QoS Config and Interface Output Drops

[martino-cisco]

Hello,

Can I get some input from some of the QoS pros please. I've noticed interface Output drops on some ports on a 2960X switch. These ports have policy-maps. I've been trying to dig into the QoS side of these and have hit some confusing points.

1. When I run the command 'sh mls qos int gi1/0/1 statistics', a portion of it shows the below:

output queues dropped:
queue: threshold1 threshold2 threshold3
-----------------------------------------------
queue 0:       0             0                  0
queue 1:       0             0                  0
queue 2:       0             0                  0
queue 3:   20744         0                370

So I can see drops on queue3/threshold3.

Is there a way to see and modify buffers on queue3/threshold3? When I run 'sh mls qos queue-set', I get output for queue sets 1 and 2. Each Queue set has queues 1-4. But I only see values for thresholds 1 and 2...Or are these referring to different things entirely (see output below)

Queueset: 1
Queue :          1           2                3              4
----------------------------------------------
buffers :         15        25            40               20
threshold1:   100      125          100              60
threshold2:   100      125          100             150
reserved :      50       100          100              50
maximum :    200      400          400            200
Queueset: 2
Queue :         1           2               3               4
----------------------------------------------
buffers :         25       25             25              25
threshold1:    100    200           100            100
threshold2:    100    200           100            100
reserved :       50     50              50              50
maximum :     400   400            400            400

Your feedback is appreciated.

[Mark Malone]

Hi

If you want to see what the qos queues map to for drops in terms of dscp use the command

show mls qos maps dscp-output-q

so above 20744 is actually queue 4 threshold 1 in the dscp-output-q

[martino-cisco]

Hi Mark,

Could you please explain how you've determined queue3/Threshold3 is actually queue 4/Threshold1?

See output of 'dscp-output-q' 

sh mls qos maps dscp-output-q

Dscp-outputq-threshold map:
d1 :d2 0 1 2 3 4 5 6 7 8 9
------------------------------------------------------------
0 : 03-03 03-03 03-03 03-03 03-03 03-03 03-03 03-03 04-01 04-01
1 : 04-02 04-01 04-02 04-01 04-02 04-01 02-01 02-01 02-01 02-01
2 : 02-01 02-01 02-01 02-01 02-02 03-01 02-01 02-01 02-01 02-01
3 : 02-01 02-01 01-03 01-03 02-01 02-01 02-01 02-01 02-01 02-01
4 : 01-03 01-03 01-03 01-03 01-03 01-03 01-03 01-03 02-03 02-03
5 : 02-03 02-03 02-03 02-03 02-03 02-03 02-03 02-03 02-03 02-03
6 : 02-03 02-03 02-03 02-03

[Mark Malone]

Hey

even though its 0-3 on the left in your mls statistics output you read it as 1-4 , if it was queue 0 theres nothing to map to in your sh mls qos maps dscp-output-q

so anywhere you see 04-01 in your output is the dscp value being dropped

queue 0 or 1 really is not where you want to be seeing any drops

queue 0:       0             0                  0

so say you wanted to see if voice is the dscp being dropped we know its dscp 46 , so on the left we drop down to 4 move across to 6 and it shows us queue 1 threshold 3 is where the dscp 46 is and we see that has 0 drops in our mls statisctic which is what we expect as its voice high priority and in the priority queue 0/1

This will help

http://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/116089-technote-switches-output-drops-qos-00.html

[martino-cisco]

Hi Mark,

That queue re-numbering in your explanation helped. Thanks. But about about the Threshold? Why does it become Threshold 1? With your explanation, I would have thought the drops should be described as being on 'Queue 4/Threshold 3'?

[Mark Malone]

Hi yes theres drops in queue 4 both thresholds 1 and 3 mostly 1 though 20744 , each of these thresholds then have certain dscps mapped to them based on the mls qos maps dscp-output-q

[Joseph W. Doherty]

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 wha2tsoever (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

On your platform, QoS supports four queues per interface, and each queue also supports 3 drop limits, thresholds 1, 2 and max.  (NB: packets can be managed by one of the threshold settings, so such packets might be dropped sooner that other packets, as the queue fills.)

What's confusing, on this platform (and the 3560/3750), sometimes those four queues are numbered 0..3, and sometimes 1..4.

The two queue sets are just a way to select one of two global settings that apply to the interfaces.  By default, all interfaces use queue set one, but you can configure interfaces to use queue set two.

In your case, some packets are being dropped by your 4th queue's 1st threshold setting.  If this isn't desired, there are several ways to modify your configuration.  You might change packet management, so they aren't managed by this threshold, or you might increase the threshold setting to match the other threshold and/or max settings.  Which, depends on what you're trying to accomplish via your (logical) QoS policy.