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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
78-11380-12
Chapter 29 Configuring QoS
Configuring Standard QoS
Table 29-6 lists the port configuration when an IP phone is present or absent.
Enabling Pass-Through Mode
In software releases earlier than Cisco IOS Release 12.1(11)EA1, the switch is in pass-through mode. It
uses the CoS value of incoming packets without modifying the DSCP value and sends the packets from
one of the four egress queues. You cannot enable or disable pass-through mode if your switch is running
a software release earlier than Cisco IOS Release 12.1(11)EA1.
In Cisco IOS Release 12.1(11)EA1 or later, the switch assigns a CoS value of 0 to all incoming packets
without modifying the packets. The switch offers best-effort service to each packet regardless of the
packet contents or size and sends it from a single egress queue.
Beginning in privileged EXEC mode, follow these steps to enable pass-through mode:
To disable pass-through mode, use the no mls qos trust pass-through dscp interface configuration
command.
If you enter the mls qos cos override and the mls qos trust [cos | dscp] interface commands when
pass-through mode is enabled, pass-through mode is disabled.
Table 29-6 Port Configurations When Trusted Boundary is Enabled
Port Configuration When a Cisco IP Phone is Present When a Cisco IP Phone is Absent
The port trusts the CoS value
of the incoming packet.
The packet CoS value is trusted. The packet CoS value is assigned
the default CoS value.
The port trusts the DSCP
value of the incoming packet.
The packet DSCP value is trusted. For tagged non-IP packets, the
packet CoS value is set to 0.
For untagged non-IP packets, the
packet CoS value is assigned the
default CoS value.
The port assigns the default
CoS value to incoming
packets.
The packet CoS value is assigned
the default CoS value.
The packet CoS value is assigned
the default CoS value.
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
interface interface-id Specify the interface on which pass-through mode is enabled, and enter
interface configuration mode.
Valid interfaces include physical interfaces.
Step 3
mls qos trust cos pass-through dscp Enable pass-through mode. The interface is configured to trust the CoS
value of the incoming packets and to send them without modifying the
DSCP value.
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.
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