178 Chapter 6: QoS Features Available on the Catalyst 2950 and 3550 Family of Switches
Use the following command to display the CoS-to-transmit queue mapping:
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The Catalyst 2950 Family of switches allows for custom configuration of the CoS-to-
transmit queue mapping on a global basis. The Catalyst 3550 Family of switches supports
CoS-to-transmit queue mapping on a per-interface basis. Use the following global and
interface configuration command to configure the CoS-to-transmit queue mapping:
wrr-queue cos-map
qid cos1..cosn
q_id represents one of the four transmit queues. cos1..cosn represents configuration for up
to eight CoS values to map to a transmit queue.
Example 6-26 illustrates a user custom configuring the CoS-to-transmit queue mapping
table and verifying the DSCP-TxQueue mapping table configuration.
Strict-Priority Queuing
By default, the Catalyst 2950 Family of switches schedules packets from transmit queues
using the strict-priority algorithm. Using this algorithm, the switch transmits all packets out
of the higher-priority queues before servicing lower-priority queues.
The Catalyst 3550 Family of switches utilizes the strict-priority queue configuration with
WRR for scheduling all the packets out of transmit queue four before servicing any other
queue. The switch still utilizes WRR for scheduling packets out of the remaining queues.
The behavior of this configuration is slightly different from the behavior on the Catalyst
2950 Family of switches. Priority Queuing on the Catalyst 2950 Family of switches
services higher-priority queues before lower-priority queues in order and does not support
the concept of a single expedite queue.
To configure the Catalyst 3550 Family of switches for the strict-priority scheduling of
transmit queue four, use the following interface command:
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As a result of the strict-priority scheduling algorithm on both the Catalyst 2950 and 3550
Family of switches, the switches may never transmit traffic out of lower-priority queues
when line-rate traffic exists in higher-priority queues. Although this behavior is warranted
for high-priority traffic such as VoIP, many applications of strict-priority queuing may
starve lower-priority traffic. To remedy this situation, consider configuring the switch for
WRR bandwidth and WRED as discussed in the next section.
Example 6-26 User Configuring and Verifying the DSCP-to-Transmit Queue Mapping Table
Switch#configure terminal
Switch(config)#wrr-queue cos-map 1 0 1 2
Switch(config)#wrr-queue cos-map 2 3 4
Switch(config)#wrr-queue cos-map 3 5
Switch(config)#end
Switch#show wrr-queue cos-map
CoS Value : 0 1 2 3 4 5 6 7
Priority Queue : 1 1 1 2 2 3 4 4