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3alee
Beginner

Question regarding fine tuning of auto-qos

Could anyone let me know what changes should be made to queue-set 1 configurations as follows so that traffic with dscp 0 can get all the remaining bandwidth and resources, without affecting traffic with DSCP 46, 34 & 26 (keep the same performance as before)?

mls qos srr-queue output dscp-map queue 1 threshold 3  40 41 42 43 44 45 46 47

mls qos srr-queue output dscp-map queue 2 threshold 3  24 25 26 27 28 29 30 31

mls qos srr-queue output dscp-map queue 2 threshold 3  48 49 50 51 52 53 54 55

mls qos srr-queue output dscp-map queue 2 threshold 3  56 57 58 59 60 61 62 63

mls qos srr-queue output dscp-map queue 3 threshold 3  16 17 18 19 20 21 22 23

mls qos srr-queue output dscp-map queue 3 threshold 3  32 33 34 35 36 37 38 39

mls qos srr-queue output dscp-map queue 4 threshold 1  8

mls qos srr-queue output dscp-map queue 4 threshold 2  9 10 11 12 13 14 15

mls qos srr-queue output dscp-map queue 4 threshold 3  0 1 2 3 4 5 6 7

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

!

interface FastEthernet0/1

srr-queue bandwidth share 10 10 60 20

srr-queue bandwidth shape  10  0  0  0

priority-queue out

mls qos trust dscp

auto qos voip trust

Thanks!

9 REPLIES 9
shillings
Enthusiast

Egress Queue 1 is the PQ (Priority Queue). Queue 1 is assigned 10% of egress port bandwidth using command:

     srr-queue bandwidth shape 10 0 0 0

Any Queue 1 traffic exceeding 10% is shaped (i.e. dropped).

An egress schedular is tasked with servicing each of the 4 egress queues. It will always service the Priority Queue first (i.e. Queue 1)

From the three DSCP values that concern you, only DSCP 46 is alligned to Queue 1, here: -

     mls qos srr-queue output dscp-map queue 1 threshold 3 40 41 42 43 44 45 46 47

DSCP 26 is aligned to Queue 2

DSCP 34 is aligned to Queue 3

Once Queue 1 (the Priority Queue) has been serviced and is empty, then the scheduler will share its time between the remaining 3 queues. It shares its time based upon the following command: -

     srr-queue bandwidth share 10 10 60 20

'10' relates to queue 2, '60' relates to queue 3 and '20' relates to queue 3. These values represent how the schedular allocates its time. They are not true % link utilisation values, but do relate to one another in terms of time spent by the schedular to empty each queue. It's hard to find the right words to explain this element.

To answer your question, you could align DSCP 34 to Queue 2 (DSCP 26 is already aligned to Queue 2) and allocate Queue 2 a larger chunk of schedular time, i.e. 60 20 10: -

     srr-queue bandwidth share 10 60 20 10

Once the PQ is empty, then Queue 2 would receive much more of the schedulers time, but only if Queue 2 requires it.

Is that what you want? What would be the impact on other traffic types?

Hope I've not confused you too much explaining how it works, but I think it's necessary in order to help you decide how to proceed, because I don't think 3750 LAN QoS will do exactly what you what. If I'm wrong, then I hope someone will correct me there.

Are you familiar with MQC (router QoS) ? This might be a better way forward if you're going to tune QoS beyond basic AutoQoS.

Hello

Sorry that's NOT what I was asking for.  Thanks for your advice anyway.

Let's focus on this:

mls qos srr-queue output dscp-map queue 4 threshold 3  0 1 2 3 4 5 6 7

I want to maximize the performance of traffic with DSCP value of "0", i.e. queue 4.  For the other 3 queues just leave them untouched.

What should I do?

Thanks.

OK, yes I see it now. My reasoning of written text is not always very good - great for exams (not)

What do you mean by 'maximize the performance'? You could give Queue 4 (i.e. DSCP 0) more egress schedular time than Queues 2 and 3? This can be done as follows: -

     srr-queue bandwidth share 10 10 20 60

You could also tweak WTD thresholds.

I've tried even with "srr-queue bandwidth share 10 5 5 80".  The result seems a lot better but my client still didn't satisfied.

'maximize the performance' here means "sends out traffic with DSCP 0 as good as possible with minimal packet drops and also NOT affecting the quality of dscp 46, 26 & 34".

Can it be done?

Also, what's the meaning of :"tweak WTD thresholds"?

Thanks!

I think the best way forward is to switch to MQC (Modular QoS Configuration), if it's available in your IOS. You can then actually classify the particular traffic that needs priority. DSCP 0 could be anything and some of it might not need any prioritisation.

Weighted Tail Drop (WTD) is a congestion avoidance mechanism whereby traffic is dropped before the buffers fill. This way, the switch can actually choose which traffic class is dropped (i.e. low priority traffic). Traffic in threshold 1 is dropped first, then if more traffic must be dropped, it moves onto threshold 2. Threshold 3 is never dropped by the WTD mechanism.

DSCP 0 is already in threshold 3: -

mls qos srr-queue output dscp-map queue 4 threshold 3 0 1 2 3 4 5 6 7

Queues 2 and 3 have slightly different WTD settings. I'm not sure if these will suit your DSCP 0 traffic better or not. If you want to try, then move DSCP 0 up to Queue 2, threshold 3 as follows: -

mls qos srr-queue output dscp-map queue 2 threshold 3 0

However, I still think MQC is a better choice, if you have the capability within the current IOS.

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Posting

'maximize the performance' here means "sends out traffic with DSCP 0 as good as possible with minimal packet drops and also NOT affecting the quality of dscp 46, 26 & 34".

Can it be done?

Possibly not - reason being QoS really juggles finite resources.  Providing any one type of traffic more of any kind of resource generally reduces that resource to other traffic types.

Any thing you do that might improve DSCP 0 traffic performance risks worsening other traffic performance.

It's possible your other traffic is reserving resources that it doesn't actually need to maintain its current performance, and if you change these reservations you may see DSCP 0 performance improve without any (immediate) adverse impact, but again, by changing reservations you're changing service guarantees.

3alee
Beginner

Thanks again.

Forgot to mention, it's a Cat3560 with IOS version 12.2(53)SEE2 with auto-qos enabled.  Can it do MQC?

According to Feature Navigator, only the 3560 Compact and 3560-X series switches are compatible.

You could confirm by trying to configure the class-map in Figure 1, below: -

http://www.cisco.com/en/US/technologies/tk543/tk545/technologies_white_paper09186a0080123415.html

If that does work, which I doubt, then don't go as far as applying a service-policy to an interface until you're confident about what your configuring.

Not likely to make such a change since there're over 400 switches in the network and such qos model has been operating for over 4 yrs.

Can we tune anything on the buffers which helps?

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