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MLS QoS Thresholds

Hi all,

 

I wanted to get to know more about QoS and thought that there was no better way than document and understand the output of auto qos.

However I want to verify some things to check that I understand it correctly.

The 2 commands I want to check on are the following:

mls qos queue-set output 1 buffers 15 25 40 20

mls qos queue-set output 1 threshold 4 60 150 50 200

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mls qos queue-set output 1 buffers 15 25 40 20, this command divides the port buffer space between the 4 egress queues.

1. Does it always need to add up to 100%? Or if it doesn't, will the remaining percentages be added to the common pool?

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mls qos queue-set output 1 threshold 4 60 150 50 200

Are the following statements correct?

2. packets belonging to queue 4 threshold 1, will be dropped when the port buffer for queue 4 is 60% full (60% of 20 buffers = 12 buffers)?

3. packets belonging to queue 4 threshold 2, will be dropped when the port buffer for queue 4 is 150% full (150% of 20 buffers = 30 buffers)?

3b. how does this work? since this are more buffers than reserved or available for the queue? what if no more buffers remain in the common pool?

4. packets belonging to queue 4 threshold 3, will be dropped when the port buffer for queue 4 is 100% full (100% of 20 buffers = 20 buffers)? 

5.the queue can steal up to 200% buffers from the common pool, can this ever be reached? since we will be dropping at 60% (th1), 150% (th2) and 100% (th3) so we will max reach 150% and not 200%?

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Posting

#1 It's recommended sum of all four queues buffers equal 100%.  I don't know what happens if they they don't.

 

#2 Yes and no, it's 60% of 20%.

 

#3 Yes and no, it's 150% of 20%.

 

#3b If not buffers to borrow from common pool, it will drop.

 

#4 No, it's 200% of 20%.

 

#5 Yes, buffers are borrowed are (attempted) borrowed from common pool when reserve buffers are all used (and WTD not reached).  Th3 is 200%, not 100% of 20%.

 

Great document on 3560/3750 buffering: https://supportforums.cisco.com/document/31581/egress-qos

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Super Bronze

DisclaimerThe Author of this

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

#1 It's recommended sum of all four queues buffers equal 100%.  I don't know what happens if they they don't.

 

#2 Yes and no, it's 60% of 20%.

 

#3 Yes and no, it's 150% of 20%.

 

#3b If not buffers to borrow from common pool, it will drop.

 

#4 No, it's 200% of 20%.

 

#5 Yes, buffers are borrowed are (attempted) borrowed from common pool when reserve buffers are all used (and WTD not reached).  Th3 is 200%, not 100% of 20%.

 

Great document on 3560/3750 buffering: https://supportforums.cisco.com/document/31581/egress-qos

Thanks for your clarification

Thanks for your clarification on this!

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