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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 26 Configuring QoS Configuring QoS

Enabling Pass-Through Mode

In software rel eases earl ier than Re lease 12.1(11) EA1, the swit ch is i n pass-thr ough mode . It uses the
CoS value of incoming pa ckets witho ut mod ifying the DSCP value and sen ds the packets fr om one of
the four egre ss queues. You canno t enable or disabl e pass-throu gh mode if your switch is runni ng a
software releas e earlie r than Rel ease 12.1(1 1)EA 1.
In 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
conte nts or size and sends i t from a single egress queu e.
Beginning i n privileged E X EC mo de , fol low these s teps to e nabl e p ass-t hro ugh m ode :
To d isab le pa ss-thr oug h mode , use the no mls qos trust pass-through dscp interface configuration
comm an d.
If you ente r the mls qos cos override and th e mls qos trust [cos | dscp] interface commands when
pass-throug h mode is ena bled, pass-t hrough mod e is disabled.
If you ente r the mls qos trust c os pass- through dscp in terfa ce configurat ion comm an d when the mls
qos cos override and the mls qos trust [cos | dscp] interface commands are already configured,
pass-throug h mode is di sabled.
Table 26-2 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 pac ket DSC P value is tr uste d. For tagged no n-I P p ac kets, the
packet CoS value is set to 0.
For untagged no n-IP packets, t he
packet CoS value is assigned t he
default CoS va lue.
The port assigns the default
CoS value to incoming
packets.
The packet CoS value is assigne d
the default CoS value. The packet CoS value is assigned
the default CoS value.
Command Purpose
Step 1 configure terminal Enter globa l configurati on mode.
Step 2 interface interface-id Enter interface configuration mode, and specify the interface on which
pass-t hrough m ode is e nab led .
Valid inte rface s i ncl ude physi cal in ter faces.
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
DSC P valu e.
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 (Optiona l) Save your entries in the co nfigurati on file.