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Catalyst 3560 Switch Software Configuration Guide
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Chapter 34 Configuring QoS
Configuring Standard QoS
Enabling DSCP Transparency Mode
In software releases earlier than Cisco IOS Release 12.2(25)SE, if QoS is disabled, the DSCP value of
the incoming IP packet is not modified. If QoS is enabled and you configure the interface to trust DSCP,
the switch does not modify the DSCP value. If you configure the interface to trust CoS, the switch
modifies the DSCP value according to the CoS-to-DSCP map.
In Cisco IOS Release 12.2(25)SE or later, the switch supports the DSCP transparency feature. It affects
only the DSCP field of a packet at egress. By default, DSCP transparency is disabled. The switch
modifies the DSCP field in an incoming packet, and the DSCP field in the outgoing packet is based on
the quality of service (QoS) configuration, including the port trust setting, policing and marking, and the
DSCP-to-DSCP mutation map.
If DSCP transparency is enabled by using the no mls qos rewrite ip dscp command, the switch does not
modify the DSCP field in the incoming packet, and the DSCP field in the outgoing packet is the same
as that in the incoming packet.
Note Enabling DSCP transparency does not affect the port trust settings on IEEE 802.1Q tunneling ports.
Regardless of the DSCP transparency configuration, the switch modifies the internal DSCP value of the
packet, which the switch uses to generate a class of service (CoS) value that represents the priority of
the traffic. The switch also uses the internal DSCP value to select an egress queue and threshold.
Beginning in privileged EXEC mode, follow these steps to enable DSCP transparency on a switch:
To configure the switch to modify the DSCP value based on the trust setting or on an ACL by disabling
DSCP transparency, use the mls qos rewrite ip dscp global configuration command.
If you disable QoS by using the no mls qos global configuration command, the CoS and DSCP values
are not changed (the default QoS setting).
If you enter the no mls qos rewrite ip dscp global configuration command to enable DSCP transparency
and then enter the mls qos trust [cos | dscp] interface configuration command, DSCP transparency is
still enabled.
Configuring the DSCP Trust State on a Port Bordering Another QoS Domain
If you are administering two separate QoS domains between which you want to implement QoS features
for IP traffic, you can configure the switch ports bordering the domains to a DSCP-trusted state as shown
in Figure 34-13. Then the receiving port accepts the DSCP-trusted value and avoids the classification
Command Purpose
Step 1 configure terminal Enter global configuration mode.
Step 2 mls qos Enable QoS globally.
Step 3 no mls qos rewrite ip dscp Enable DSCP transparency. The switch is configured to not modify the
DSCP field of the IP packet.
Step 4 end Return to privileged EXEC mode.
Step 5 show mls qos interface [interface-id]Verify your entries.
Step 6 copy running-config startup-config (Optional) Save your entries in the configuration file.