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Beginner

## Help Understanding QoS Threshold

Can some explain what the following would do?

mls qos queue-set output 1 threshold 2 400 400 100 400---Are these based on percentages? If so, from what?
mls qos queue-set output 1 buffers 15 45 20 20----I don't understand this at all

Below is the configuration I am using on the 3750:

srr-queue bandwidth share 1 70 25 5
srr-queue bandwidth shape 3 0 0 0

mls qos srr-queue output cos-map queue 1 threshold 3  5
mls qos srr-queue output cos-map queue 2 threshold 1  2 4
mls qos srr-queue output cos-map queue 2 threshold 2  3
mls qos srr-queue output cos-map queue 2 threshold 3  6 7
mls qos srr-queue output cos-map queue 3 threshold 3  0
mls qos srr-queue output cos-map queue 4 threshold 3  1
mls qos srr-queue output dscp-map queue 1 threshold 3  46
mls qos srr-queue output dscp-map queue 2 threshold 1  16 18 20 22 25 32 34 36
mls qos srr-queue output dscp-map queue 2 threshold 1  38
mls qos srr-queue output dscp-map queue 2 threshold 2  24 26
mls qos srr-queue output dscp-map queue 2 threshold 3  48 56
mls qos srr-queue output dscp-map queue 3 threshold 3  0
mls qos srr-queue output dscp-map queue 4 threshold 1  8
mls qos srr-queue output dscp-map queue 4 threshold 3  10 12 14
mls qos queue-set output 1 threshold 2 400 400 100 400
mls qos queue-set output 1 buffers 15 45 20 20

23 REPLIES 23
Highlighted
Beginner

Hi Jack,

to map CoS 1 to Queue 1 and CoS 3 to Queue 2, enter:

mls qos srr-queue output cos-map 1 1

mls qos srr-queue output cos-map 2 3

And then verify with show mls qos map cos-output-q

To drop CoS 1 traffic in Queue 1 when output queue level hits 60%, enter:

Q    Th1    Th2     Res    Max

mls qos queue-set output 1 threshold     1     60     100     100     400

Th1  CoS

mls qos srr-queue output cos-map queue 1 threshold     1     1

Th = Threshold

Res = Reserved

Max = Maximum

I don't know why the %'s max value is 3200.

Buffer weights determine what percentage of port buffer are available for each queue. The threshold, reserved,

and maximum values are all percentages of the buffer assigned to the queue. By default, each queue reserves

half of its buffer space assigned. What it's not reserved goes into the common pool. Thresholds can be

configured above 100% by allowing the queue to borrow from the common pool.

Highlighted
Contributor

Alirio,

Thank you so much for your response. I'm a little confused on when to use "output" and when to use "input" keyward. Considering the packets we are mapped when coming in, shouldn't we use "input" instead? I noticed "mls qos queue-set" only supports "output" on my 3750 switch. Please explain the directions as well.

mls qos srr-queue input cos-map 1 1

mls qos srr-queue input cos-map 2 3

mls qos queue-set output 1 threshold     1     60     100     100     400

mls qos srr-queue input cos-map queue 1 threshold     1     1

Highlighted
Beginner

Hi Jack,

You would use the input keyword to configure the actual input queues on switches that support them. I can't remember the hardware platforms, but on newer switches and bigger platforms, there are two input queues that could buffer frames in case the switching engine can't process frames or it's so busy that can't handle more incoming frames. Therefore, input queues are used to hold the frames until the switching engine is ready for those frames. In this way, you prevent dropping frames. Once frames are switched and ready to leave the switch, you use the output keyword you configure the egress queues in the outbound direccion.

The input queues are rarely utilized, however.

I'm going to send you a text file where I summarized Cat QoS and that may help you.

Highlighted
Contributor

More I think, more questions I have ...

What's the difference between this two commands? Under what condition we need to use "threshold"?

mls qos srr-queue output cos-map queue 1 1

mls qos srr-queue output cos-map queue 1 threshold     1     1

How do I accomplish this requirement?

• Set Queue 1 to drop threshold in case that the traffic in the queue 1 reached 60%
• Set Queue 2 to drop threshold in case that the traffic in the queue 2 is full
Highlighted
Beginner

Hi Alirio,

Very good explanation, I have a doubt, what does the common pool memory comprise of?

what is the memory assigned to each port ?

I read that there are two segements of memory available in switches one segment equally assigned to each port and common pool memory shared by all the ports.

how to find memory size in common pool and each port buffer?

Cheers,

Shaggy.

Highlighted
Beginner

Your explanation was so handful but now I am trying to test MLS feature on my Cisco switch and I was surprised after see that the threshold of Queue 1 appears automatically:

I would like to know how calculate the Queue1 values.

Highlighted
Beginner

it all sounds good but when you get to the fact that the first threshold is configured at 400% and second thresold is also configured at 400% it gets confusing. how can you go over 100% in the first place and why you have 2 thresholds on the same queue with same percentages.

I guess my question is how is it possible to have a threshold configured over 100% ??

thanks in advance for any clarafication.

Highlighted
Beginner

Hey Atef,

Apologies for the delayed response, I was out of station, as I mentioned earlier, there are two segments or portions of memory used in switches, one portion comprises of memory assigned to each port ( equally distributed ) for queuing and other portion comprises of memory ( it used when threshold is configured more than 100% ).

Cheers,

RS

Highlighted
VIP Expert

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See if this document helps: https://supportforums.cisco.com/docs/DOC-8093