QoS buffer tuning

Unanswered Question
Jan 18th, 2008
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On the 3560 and 3750 switches, you can tune the buffer allocation between queues with the:

mls qos queue-set output <qset-id> buffers <alloc1> <alloc2> <alloc3> <alloc4>

command. The alloc numbers are percentages and not absolute values.

I've searched high and low looking for the size of the (absolute) total buffer space on the 3560, but I have not been able to find it.

I would guess this buffer size is the output queue size of the interface, which you can tune with the hold-queue out <size> command? It defaults to 40 frames, if I'm not mistaken.

Anyway got experience with this?

Best regards,

Donald Heering.

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Hieu Cao Fri, 01/18/2008 - 15:31
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You won't find the absolute values because they're not there, and those values are only percentages.

There are 4 egress queues with 3 threshold values per switch port. From the mls qos statement above, yes, the buffer is set at certain percentages for 4 egress queue in a particular port using the qset-id number.

In order for the switchport to work properly using the mls qos statement above, you would need to assign your switch port a particular qset-id.

Take a look at this document for more info.




DONALD HEERING Mon, 01/21/2008 - 12:24
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Hello Hieu,

Thanks! Useful document.

Still, there must be a limitation of memory available for the 4 egress queues somewhere :-). It seems on of the best kept secrets about the 3560...

Best regards,



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