[c-nsp] Cisco 3560 LAN QoS egress queing shaping/sharing questions

[Roger Wiklund]

Correction!

It should be 1/25th of 100meg = 4 meg. Thats really strange to have such a
small limit.

Found this also:
http://www.gossamer-threads.com/lists/cisco/nsp/113754

Regards
Roger

On Mon, Aug 17, 2009 at 4:27 PM, Roger Wiklund <copse at xy.org> wrote:

> Hi
>
> Im a bit confused regarding 3560 egress QoS.
>
> This is the default setting on a 3560, only "mls qos" is enabled globally.
>
> FastEthernet0/4
> Egress Priority Queue : disabled
> Shaped queue weights (absolute) :  25 0 0 0
> Shared queue weights  :  25 25 25 25
> The port bandwidth limit : 100  (Operational Bandwidth:100.0)
> The port is mapped to qset : 1
> So after reading the document, the 4 egress queues are configure with 25%
> bandwith each. and they are in shared mode, which means that they have a
> minimum of 25% but can also use more from the other queues if available.
>
> But then we have the shaped queue. 25 0 0 0.
> This is from the documentation:
>
> In shaped mode, the egress queues are guaranteed a percentage of the
> bandwidth, and they are rate-limited to that amount. Shaped traffic does not
> use more than the allocated bandwidth even if the link is idle. Shaping
> provides a more even flow of traffic over time and reduces the peaks and
> valleys of bursty traffic. With shaping, the absolute value of each weight
> is used to compute the bandwidth available for the queues.
>
> By default, weight1 is set to 25; weight2, weight3, and weight4 are set to
> 0, and these queues are in shared mode.
>
> For weight1 weight2 weight3 weight4, enter the weights to control the
> percentage of the port that is shaped. The inverse ratio (1/weight) controls
> the shaping bandwidth for this queue. Separate each value with a space. The
> range is 0 to 65535.
>
> If you configure a weight of 0, the corresponding queue operates in shared
> mode. The weight specified with the srr-queue bandwidth shape command is
> ignored, and the weights specified with the srr-queue bandwidth share
> interface configuration command for a queue come into effect. When
> configuring queues in the same queue-set for both shaping and sharing, make
> sure that you configure the lowest number queue for shaping.
> The shaped mode overrides the shared mode.
>
> Does this then mean that per default, the egress queue 1, handling COS
> 5, EF etc, only has 25mbit on a fastethernet port. Everything above that
> gets dropped.
>
> And also:
>
> Priority-queue out
>
> When you configure this command, the SRR weight and queue size ratios are
> affected because there is one less queue participating in SRR. This means
> that weight1 in the srr-queue bandwidth shape or the srr-queue bandwidth
> share command is ignored (not used in the ratio calculation).
>
> And also, when enabling egress prio queue, that queue qets 100% of the
> bandwith? That will starve all the other traffic. Im reading in the Cisco
> QoS book where you can have strict prio + weighted round robin. But it looks
> like thats not available on the 3560.
>
> Thanks
>
> /Roger
>
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