Chapter 30 Configuring Qo; Understanding Qos - Cisco 2950 - Catalyst Switch Configuration Manual

Understanding QoS

You can also use these wizards to configure QoS only if your switch is running the EI:
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This chapter consists of these sections:
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Understanding QoS
This section describes how QoS is implemented on the switch. If you have the SI installed on your
switch, some concepts and features in this section might not apply. For a list of available features, see
Table 30-1 on page
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 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 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 6 bits from the deprecated IP
type-of-service (ToS) field to carry the classification (class) information.
Classification can also be carried in the Layer 2 frame. These special bits in the Layer 2 frame or
a Layer 3 packet are described here and shown in
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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
30-2
Priority data wizard—Lets you assign priority levels to data applications based on their TCP or UDP
ports. It has a standard list of applications, and you select the ones that you want to prioritize, the
priority levels, and the interfaces where the prioritization occurs. Refer to the priority data wizard
online help for procedures about using this wizard.
Video wizard—Gives traffic that originates from specified video servers a higher priority than the
priority of data traffic. The wizard assumes that the video servers are connected to a single device
in the cluster. Refer to the video wizard online help for procedures about using this wizard.
Understanding QoS, page 30-2
Configuring Auto-QoS, page 30-9
Displaying Auto-QoS Information, page 30-15
Auto-QoS Configuration Example, page 30-16
Configuring Standard QoS, page 30-18
Displaying Standard QoS Information, page 30-39
Standard QoS Configuration Examples, page 30-39
30-1.
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.
Figure 30-1:
Chapter 30 Configuring QoS
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