Auto QoS

[izackvail]

I was hoping to clear up a point of confusion with the standard QoS configuration on the 2960/3560/3750.

I noticed that when the ingress WTD thresholds are configured they are setup as a percentage of the total buffs.

mls qos srr-queue input bandwidth 90 10

mls qos srr-queue input threshold 1 8 16

**Thresholds for Q1 are 8%/16%/100%

mls qos srr-queue input threshold 2 34 66

**Thresholds for Q2 are 34%/66%/100%

mls qos srr-queue input buffers 67 33

But when the WTD Thresholds are setup for the Egress Queues they are setup as a number of buffers regardless of the switch platform / model.

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

** 138 is not a percentage. It represents 138 of the total available buffers.

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

My question is why? When a 2960 has 400 available buffers and a 3750 has 3200 available it seems like a percentage would make a lot more sense in this situation.

Example:

Cat2960

SW3(config)#mls qos queue-set output 1 threshold 1 ?

<1-400> enter drop threshold1 1-400

Cat3750

SW1(config)#mls qos queue-set output 1 threshold 1 ?

<1-3200> enter drop threshold1 1-3200

Thanks!

[izackvail]

Let me ask this another way. I don't understand why auto qos would configure the same thresholds for a switch that has 400 available buffers as a switch that has 3200?

For example, on a 3750 in queue 1 auto qos configures 138/3200 buffers for threshold 1 and 138/3200 buffers for theshold 2.

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

For a 2960 auto qos configures 138/400 buffers for queue 1 and 138/400 buffers for queue 2.

How can both of these configurations be valid? I know I must be missing something here!

[h.parsons]

The thresholds are in percentage. This is out of the configuration guide:

Two WTD thresholds expressed as a percentage of the allocated memory of the queue. The range is 1 to 3200 percent.

[izackvail]

Thank you. You are absolutely right.

In actuality it is a percentage of the allocated buffer space for that queue. For example if the buffer space was allocated like this:

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

and there are 1000 total buffers you are allocating 100 total buffers to Q1. If the thresholds are set like this:

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

It is saying to allow Q1T1 138% of the 100 allocated buffers or 138 buffers before dropping traffic. The same would be true of Q1T2 and Q1T3. Of the 138 buffers 92 of them would come from the reserved pool and 46 would come from the common pool. The maximum memory allocation for Q1 is also 138.

In other words for Q1 there is no destinction between the 3 available thresholds for the queue. It will drop traffic placed in T1 T2 or T3 when they reach 138% of the allocated buffers for the queue.