/2009 - 14:43
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Hi rjp,

The DSCP/CoS markings are set originally at endpoint. The access layer port is generally where the decision is made to trust, not trust (set everything to 0), or do some type of mapping.

The router can mark, but will not by default. This is done with the 'set dscp'/'set tos' command within a class in a policy map.

You can match traffic for a class-map on many other things than DSCP, so depending on what you're trying to match you can get around the problem as well.



rjpaolello Tue, 03/10/2009 - 15:58
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First, thanks for the quick response. To clarify, if I'm not using the 'set dscp or set tos' commands in my policy map, my router is not changing tags received from the switch. Is this correct?

Thanks again,


Nicholas Matthews Tue, 03/10/2009 - 17:07
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Hi rjp,

This is correct. The only time a router will change the DSCP byte is when you tell it to via a route map or policy map.


rjpaolello Tue, 03/10/2009 - 17:55
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Thank you so much for your help.

Best Regards,



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