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

Configuring the CoS Value for an Interface

QoS assigns the CoS v al ue specif ied with the mls qos cos interfac e conf igu ration c ommand to unta gged
frames re ceived on trusted and untrust ed port s.
Beginning in privileged EXEC mode, follow these steps to define the default CoS value of a port or to
assign the defa ult CoS to all incoming packets on the port:
To return to the default setting, use the no mls qos cos {default-cos | override} in ter face co nfigurati on
command.

Configuring Trusted Boundary

In a t ypic al network , you c onn ect a C i sco IP ph one to a sw it ch po rt as shown in Figu re 26-3 on
page 26-11. Traffic sent from the telephone to the switch is typically marked with a tag that uses the
802.1 Q header. The heade r contai ns the VL AN info rmation a nd the CoS 3-bi t field, which de termi nes
the priority of the packet. For most Cisco IP phone configurations, the traff ic sent from the telephone to
the switch is trusted to ensure that voice traffic is properly prioritized over other types of traffic in the
network. By using the mls qos trust cos inter face co nf igurat ion comman d, you can co nf igure the s witch
port to which the telephon e is connected to trust the CoS labels of all traffic received on that port.
In some situations, you also might connect a PC or workstation to the IP phone. In these cases, you can
use the switchport priority extend cos interface c on figurati on co mm an d to c onfigure the te lep hone
through the switch CLI to override the priority of the traffic received from the PC. With this command,
you can prevent a PC from takin g advantage of a high -priori ty data que ue.
Command Purpose
Step 1 configure terminal Enter globa l configurati on mode.
Step 2 interface in terface-id Enter inter face configuration mode, and specify the interface to be trusted.
Valid i nte rfac es i ncl ude physi cal inter faces.
Step 3 mls qos cos {default-cos | override} Configure the default CoS value for the port.
•For default-cos, specify a default CoS value to be assigned to a port. If
the port is CoS trusted and packets are untagged, the default CoS value
becomes the CoS value for the packet. Th e CoS range is 0 to 7. The
default is 0.
•Use th e override keyword to override the previously configured trust
state of the incoming packets and to apply the default port CoS v alue to
all inc omi ng packe ts. B y d efaul t, C oS override is disa bled .
Use th e override keyword when all incoming packets on certain ports
deserve higher priority th an packets entering from other ports. Even if
a port was prev iously set to trust DSCP, this command overrides the
previously configured trust state, and all the incoming CoS values are
assigned the defa ult CoS value configu red with this command. If an
incoming packet is tagged, the CoS value of the packet is modified with
the de fault C oS o f th e port at the egress port .
Step 4 end Return to p rivileged EXEC mode.
Step 5 show mls qos interface Verify your e ntrie s.
Step 6 copy running-config startup-config (Optiona l) Save your entries in the co nfigurati on file.