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Catalyst 2928 Switch Software Configuration Guide
OL-23389-01
Chapter 31 Configuring QoS
Understanding QoS
The switch supports two configurable ingress queues, which are serviced by SRR only in shared mode.
describes the queues.
You assign each packet that flows through the switch to a queue and to a threshold. Specifically, you map
CoS values to an ingress queue and map CoS values to a threshold ID. You use 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 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 predefined default drop thresholds that are not changeable. For more information about how WTD
works, see the
“Weighted Tail Drop” section on page 31-4
Priority Queueing
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 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.
You can combine the commands described in this section to prioritize traffic by placing packets with
particular CoSs into certain queues. For configuration information, see the
Characteristics” section on page 31-12
Queueing on Egress Queues
shows the queueing and scheduling flowchart for egress ports.
Note
If the expedite queue is enabled, SRR services it until it is empty before servicing the other three queues.
Figure 31-5
Queueing Flowchart for Egress Ports
Each port supports four egress queues, one of which (queue 1) can be the egress expedite queue. These
queues are assigned to a queue set. All traffic exiting the switch flows through one of these four queues
and is subjected to a threshold based on the QoS label assigned to the packet.
Table 31-1
Ingress Queue Types
Queue Type
1
1.
The switch uses two nonconfigurable queues for traffic that is essential for proper network operation.
Function
Normal
User traffic that is considered to be normal priority. You can configure three different
thresholds to differentiate among the flows. You can use the
mls qos srr-queue input
cos-map
global configuration command.
Expedite
High-priority user traffic such as differentiated services expedited forwarding or voice
traffic. You can configure the bandwidth required for this traffic as a percentage of the
total traffic. You can configure the bandwidth required for this traffic as a percentage of
the total traffic by using the
mls qos srr-queue input priority-queue
global configuration
command. The expedite queue has guaranteed bandwidth.
