RE: Layer 2 QoS

Thank Carlos. The light bulb went off as I was reading a different document. I was look at these output queue maps wrong. I was thinking it was putting the traffic in that area of the queue when it's really mapping when to start dropping that traffic. In my example (the middle line) COS 3 packets won't even be eligible to be dropped until the queue fills to threshold 2 (which can be configured to be any percent of the queue). If threshold 2 is passed lower COS value traffic is dropped more aggressively and some COS 3 packets could be dropped. If threshold 3 is 100% COS 6 and 7 would only get tail dropped because the queue is absolutely full.

Thanks,
Andy

________________________________________
From: Carlos G Mendioroz [tron_at_huapi.ba.ar]
Sent: Sunday, February 27, 2011 5:25 PM
To: Diment, Andrew
Cc: 'ccielab_at_groupstudy.com'
Subject: Re: Layer 2 QoS

Thresholds are limits of buffer usage, that when exceeded cause packets
to get discarded.
Examples usually show higher values on higher ids, in the 3560
the threshold 3 fixed at 100%.
(Meaning you drop when there is absolutelly no place to hold the packet)
Having a 50% threshold would discard even though you have half of the
buffers available.

 From where do you get the impression that higher thresholds are more
drop prone ?

-Carlos

Diment, Andrew @ 27/02/2011 17:23 -0300 dixit:
> I have a quick questions on layer 2 QoS that I cannot find an answer to. In
> all of cisco's doc and QoS design guides they map higher COS traffic (or DSCP
> traffic) to higher thresholds within the queues like below.
>
> mls qos srr-queue output cos-map queue 2 threshold 1 2
> mls qos srr-queue output cos-map queue 2 threshold 2 3
> mls qos srr-queue output cos-map queue 2 threshold 3 6 7
>
> My understanding of thresholds is the higher the threshold breached the more
> likely packets get dropped. The way Cisco says do to this mapping seem
> opposite to me. I would think you would want the more critical traffic in the
> lower threshold...not the higher. I know COS 5 goes to the priority queue so
> I'm just looking at the non-priority queues.
>
> Thanks,
> Andy
>
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--
Carlos G Mendioroz  <tron_at_huapi.ba.ar>  LW7 EQI  Argentina
This communication is the property of Qwest and may contain confidential or
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prohibited and may be unlawful.  If you have received this communication
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