Cisco Systems 3560 Manual De Usuario

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. 

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. 

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 

. Then the receiving port accepts the DSCP-trusted value and avoids the classification 

Enter global configuration mode.

Enable QoS globally.

Enable DSCP transparency. The switch is configured to not modify the 
DSCP field of the IP packet.

Return to privileged EXEC mode.

show mls qos interface [interface-id] Verify your entries.

(Optional) Save your entries in the configuration file.