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Cisco Catalyst Blade Switch 3130 for Dell Software Configuration Guide
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Chapter36 Configuring QoS Understanding QoS

WTD Thresholds

You can assign each packet that flows through the switch to a queue and to a threshold. Specifically, you
map DSCP or CoS values to an egress queue and map DSCP or CoS values to a t hresh old ID . You use
the mls qos srr-queue output dscp-map queue queue-id {dscp1...dscp8 | threshold threshold-id
dscp1...dscp8} or the mls qos srr-queue output cos-map queue queue-id {cos1...cos8 | threshold
threshold-id cos1...cos8} global configuration command. You can display the DSCP output queue
threshold map and the CoS output queue threshold map by using the show mls qos maps privileged
EXEC command.
The queues use WTD to support distinct drop percentages for different traffic classes. Each queue has
three drop thresholds: two configurable (explicit) WTD thresholds and one nonconfigurable (implicit)
threshold preset to the queue-full state. You assign the two WTD threshold percentages fo r thre shold
ID 1 and ID 2. The drop threshold for threshold ID 3 is preset to the queue -f ull st ate, a nd you c annot
modify it. For more information about how WTD works, see the “Weighted Tail Drop” section on
page 36-13.

Shaped or Shared Mode

SRR services each queue-set in shared or shaped mode. You map a port to a queue-set by using the
queue-set qset-id interface configuration command. You assign shared or shaped weights to the port by
using the srr-queue bandwidth share weight1 weight2 weight3 weight4 or the srr-queue bandwidth
shape weight1 weight2 weight3 weight4 interface configuration command. For an explanation of the
differences between shaping and sharing, s e e t he “ SRR Shaping and Sha ring” sec tion on page 3 6-14.
The buffer allocation together with the SRR weight ratios control how much data can be buffered and
sent before packets are dropped. The weight ratio is the ratio of the fre qu en cy in whi ch the SRR
scheduler sends packets from each queue.
All four queues participate in the SRR unless the expedite queue is enabled, in which case the first
bandwidth weight is ignored and is not used in the ratio calculation. The expedite queue is a priority
queue, and it is serviced until empty before the other queues are serviced. You enable the expedite queue
by using the priority-queue out interface configuration command.
You can combine the commands described in this section to prioritize traffic by placing packets with
particular DSCPs or CoSs into certain queues, by allocating a large queue size or by servicing the queue
more frequently, and by adjusting queue thresholds so that packets with lower priorities are dropped. For
configuration information, see the “Configuring Egress Queue Characteristics” section on page 36-70.
Note The egress queue default settings are suitable for most situations. You should change them only when
you have a thorough understanding of the egress queues and if these settings do not meet your QoS
solution.
Packet Modification
A packet is classified, policed, and queued to provide QoS. Packet modifications can occur during this
process:
•For IP and non-IP packets, classification involves assigning a QoS label to a packet based on the
DSCP or CoS of the received packet. However, the packet is not modified at this stage; only an
indication of the assigned DSCP or CoS value is carried along. The reason for this is that QoS
classification and forwarding lookups occur in parallel, and it is possible that the p acket is forwarded
with its original DSCP to the CPU where it is again processed through software.