A diagnostic tool for identifying priority errors within a data network is realized using a priority trace data packet. The priority trace data packet is originated at an originating node of interest and terminated at a terminating node of interest. At various intermediate network nodes along the data path between the originating and terminating nodes, the priority trace packet is inspected to determine the current priority value of the priority trace packet. The current priority value is stored in a priority trace field that can be analyzed by a network management node to identify one or more sources of priority errors.