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Configuring QoS
This chapter describes how to configure quality of service (QoS) by using automatic QoS (auto-QoS) commands or by using standard QoS commands. With QoS, you can give preferential treatment to certain traffic at the expense of others. Without QoS, the Catalyst 3550 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.


For complete syntax and usage information for the commands used in this chapter, refer to the command reference for this release. This chapter consists of these sections:
• • • • • • •

Understanding QoS, page 28-2 Configuring Auto-QoS, page 28-17 Displaying Auto-QoS Information, page 28-22Auto-QoS Configuration Example, page 28-23 Configuring Standard QoS, page 28-24 Displaying Standard QoS Information, page 28-68 Standard QoS Configuration Examples, page 28-68


When you are configuring QoS parameters for the switch, in order to allocate system resources to maximize the number of possible QoS access control entries (ACEs) allowed, you can use the sdm prefer access globalconfiguration command to set the Switch Database Management feature to the access template. For more information on the SDM templates, see the “Optimizing System Resources for User-Selected Features” section on page 7-27.

Catalyst 3550 Multilayer Switch Software Configuration Guide 78-11194-06


Chapter 28 Understanding QoS

Configuring QoS

Understanding QoS
Typically, networks operateon 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 QoS, you can select specific network traffic, prioritize it according to its relative importance, and use congestion-management and congestion-avoidance techniques togive 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 inthe 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 in the Layer 3 packet are described here and shown in Figure 28-1:

Prioritization bits in Layer 2 frames: Layer 2 Inter-Switch Link (ISL) frame headers have a1-byte User field that carries an IEEE 802.1P class of service (CoS) value in the three least-significant bits. On interfaces 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 interfaces configured asLayer 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 Differentiated Services Code Point (DSCP) value. QoS supports the use of eithervalue 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.

Catalyst 3550 Multilayer Switch Software Configuration Guide



Chapter 28

Configuring QoS Understanding QoS

Figure 28-1 QoS Classification Bits in Frames and Packets

Encapsulated Packet Layer 2 header IP...
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