A method and associated system for testing a virtual network function by a virtual network tester, wherein a predefined certification context specification specifies services based on the virtual network function in form of different usage patterns, and wherein a predefined test case selection matrix specifies applicable test cases for each usage pattern. In response to selecting a usage pattern from the different sage patterns, the applicable test cases are determined from the test case selection matrix and control parameters of the test case selection matrix based on the selected usage pattern.