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39-5

Catalyst 3750-X and 3560-X Switch Software Configuration Guide

OL-21521-01

Chapter 39 Configuring QoS

Understanding QoS

Figure 39-2 Basic QoS Model

Classification

Classification is the process of distinguishing one kind of traffic from another by examining the fields

in the packet. Classification is enabled only if QoS is globally enabled on the switch. By default, QoS is

globally disabled, so no classification occurs.

During classification, the switch performs a lookup and assigns a QoS label to the packet. The QoS label

dentifies all QoS actions to be performed on the packet and from which queue the packet is sent.

The QoS label is based on the DSCP or the CoS value in the pa

cket and decides the queueing and

scheduling actions to perform on the packet. The label is mapped according to the trust setting and the

packet type as shown in Figure 39-3 on page 39-7.

You specify which fields in the frame or p

acket that you want to use to classify incoming traffic. For

non-IP traffic, you have these classification options as shown in Figure 39-3:

• Trust the CoS value in the incoming frame (configure the port to trust CoS). Then use the

configurable CoS-to-DSCP map to generate a DSCP value for the packet. Layer 2 ISL frame headers

carry the CoS value in the 3 least-significant bits of the 1-byte User field. Layer 2 802.1Q frame

headers carry the CoS value in the 3 most-significant bits of the Tag Control Information field. CoS

values range from 0 for low priority to 7 for high priority.

• Trust the DSCP or trust IP precedence value in the incoming frame. These configurations are

meaningless for non-IP traffic. If you configure a port with either of these options and non-IP traffic

is received, the switch assigns a CoS value and generates an internal DSCP value from the

CoS-to-DSCP map. The switch uses the internal DSCP value to generate a CoS value representing

the priority of the traffic.

• Perform the classification based on a configured Layer 2 MAC access control list (ACL), which can

examine the MAC source address, the MAC destination address, and other fields. If no ACL is

configured, the packet is assigned 0 as the DSCP and CoS values, which means best-effort traffic.

Otherwise, the policy-map action specifies a DSCP or CoS value to assign to the incoming frame.

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