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3750 Egress Queue Selection

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?  Is it possible to control this behavior via configuration? 

Chad Peterson
Cisco Employee

Ultimatly it will be determined by the trust statement at ingress:

show mls qos int

You can see which COS values and DSCP values go to which queues here:

mls qos maps cos-output-q

mls qos maps dscp-output-q

This page is a pretty good reference for general qos on 3750.  Feel free to ask more questions if this didn't clear it up

Thanks Chad, I do have that document but it was not clear on how the egress selection was made when both COS and DSCP

are present.  I do have two other questions if I may!

1.  The documentation for INGRESS states.

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.”

    however there is no mention if this practice should be adhered to for Egress queueing and they certainly do not "sync" on their own???  Should these settings be consistent on egress.  In other words should COS and corresponding DSCP values be manually configured to insure the same egress queue and thresholds are selected?

2. Sharing Ratio Calculation.  Do both the configurations below accomplish the same thing.

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  <<<<<<<<<<< First value not used due to shaping command therefor 23 out of 75 packets assigned to queue 2 (23=30% of 75)

Q1- the egress q selection depends on whether layer 2 or layer 3 was used on the ingress port. If you "trust cos" on the ingress side then the qos label for that frame is based on L2 and ingress AND EGRESS q selection will be dependent on COS- the "dscp-maps" on both ingress and egress are irrelevant in this case. However other ports could use "trust dscp" in which case "cos-maps" are redundant and "dscp-maps" are used for ingress and egress queue selection. Therefore it is adviseable that they both match.

Q2- yes- but one correction. 23/76 is not exactly 30%. But conceptually you are correct- "srr-queue band share 3 4 3" is the same for example.