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Cisco IE 2000 Switch Software Configuration Guide
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Chapter 38 Configuring Standard QoS
Information About Standard QoS
The default DSCP-to-DSCP-mutation map is a null map, which maps an incoming DSCP value to the
same DSCP value.
The default policed-DSCP map is a null map, which maps an incoming DSCP value to the same DSCP
value (no markdown).
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
identifies 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 packet 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 38-3 on page 38-12.
You specify which fields in the frame or packet that you want to use to classify inco ming traffic. For
non-IP traffic, you have these classification options as shown in Figure 38-3:
• 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 IEEE 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 option s 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.
For IP traffic, you have these classification options as shown in Figure 38-3:
• Trust the DSCP value in the incoming packet (configure the port to trust DSCP), and assign the same
DSCP value to the packet. The IETF defines the 6 most-significant bits of th e 1-byte ToS field as
the DSCP. The priority represented by a particular DSCP value is configurable. DSCP values range
from 0 to 63.
For ports that are on the boundary between two QoS administrative domain s, you can modify the
DSCP to another value by using the configurable DSCP-to-DSCP-mutation map.
• Trust the IP precedence value in the incoming packet (configure the port to trust IP precedence), and
generate a DSCP value for the packet by using the configurable IP-precedence-to-DSCP map. The
IP Version 4 specification defines the 3 most-significant bits of the 1-byte ToS field as the IP
precedence. IP precedence values range from 0 for low priority to 7 for high priority.
• Trust the CoS value (if present) in the incoming packet, and generate a DSCP value for the packet by
using the CoS-to-DSCP map. If the CoS value is not present, use the default port CoS value.