Enabling Pass-Through Mode - Cisco Catalyst 3550 series Software Configuration Manual

Chapter 29 Configuring QoS

Enabling Pass-Through Mode

You can use the pass-through mode to enable the CoS and DSCP setting to be independent for packets
that contain both values. Use the pass-through mode when you do not want the other value (CoS or
DSCP) to be modified when using the mls qos trust [cos | dscp] interface configuration command.
By default, in software releases earlier than Cisco IOS Release 12.1(11)EA1, if you configure the
interface to trust the DSCP, the switch does not modify the DSCP field of the IP packet. However, the
switch modifies the CoS value of the packet according to the DSCP-to-CoS map. If you configure the
interface to trust the CoS, the switch does not modify the CoS field of the packet. However, the switch
modifies the DSCP according to the CoS-to-DSCP map if the packet is an IP packet.
In Cisco IOS Release 12.1(11)EA1 or later, you configure the interface for pass-through mode. The
interface trusts the DSCP, and the switch sends the packet without modifying the CoS value (the
DSCP-to-CoS map is ignored). Otherwise, the interface trusts the CoS, and the switch sends the packet
without modifying the DSCP value. The CoS-to-DSCP map is ignored.
Beginning in privileged EXEC mode, follow these steps to enable pass-through mode on an interface:
Command
Step 1 configure terminal
Step 2 interface interface-id
Step 3
mls qos trust cos pass-through dscp
or
mls qos trust dscp pass-through cos
Step 4
end
Step 5 show mls qos interface [interface-id] Verify your entries.
Step 6 copy running-config startup-config
To disable pass-through mode, use the no mls qos trust cos pass-through dscp or the no mls qos trust
dscp pass-through cos interface configuration command.
If you configure the mls qos trust [cos pass-through dscp | dscp pass-through cos] interface
configuration command and then configure the mls qos trust [cos | dscp] interface configuration
command, pass-through mode is disabled.
Note If you configure an interface for DSCP pass-through mode by using the mls qos trust cos pass-through
dscp interface configuration command and apply the DSCP-to-DSCP mutation map to the same
interface, the DSCP value changes according to the mutation map.
78-11194-09
Purpose
Enter global configuration mode.
Enter interface configuration mode, and specify the interface on which
pass-through mode is enabled.
Valid interfaces include physical interfaces.
Enable pass-through mode. The interface is configured to trust the CoS
value of the incoming packets. The switch does not modify the DSCP value.
or
Enable pass-through mode. The interface is configured to trust the DSCP
value of the incoming packets. The switch does not modify the CoS value.
Return to privileged EXEC mode.
(Optional) Save your entries in the configuration file.
Catalyst 3550 Multilayer Switch Software Configuration Guide
Configuring Standard QoS
29-33