Syntax Description

Parameter

Description

values

Eight DSCP values, separated by spaces, corresponding to the CoS values; valid values are from 0 to 63.

DSCP-to-CoS Default Mapping

Marker

Value

DSCP

0

8, 10

16, 18

24, 26

32, 34

40, 42

48

56

Values

CoS Values

0

1

2

3

4

5

6

7

To define an egress DSCP-to-CoS mapping, use the mls qos map dscp-cos command. The DSCP-to-CoS map is used to map DSCP values in incoming packets to a CoS value, which is used to select one of the four egress queues. The CoS mapped value is written into the ISL header or 802.1Q tag of the transmitted frame on trunk interfaces. You can enter up to eight DSCP values separated by a space, and up to eight CoS values separated by a space. Use the no form of this command to remove a prior entry.

mls qos map dscp-cos dscp-values to cos-values

Syntax Description

Parameter

Descri ption

dscp-values

DSCP values; valid values are from 0 to 63.

to

Defines mapping.

cos-values

CoS values; valid values are from 0 to 63.

The figure shows a configuration example on a Catalyst 2950 switch where the CoS-to-DSCP map has been changed from the default.

The default map is:

Marker

Value

CoS Values

0

1

2

3

4

5

6

7

DSCP Values

0

8

16

24

32

40

48

56

And the map after configuration is:

Marker

Value

CoS Values

0

1

2

3

4

5

6

7

DSCP Values

0

10

18

26

34

46

48

56

The interface has been set to trust the CoS value using the mls qos trust command, using both the cos and cisco-phone options. The result of the configuration is that the switch interface trusts CoS only when a Cisco IP Phone is attached. The switch uses Cisco Discovery Protocol (CDP) to detect if a Cisco IP Phone is attached, and also passes the voice VLAN ID information to the Cisco IP Phone using CDP.

Use the switchport priority extend cos 0 interface configuration command to enable the IP Phone to override the CoS marking from the PC attached to the IP Phone with a CoS value of 0.

Use the switchport priority extend interface configuration command to set a port priority for the incoming untagged frames or the priority of frames received by the IP Phone connected to the specified port. Use the no form of this command to return to the default setting.

switchport priority extend {cos value | trust}

Syntax Description

Parameter

Descri ption

cos value

Sets the IP Phone port to override the priority received from PC or the attached device.

The CoS value is a number from 0 to 7. 7 is the highest priority. The default is 0.

trust

Sets the IP Phone port to trust the priority received from PC or the attached device.

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