A method monitors at least one avionic system connected to a communication medium that includes at least one active communication switch component. The method includes determining a state of the switch component and evaluating an indicator of a state of the communication medium from the state of the switch component and from a predetermined modeling of communication flows of the communication medium. The method also includes selecting and activating an alarm according to the evaluated state of the communication medium and according to a predetermined modeling of consequences of the evaluated state of the communication medium on a function of the at least one avionic system. A corresponding device and a method for determining conditions for alarm activation are also disclosed.