Cisco 2950 Software Configuration Guide - Page 565

Chapter 30 Configuring QoS Understanding QoS Classification Note This feature is available only if your switch is running the EI. Classification is the process of distinguishing one kind of traffic from another by examining the fields in the packet. Classification occurs only on a physical interface basis. No support exists for classifying packets at the VLAN level. You specify which fields in the frame or packet that you want to use to classify incoming traffic. For non-IP traffic, you have these classification options: • Use the port default. If the frame does not contain a CoS value, the switch assigns the default port CoS value to the incoming frame. • Trust the CoS value in the incoming frame (configure the port to trust CoS). Layer 2 802.1Q frame headers carry the CoS value in the three most-significant bits of the Tag Control Information field. CoS values range from 0 for low priority to 7 for high priority. The trust DSCP configuration is meaningless for non-IP traffic. If you configure a port with this option and non-IP traffic is received, the switch assigns the default port CoS value and classifies traffic based on the CoS value. For IP traffic, you have these classification options: • Trust the IP DSCP in the incoming packet (configure the port to trust DSCP). The switch assigns the same DSCP to the packet for internal use. The IETF defines the 6 most-significant bits of the 1-byte ToS field as the DSCP. The priority represented by a particular DSCP value is configurable. The supported DSCP values are 0, 8, 10, 16, 18, 24, 26, 32, 34, 40, 46, 48, and 56. • Trust the CoS value (if present) in the incoming packet. The switch generates the DSCP by using the CoS-to-DSCP map. Note An interface can be configured to trust either CoS or DSCP, but not both at the same time. Classification Based on QoS ACLs You can use IP standard, IP extended, and Layer 2 MAC access control lists (ACLs) to define a group of packets with the same characteristics (class). In the QoS context, the permit and deny actions in the access control entries (ACEs) have different meanings than with security ACLs: • If a match with a permit action is encountered (first-match principle), the specified QoS-related action is taken. • If no match with a permit action is encountered and all the ACEs have been examined, no QoS processing occurs on the packet. • If multiple ACLs are configured on an interface, the packet matches the first ACL with a permit action, and QoS processing begins. 78-11380-10 Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide 30-5