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3560 DSCP & Precedence Trusts

With regards to IP precedence on a 3560 is it fair to say that 'mls qos trust dscp' and 'mls qos trust precedence' both achieve the same thing?  Trust DSCP trusts the Class Selector whereas trust precedence trusts the IP precedence which is the same as the DSCP class selector.

Also there are no PREC-->COS maps so I am presuming that the DSCP-->COS map applies when IP precedence is trusted?  Therefore the precedence values (precedence) map to the same values as DSCP CS?

Thanks

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Bronze

Re: 3560 DSCP & Precedence Trusts

Hi Richard,

I think the answer to your question is maybe, or possibly depending on whether the outcome actually is the same thing.

Have a look at this link --> http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a00800949f2.shtml

There's a table which shows matching DCSP/Precident values which you'll be able to use I'm sure.

Adam

New Member

Re: 3560 DSCP & Precedence Trusts

It depends what who intend to do.

If you trust PREC, the internal DSCP will get one of the CS1 -CS7 values.

The values are all multiple of 8.

Then on the egress, the original DSCP will be replaced by the corresponding CSn.

For instance, VoIP Express Forwarding traffic (DSCP 46)  will have instead CS5 (DSCP 40).

So, depending on the CoS behavior, the result could be different.

For example, I once designed a network where priority queue in routers was only for DSCP 46.

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