Classification - Cisco Catalyst 3550 series Software Configuration Manual

Chapter 29 Configuring QoS

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.
Note Classification occurs on a physical interface or on a per-port per-VLAN basis. No support exists for
classifying packets at the switch virtual interface level.
You specify which fields in the frame or packet that you want to use to classify incoming traffic.
For non-IP traffic, these are the classification options as shown in
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For IP traffic, these are the classification options as shown in
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For information on the maps described in this section, see the
For configuration information on port trust states, see the
Trust States" section on page
78-11194-09
Use the port default. If the frame does not contain a CoS value, the switch assigns the default port
CoS value to the incoming frame. Then, the switch uses the configurable CoS-to-DSCP map to
generate the internal DSCP value.
Trust the CoS value in the incoming frame (configure the port to trust CoS). Then, the switch uses
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.
The trust DSCP and trust IP precedence 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 the
default port CoS value and generates the internal DSCP from the CoS-to-DSCP map.
Perform the classification based on the configured Layer 2 MAC access control list (ACL), which
can examine the MAC source address, the MAC destination address, and the Ethertype field. If no
ACL is configured, the packet is assigned the default DSCP of 0, which means best-effort traffic;
otherwise, the policy map specifies the DSCP to assign to the incoming frame.
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 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.
Trust the IP precedence in the incoming packet (configure the port to trust IP precedence), and
generate a DSCP 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 the DSCP by using the
CoS-to-DSCP map.
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 the default DSCP
of 0, which means best-effort traffic; otherwise, the policy map specifies the DSCP to assign to the
incoming frame.
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Figure
Figure
"Mapping Tables" section on page
"Configuring Classification By Using Port
Catalyst 3550 Multilayer Switch Software Configuration Guide
Understanding QoS
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