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| 8.3 |  | Classification at the Access Layer |
| 8.3.2 |  | Trusting the CoS |

The three bit CoS field in an ISL or 802.1Q/P frame allows for a range of CoS values between 0 and 7, where 0 is the lowest priority and 7 is the highest. If the edge device, such as an IP phone or a PC application, is capable of setting the CoS bits then the network designer must decide whether to trust the device or not. The default action is for a switch with QoS features activated not to trust edge devices and any frames that enter the switch have their CoS re-written to the lowest priority of zero. If the edge device can be trusted, this default behaviour must be overridden and the access switch must be configured to switch the frame, leaving the CoS bits untouched.