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

[Roger Wiklund]

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