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3750 QOS Questions (3)

Hello, I was looking for clarification on the following items all related to EGRESS queuing on 3750 switch.  Thank you.

Question 1

If a packet is received with BOTH COS and DSCP markings which QOS label is used as a basis for selection of the egress queue and threshold….COS or DSCP?  How can I control this?  I have read it is based on the trust setting of the port which is confusing as trust is ingress not egress. 

Question 2

On egress, should the  mls qos maps cos−output−q” and “mls qos maps DSCP−output−q” be consistent?  The documentation indicates this should be the case on ingress, see below.  However there is no mention of this for egress queues?

Therefore, the Dscp−inputq−threshold map is configured to sync with the Cos−inputq−threshold map. These mappings should be as consistent as possible in order to ensure predictable behavior and to simplify troubleshooting.”

Question 3

Sharing Ration Calculation.  I am a little unsure of the math here.  On egress if I am using both shaping  and sharing and I want to shape queue 1 to 25%  and insure queue 2 is given a minimum of 30% during periods of congestion do the below configurations both accomplish this.

srr queue bandwidth shape 4 0 0 0

srr queue bandwidth share  1 30 40 30

srr queue bandwidth shape 4 0 0 0

srr queue bandwidth share 25 23 30 23 

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