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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 20 Configuring QoS
Understanding QoS
For IP traffic, you have these classification options as shown in
Figure 20-3
:
•
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 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.
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.
For information on the maps described in this section, see the
“Mapping Tables” section on page 20-11
.
For configuration information on port trust states, see the
“Configuring Classification Using Port Trust
States” section on page 20-21
.
