Autoqos queue-set on 2960/3560/3750

Unanswered Question
Mar 26th, 2009

Does anyone have an idea or an explanation what the considerations behind the autoqos settings of the thresholds were? It is very unclear to me.

Any hint is appreciated.

Mat

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

mls qos queue-set output 1 buffers 10 10 26 54

mls qos queue-set output 2 buffers 16 6 17 61

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MATTHIAS SCHAERER Thu, 03/26/2009 - 08:37

Hi Andrew,

thanks for the answer. Where in the SRND have you found an explanation that goes beyond the command syntax?

Regards,

Mat

MATTHIAS SCHAERER Thu, 03/26/2009 - 09:11

Hi Andrew,

I did some time ago and I did it again but apart from saying that you assign thresholds I can't see anything that gives me a clue about why the Cisco programmer came to the solution with the values that are present in Autoqos. There sure must be some considerations not to allocate the full 100 percent of buffers initially in some queues or to go up to 318% of buffer usage in queue. But I have not yet found an explication for that.

Kind regards,

Mat

Mat,

I cannot really comment on what the programmer was thinking with the autoqos script. I however do not use it, but do use the recommended settings per qos model in the SRND guide.

To answer "There sure must be some considerations not to allocate the full 100 percent of buffers initially in some queues or to go up to 318% of buffer usage in queue"

The egress queue default settings are suitable for most situations. You should change them only when you have a thorough understanding of the egress queues and if these settings do not meet your QoS solution.

Two queue sets are configured and queue set 1 is assigned to all the ports by default. Each queue is allocated 25 percent of the total buffer space. Each queue is reserved 50 percent of allocated buffer space which is 12.5 percent of the total buffer space. The sum of all the reserved buffers represents the reserved pool, and the remaining buffers are part of the common pool. The default configuration sets 400 percent as the maximum memory that this queue can have before packets are dropped.

HTH>

Pavel Bykov Mon, 04/27/2009 - 06:45

This explanation, although seen in many places, does not work in real life.

I am still trying to reach final conclusion.

I have set up tests in the lab, and found out, that for example

mls qos queue-set output 1 threshold 1 100 200 100 200

has exactly same result as

mls qos queue-set output 1 threshold 1 100 200 100 300

and

mls qos queue-set output 1 threshold 1 100 200 100 400

Funny thing is that although command allows to specify drop thresholds and max thresholds up to 3200, It was impossible to pass the 500% mark - buffers were not allocated beyond that even when the queues were empty. I have yet to experiment with disallocating buffers from the other ports in the common pool, because maybe the common pool is just not big enough. Using testing I found out many things, including how many actual BYTES does each command allocate,but I have yet to discover why AutoQoS specifies for example: "mls qos queue-set output 2 threshold 4 42 72 100 242", because buffer system will never allocate more than 100% for Drop Threshold 3.

E.G.:

"mls qos queue-set output 1 threshold 1 100 200 100 200" command allows packets of Queue 1 and Drop Threshold 3 to use up to 200% of allocated buffers.

And Also:

"mls qos queue-set output 1 threshold 1 100 200 100 300" command allows packets of Queue 1 and Drop Threshold 3 to use up to 200% of allocated buffers.

As you can see, the MAX has been raised from 200 to 300, but still buffers are not allocate over that thresholds, because it is the DROP threshold, so everything over that is dropped.

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