On the Catalyst fixed configuration switches, you can configure QoS by issuing automatic QoS commands or by issuing standard QoS commands. Catalyst 2940 is the only exception since it does not support automatic QoS commands.
With QoS, you can give preferential treatment to certain types of traffic at the expense of others. Without QoS, the Catalyst switches offer 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.
When you configure the QoS feature, you can select specific network traffic, prioritize it according to its relative importance and 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 Differentiated Services (DiffServ) architecture, an emerging standard from the Internet Engineering Task Force (IETF). This architecture specifies that each packet is classified upon entry into the network. The classification is carried in the IP packet header, using six bits from the deprecated IP Type of Service (ToS) field to carry the classification (class) information.
Implementing QoS in your network can be a simple or complex task and depends on the QoS features offered by your internetworking devices, the traffic types and patterns in your network, and the granularity of control that you need over incoming and outgoing traffic.
To use many QoS features, you must have the Enhanced Image (EI) installed on your switch. If you have the Standard Image (SI) installed on your switch, you cannot configure some of the features.
For more information on how to configure the available QoS features, refer to these documents: