Cisco Systems 3750E manual No mls qos rewrite ip dscp, 36-43

Chapter 36 Configuring QoS

Configuring Standard QoS

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:

	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.

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 36-16. Then the receiving port accepts the DSCP-trusted value and avoids the classification stage of QoS. If the two domains use different DSCP values, you can configure the DSCP-to-DSCP-mutation map to translate a set of DSCP values to match the definition in the other domain.

	Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02		36-43