Content

| 7.5 |  | Voice Quality Issues |
| 7.5.4 |  | Trust boundaries |

Trust Boundaries
The concept of trust is an important and integral one to implementing QoS. Once the end devices have a set class of service (CoS) or type of service (ToS), the switch has the option of trusting them or not. If the switch trusts the settings, it does not need to do any reclassification; if it does not trust the settings, then it must perform reclassification for appropriate QoS.

The notion of trusting or not trusting forms the basis for the trust boundary. Ideally, classification should be done as close to the source as possible. If the end device is capable of performing this function, then the trust boundary for the network is at the access layer in the wiring closet. If the device is not capable of performing this function, or the wiring closet switch does not trust the classification done by the end device, the trust boundary may shift. How this shift happens, depends on the capabilities of the switch in the wiring closet. If the switch can reclassify the packets, then the trust boundary remains in the wiring closet. If the switch cannot perform this function, then the task falls to other devices in the network going toward the backbone. In this case, the rule of thumb is to perform reclassification at the distribution layer. This means that the trust boundary has shifted to the distribution layer. It is more than likely that there is a high-end switch in the distribution layer with features to support this function. If possible, try to avoid performing this function in the core of the network.