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Understanding QoS
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 6 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.
Beginning with Cisco IOS Release 12.2(52)SE, there is an option to classify IP traffic based on IPv6
DSCP.
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.
Beginning with Cisco IOS Release 12.2(52)SE, there is an option to classify IP traffic based on IPv6
IP precedence.
•
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.
Override the configured CoS of incoming packets, and apply the default port CoS value to them. For
•
IPv6 packets, the DSCP value is rewritten by using the CoS-to-DSCP map and by using the default
CoS of the port. In Cisco IOS Release 12.2(52)SE and later, you can do this for both IPv4 and IPv6
traffic.
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.
Cisco Catalyst Blade Switch 3130 and 3032 for Dell Software Configuration Guide
36-6
36-37.
Chapter 36
Figure
36-3:
"Mapping Tables" section on page
"Configuring Classification Using Port Trust
Configuring QoS
36-13.
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