Catalyst 2960/3560/3750 AutoQoS Question

[jguemes]

I have seen that when configuring AutoQoS on a Cat 2960 / 3560 / 3750 the default for queue-set 1 is changed but not for queue-set 2, and when applying an Auto QoS command to an interface, queue-set 1 is configured for a Fastethernet interface but queue-set 2 for a Gigabit interface.

I can´t figure out why is it. It is supossed that Gigabit interfaces suports all accumulative traffic (link to the backbone) and that a more accurate QoS configuration is needed for that kind of links.

Does anyone know te reason for this? I don´t think it´s an IOS programmer mistake.

I'm using IOS 12.2(55)SE3 and auto qos srnd4.

Regards.

[Edison Ortiz]

It depends on the interface capabilities, if they have 4Q3T, it would use queue-set 1

If they have 1P3Q3T, they would use queue-set 2

You can find out the queue type with the command:

show interface x/x capabilities

[jguemes]

Hi Edison:

Thanks for the answer but I can´t agree:

On a Cat 2960 model WS-C2960-48PST-L all the interfaces appears as 4Q3T, Gigabit or Fast ethernet, but the config generated by auto qos trust cos is different (output from debug auto qos):

Gigabit:

Jun 18 10:49:26:   mls qos trust cos

Jun 18 10:49:26:  no queue-set 1

Jun 18 10:49:26:   queue-set 2

Jun 18 10:49:26:  priority-queue out

Jun 18 10:49:26:  srr-queue bandwidth share 1 30 35 5

FastEthernet:

Jun 18 10:47:03:   mls qos trust cos

Jun 18 10:47:04:  no queue-set 1

Jun 18 10:47:04:  priority-queue out

Jun 18 10:47:04:  srr-queue bandwidth share 1 30 35 5

Regards.

[Joseph W. Doherty]

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Posting

I think you may already answered your own question.  Using different queue sets for FastE vs. GigE allows different resource allocation for the two different bandwidths.  Everything else being equal, BDP (bandwidth delay product) is larger for GigE hence to support it correctly requires larger buffers.