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Catalyst 2960 and 2960-S Switches Software Configuration Guide, Release 15.0(1)SE
OL-26520-01
Chapter 33 Configuring QoS
Understanding QoS
For IP traffic, you have these classification options as shown in Figure 33-3:
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. You can also classify IP traffic based on IPv6 DSCP.
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 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. You can also
classify IP traffic based on IPv6 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. You can do this for both IPv4 and IPv6 traffic.
For information on the maps described in this section, see the “Mapping Tables” section on page 33-11.
For configuration information on port trust states, see the “Configuring Classification Using Port Trust
States” section on page 33-42.
After classification, the packet is sent to the policing, marking, and the ingress queueing and scheduling
stages.
Note Catalyst 2960-S switches do not support ingress queueing.
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