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| 8.3 |  | Classification at the Access Layer |
| 8.3.3 |  | Configuring CoS trust using the IOS |

Depending on the switch model, it may be necessary to first activate QoS using the following command:

> switch(config)#mls qos

This command is required on both the Catalyst 3550 and the Catalyst 6500. The Catalyst 2950 has QoS enabled by default.

The trust is configured on the switch port using the command:

> switch(config-if)#mls qos trust cos

Any ISL or 802.1Q/p frames that enter the switch port will now have its CoS passed, untouched, through the switch. If an untagged frame arrives at the switch port, the switch will assign a default CoS to the frame before forwarding it. By default untagged frames are assigned a CoS of zero. This can be changed using the interface configuration command:

> switch(config-if)#mls qos cos default-cos

Where default-cos is a number between 0 and 7.