LAN QoS - output queues

Unanswered Question
Sep 28th, 2008


This question is for all HW based platforms that allow for DSCP and COS based output queues.

If I have both COS and DSCP fields set, with CoS=3 and DSCP=24, and I have both DSCP and COS output queue maps like so:

mls qos srr-queue output cos-map queue 2 3

mls qos srr-queue output dscp-map queue 3 24

Which output queue will the packet fall in to?

I know what will happen if this would be an input (trust option) but there is no such thing as trust on the output, after Internal DSCP has been consulted. So how is it decided in this case?


P.S.: Interface is trunk and does have 802.1q/802.1p. There are no counters that would show the queue match, for example "show mls qos interface x/x statistics" shows output VALUES, not queues. But i know the values exactly and need to know output queues.

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Overall Rating: 2.5 (4 ratings)
bmcginn Sun, 09/28/2008 - 20:58

Hi there,

Different hardware queues for different switching blades/archtectures. The queueing command is always helpful.

show queueing interface fax/x

That shows what cos values go into what output (or input) hardware queue. If the line card supports dscp to queue mapping then that command will show that also.

Good luck with it!

Marwan ALshawi Mon, 09/29/2008 - 04:05

Although DSCP-to-queue/threshold maps override CoS-to-queue/threshold maps, these mappings should be as consistent as possible, to ensure predictable behavior and simplify troubleshooting.

if helpful Rate

Pavel Bykov Mon, 09/29/2008 - 12:44

Sorry guys, not helpful at all. I was asking a specific question, not about a recommendation.

Anyway, someone did provide me with an extremely helpful link that solved the issue:

Section 2 answers the qeustion: If both COS/DSCP are present, the input trust/mark will decide on the output queue.


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