Understanding "mls qos queue-set output"

[shikamarunara]

Hello,

     I've been working on breaking down and understanding the default auto qos configuration on a Cisco 3750 in the hopes of putting together a QoS strategy that will fit our environment.  I'm having some difficulty understanding how the "mls qos queue-set output" syntax works.

     From another post, at https://supportforums.cisco.com/thread/2004369 , the author offers the following example and explanation;

mls qos queue-set output <1/2> threshold 2 400 400 100 400

thresshold 1: 400%

thresshold 2: 400%

thresshold 3: 100% (implicit, not configurable)

reserved: 100%

max: 400%

     However, I'm having trouble understanding what is meant.  Here, it looks like it's saying, for example;

mls qos queue-set output 1 threshold 2 400 400 100 400

How come there is syntax stating "threshold 2" when in the succeeding part the 400 refers to thresshold 1 and threshold 2 again?  The syntax 400 400 is, apparently, already referring to thresshold 1 and 2, no?

[Joseph Martini · Accepted solution]

The syntax of that command is mls qos queue-set output qset-id threshold queue-id, so the threshold 2 is not threshold 2, it's threshold for queue 2, so queue 2 threshold 1, queue 2 threshold 2, queue 2 threshold 3.

[shikamarunara]

Hey Joe,

     Okay, so, breaking this down, if we look at the default set of auto qos syntax for this;

mls qos queue-set output 1 threshold 1 138 138 92 138

mls qos queue-set output 1 threshold 2 138 138 92 400

mls qos queue-set output 1 threshold 3 36 77 100 318

mls qos queue-set output 1 threshold 4 20 50 67 400

mls qos queue-set output 2 threshold 1 149 149 100 149

mls qos queue-set output 2 threshold 2 118 118 100 235

mls qos queue-set output 2 threshold 3 41 68 100 272

mls qos queue-set output 2 threshold 4 42 72 100 242

Let's pick on the first line.  "138 138 92 138" refers to, respectively, "'Threshold 1' 'Threshold 2' 'Reserved Buffer' 'Maximum Buffer Size'".  There are three thresholds per queue, but we can't reconfigure Threshold 3 here, it is implicitly set to 100%, I believe.  There are three thresholds per queue and 4 egress queues.  I'm with you so far.

So, breaking down the rest of it;

mls qos queue-set output threshold

"queue id" seems to refer to one for the 4 total egree queues (the syntax after that refers to the thresholds within that queue).  But, the queue-set id is totally confusing.  Looking at this set of syntax, why do we have two set of syntax (1 and 2) for the 4 egress queues?

[Joseph Martini · Accepted solution]

On a 3750 there are two queue-sets, which are really just two groups for adding ports to that share the same configuration settings.  You're correct there are 4 egrees queues and two queue-sets, so we have to define the settings for all 4 queues for both queue-set 1 and queue-set 2.  By default all ports are in queue-set 1.

[shikamarunara]

But then what's queue set 2 for?

[Joseph Martini · Accepted solution]

You can assign a port such as gi1/0/1 to queue-set 1 meaning these settings are applied to that port:

mls qos queue-set output 1 threshold 1 138 138 92 138

mls qos queue-set output 1 threshold 2 138 138 92 400

mls qos queue-set output 1 threshold 3 36 77 100 318

mls qos queue-set output 1 threshold 4 20 50 67 400

Then you can assign a port gi1/0/2 to queue-set 2 which would apply these settings:

mls qos queue-set output 2 threshold 1 149 149 100 149

mls qos queue-set output 2 threshold 2 118 118 100 235

mls qos queue-set output 2 threshold 3 41 68 100 272

mls qos queue-set output 2 threshold 4 42 72 100 242

It's a way to set different settings to different groups of ports.

Here's a great video if you have about 2 hours to watch it by Cisco Certified Instructor Kevin Wallace:

http://www.youtube.com/watch?v=IA4iOrn2eiU

[shikamarunara]

That rabbit hole is deep on the buffer settings, no question.  This is an awful lot of nerd nobs for something that 95% of people are very likely not going to change from the defaults.  Some of these numbers are pretty crazy, I wonder how they came up with numbers like "138%".

Having 2 sets makes sense when you think about it, but I imagine that queue set 2 doesn't get much of a workout.  Have you had a need to modify these?

[shikamarunara]

Odd, why did the forum censor out the word "nird" (mispelled here to circumvent censoring).

[Rob Huffman · Accepted solution]

Hi Shikamarunara,

Hope all is well buddy

You can aasign the indivudual ports to either queue set 1 or queue set 2. These queues share a common

config set as described quite nicely in this doc and by our friend Joe here;

"On a 3750 there are two queue-sets, which are really just two groups for adding ports to that share the same configuration settings."

Queue Configuration:

Egress queue configuration allows you to configure two queue sets. Each queue set has the option to

configure the buffer size and threshold value for the four egress queues. Then, you can apply any one

of the queue sets to any of the ports. By default, queue set 1 is assigned to all the ports when you

enable QoS on the switch.

http://www.cisco.com/image/gif/paws/91862/cat3750-qos-config.pdf

Cheers!

Rob

PS: Great answers here Joe! +5 it's always good to have you around here @ CSC

[shikamarunara]

Hey, Rob, how's it going friend?  How's the weather up there

Yeah, Joe's been helping me save my sanity with firguring out this syntax, I think I've got it worked out.  I guess my question would go to both of you (and anyone else who cares to answer); have you found any reason to modify these settings from the auto qos Cisco recommendations?

[Rob Huffman · Accepted solution]

Hi Shikamarunara,

It was actually snowing here this morning How bout you? It's been a crazy year so far but it's getting better

Yes, thankfully peeps like Joe help us all save our sanity!

Our numbers have never been adjusted from the auto qos settings.

Cheers!

Rob

[shikamarunara]

Snowing mid April?  Oh, man, that's messed.

Yeah, I figure that the auto qos settings should suffice, I can't imaging a situation where something happens and someone would all of the sudden have to start tweaking percentages on a 3750 - seems a little impractical from a systems management standpoint.

Still, important to know what the settings do.  You wouldn't happen to know if auto qos settings are different between switch models?  My understanding is that they differ.