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Catalyst 3750 Metro Switch Software Configuration Guide
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Chapter 26 Configuring QoS
Understanding QoS
This chapter consists of these sections:
• Understanding QoS, page 26-2
• Understanding Hierarchical QoS, page 26-19
• Configuring Auto-QoS, page 26-29
• Displaying Auto-QoS Information, page 26-37
• Configuring Standard QoS, page 26-37
• Displaying Standard QoS Information, page 26-75
• Configuring Hierarchical QoS, page 26-76
• Displaying Hierarchical QoS Information, page 26-101
Understanding QoS
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
provide 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 (Diff-Serv) 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 also can be carried in the Layer
2 frame. These special bits in the Layer 2 frame or in the Layer 3 packet are described here and shown
in Figure 26-1:
• Prioritization bits in Layer 2 frames:
Layer 2 Inter-Switch Link (ISL) frame headers have a 1-byte User field that carries an IEEE 802.1p
CoS value in the three least-significant bits. On ports configured as Layer 2 ISL trunks, all traffic is
in ISL frames.
Layer 2 802.1Q frame headers have a 2-byte Tag Control Information field that carries the CoS value
in the three most-significant bits, which are called the User Priority bits. On ports 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.
• Prioritization bits in Layer 3 packets:
Layer 3 IP packets can carry either an IP precedence value or a DSCP value. QoS supports the use
of either value because DSCP values are backward-compatible with IP precedence values.
IP precedence values range from 0 to 7.
DSCP values range from 0 to 63.
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