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| 8.3 |  | Classification at the Access Layer |
| 8.3.4 |  | Assigning CoS on a per-port basis |

For most network devices, such as PCs and printers, switch access ports can be assigned a default port CoS value of zero as they have no special QoS requirements. Devices requiring a specific QoS but are unable to set the CoS value, an appropriate CoS value should be applied to the switch port. Once a switch port is configured with a default CoS, traffic that passes through the port will be automatically tagged with the required CoS value.

The CoS value assigned by the switch can be changed on a port-by-port basis using the interface configuration command:

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

After applying this command and assuming the switch has been configured to trust the existing CoS value of frames, the switches behavior will be as follows:

- If the incoming frame has no CoS, value of zero, apply the default CoS.
- If the incoming frame has a CoS, maintain the same CoS.