Cisco 4500M Software Manual page 382

Overview of QoS
For non-IP traffic, you have the following classification options:
Use the port default. If the packet is a non-IP packet, assign the default port DSCP value to the
•
incoming packet.
• Trust the CoS value in the incoming frame (configure the port to trust CoS). Then use the
configurable CoS-to-DSCP map to generate the internal DSCP value. 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. If the frame does not contain a CoS
value, assign the default port CoS to the incoming frame.
The trust DSCP configuration is meaningless for non-IP traffic. If you configure a port with trust
DSCP and non-IP traffic is received, the switch assigns the default port DSCP.
For IP traffic, you have the following classification options:
Trust the IP DSCP in the incoming packet (configure the port to trust DSCP), and assign the same
•
DSCP to the packet for internal use. The IETF defines the six most-significant bits of the 1-byte
Type of Service (ToS) field as the DSCP. The priority represented by a particular DSCP value is
configurable. DSCP values range from 0 to 63.
• Trust the CoS value (if present) in the incoming packet, and generate the DSCP by using the
CoS-to-DSCP map.
• Perform the classification based on a configured IP standard or extended ACL, which examines
various fields in the IP header. If no ACL is configured, the packet is assigned the default DSCP
based on the trust state of the ingress port; otherwise, the policy map specifies the DSCP 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
section on page
Software Configuration Guide—Release 12.2(25)EW
29-6
29-40.
Chapter 29
"Mapping Tables" section on page
"Configuring the Trust State of Interfaces"
Configuring QoS
29-13.
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