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Catalyst 3750 Metro Switch Software Configuration Guide
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Chapter 26 Configuring QoS
Understanding QoS
Ingress Classification
Ingress classification is the process of distinguishing one kind of traffic from another by examining the
fields in the packet upon receipt. Classification is enabled only if QoS is globally enabled on the switch.
By default, QoS is globally disabled, so no classification occurs.
During ingress 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 26-3 on page 26-7.
You specify which fields in the frame or packet that you want to use to classify inbound traffic. For
non-IP traffic, you have these ingress classification options as shown in Figure 26-3:
• Trust the CoS value in the inbound 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 three least-significant bits of the 1-byte User field. 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.
• Trust the DSCP or trust IP precedence value in the inbound 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 a DSCP value from the CoS-to-DSCP map.
• Perform the classification based on a configured Layer 2 MAC 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 a CoS value to assign to the inbound frame.
For IP traffic, you have these ingress classification options as shown in Figure 26-3:
• Trust the DSCP value in the inbound packet (configure the port to trust DSCP), and assign the same
DSCP value to the packet. The IETF defines the six most-significant bits of the 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 domains, you can modify the DSCP
to another value through the configurable DSCP-to-DSCP-mutation map.
• Trust the IP precedence value in the inbound packet (configure the port to trust IP precedence), and
generate a DSCP value for the packet through the configurable IP-precedence-to-DSCP map. The
IP version 4 specification defines the three 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 inbound packet, and generate a DSCP value for the packet through
the CoS-to-DSCP map. If the CoS value is not present, use the default port CoS value.
• Perform the classification based on a configured IP standard or an extended ACL, which examines
various fields in the IP header. 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
a CoS value to assign to the inbound frame.
For information on the maps described in this section, see the “Mapping Tables” section on page 26-11.
For configuration information on port trust states, see the “Configuring Ingress Classification by Using
Port Trust States” section on page 26-42.
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