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Cisco Catalyst Blade Switch 3130 and 3032 for Dell Software Configuration Guide
OL-12247-04
Chapter 36 Configuring QoS
Understanding QoS
You assign each packet that flows through the switch to a queue and to a threshold. Specifically, you map
DSCP or CoS values to an ingress queue and map DSCP or CoS values to a threshold ID. You use the
mls qos srr-queue input dscp-map queue
queue-id
{
dscp1...dscp8
|
threshold
threshold-id
dscp1...dscp8
} or
the
mls qos srr-queue input cos-map queue
queue-id
{
cos1...cos8
|
threshold
threshold-id cos1...cos8
} global configuration command. You can display the DSCP input queue
threshold map and the CoS input queue threshold map by using the
show mls qos maps
privileged EXEC
command.
WTD Thresholds
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 explicit WTD threshold percentages for
threshold ID 1 and ID 2 to the ingress queues by using the
mls qos srr-queue input threshold
queue-id
threshold-percentage1 threshold-percentage2
global configuration command. Each threshold value is a
percentage of the total number of allocated buffers for the queue. The drop threshold for threshold ID 3
is preset to the queue-full state, and you cannot modify it. For more information about how WTD works,
see the
“Weighted Tail Drop” section on page 36-14
.
Buffer and Bandwidth Allocation
You define the ratio (allocate the amount of space) with which to divide the ingress buffers between the
two queues by using the
mls qos srr-queue input buffers
percentage1 percentage2
global configuration
command. The buffer allocation together with the bandwidth allocation control how much data can be
buffered and sent before packets are dropped. You allocate bandwidth as a percentage by using the
mls
qos srr-queue input bandwidth
weight1 weight2
global configuration command. The ratio of the
weights is the ratio of the frequency in which the SRR scheduler sends packets from each queue.
Priority Queueing
You can configure one ingress queue as the priority queue by using the
mls qos srr-queue input
priority-queue
queue-id
bandwidth
weight
global configuration command. The priority queue should
be used for traffic (such as voice) that requires guaranteed delivery because this queue is guaranteed part
of the bandwidth regardless of the load on the stack or internal ring.
SRR services the priority queue for its configured weight as specified by the
bandwidth
keyword in the
mls qos srr-queue input priority-queue
queue-id
bandwidth
weight
global configuration command.
Then, SRR shares the remaining bandwidth with both ingress queues and services them as specified by
the weights configured with the
mls qos srr-queue input bandwidth
weight1 weight2
global
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 Ingress Queue Characteristics” section on page 36-73
.