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CHAPTER
24-1
Catalyst 2940 Switch Software Configuration Guide
78-15507-02
24
Configuring QoS
This chapter describes how to configure quality of service (QoS) by using standard QoS commands.
With QoS, you can give preferential treatment to certain types of traffic at the expense of others. Without
QoS, the Catalyst 2940 switch offers best-effort service to each packet, regardless of the packet contents
or size. It sends the packets without any assurance of reliability, delay bounds, or throughput.
Note For complete syntax and usage information for the commands used in this chapter, refer to the command
reference for this release.
QoS can be configured either by using the Cluster Management Suite (CMS) or through the
command-line interface (CLI). Refer to the CMS online help for configuration procedures through CMS.
For information about accessing and using CMS, see Chapter 3, “Getting Started with CMS.”
This chapter consists of these sections:
• Understanding QoS, page 24-1
• Configuring QoS, page 24-3
• Displaying QoS Information, page 24-10
Understanding QoS
This section describes how QoS is implemented on the switch. Typically, networks operate on a
best-effort delivery basis, which means that all traffic has equal priority and an equal chance of being
delivered in a timely manner. When congestion occurs, all traffic has an equal chance of being dropped.
When you configure the QoS feature, you can use congestion-management and congestion-avoidance
techniques to give preferential treatment. Implementing QoS in your network makes network
performance more predictable and bandwidth utilization more effective.
The QoS implementation is based on the prioritization values in Layer 2 frames
Layer 2 802.1Q frame headers have a 2-byte Tag Control Information field that carries the class of
service (CoS) value in the three most-significant bits, which are called the User Priority bits. On
interfaces configured as Layer 2 802.1Q trunks, all traffic is in 802.1Q frames except for traffic in the
native VLAN. Other frame types cannot carry Layer 2 CoS values. Layer 2 CoS values range from 0 for
low priority to 7 for high priority.