Merge pull request #51747 from neerajks77/patch-4

Possibility of device to cloud communication failure even when the device shows as running

Author: ktoliver

articles/iot-hub/iot-hub-arduino-iot-devkit-az3166-get-started.md

@@ -323,6 +323,17 @@ The sample application is running successfully when you see the following result
 
 ![Serial monitor output](media/iot-hub-arduino-devkit-az3166-get-started/getting-started/result-serial-output.png)
 
+> [!NOTE]
+> You might encounter an error during testing in which the LED isn't blinking, the Azure portal doesn't show incoming data from the device, but the device OLED screen shows as **Running...**. To resolve the issue, in the Azure portal, go to the device in the IoT hub and send a message to the device. If you see the following response in the serial monitor in VS Code, it's possible that direct communication from the device is blocked at the router level. Check firewall and router rules that are configured for the connecting devices. Also, ensure that outbound port 1833 is open.
+> 
+> ERROR: mqtt_client.c (ln 454): Error: failure opening connection to endpoint  
+> INFO: >>>Connection status: disconnected  
+> ERROR: tlsio_mbedtls.c (ln 604): Underlying IO open failed  
+> ERROR: mqtt_client.c (ln 1042): Error: io_open failed  
+> ERROR: iothubtransport_mqtt_common.c (ln 2283): failure connecting to address atcsliothub.azure-devices.net.  
+> INFO: >>>Re-connect.  
+> INFO: IoThub Version: 1.3.6  
+
 ### View the telemetry received by Azure IoT Hub
 
 You can use [Azure IoT Tools](https://marketplace.visualstudio.com/items?itemName=vsciot-vscode.azure-iot-tools) to monitor device-to-cloud (D2C) messages in IoT Hub.