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Chapter 32
Configuring QoS
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 as shown in
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 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 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 a DSCP value from the CoS-to-DSCP map.
•
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
•
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 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 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 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 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.
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
CoS value to assign to the incoming frame.
For information on the maps described in this section, see the
For configuration information on port trust states, see the
States" section on page
After classification, the packet is sent to the policing, marking, and the ingress queueing and scheduling
stages.
78-16180-02
32-32.
Catalyst 3750 Switch Software Configuration Guide
Figure
32-3:
Figure
32-3:
"Mapping Tables" section on page
"Configuring Classification Using Port Trust
Understanding QoS
32-10.
32-5
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