The MQTT broker Mosquitto is part of all Edge Gateway's by default.
You can activate and deactivate (default) this service in the control panel of the gateway's web management UI.
The broker can be addressed locally over the IP address 127.0.0.1:1883 via Node-RED where you configure the topics the broker shall subscribe to and distribute their data if received.
The broker listens to port 1883 respectively port 8883(ssl) on all active IP networks.