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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
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Chapter 29 Configuring QoS
Understanding QoS
You can also use these wizards to configure QoS only if your switch is running the EI:
• 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. See 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. See the video wizard online help for procedures about using this wizard.
This chapter consists of these sections:
• Understanding QoS, page 29-2
• Configuring Auto-QoS, page 29-9
• Displaying Auto-QoS Information, page 29-14
• Auto-QoS Configuration Example, page 29-14
• Configuring Standard QoS, page 29-16
• Displaying Standard QoS Information, page 29-36
• Standard QoS Configuration Examples, page 29-37
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 29-1 on page 29-1.
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 Figure 29-1:
• Prioritization values in Layer 2 frames
Layer 2 IEEE 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 IEEE 802.1Q trunks, all traffic is in IEEE 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.
• Prioritization bits in Layer 3 packets