In this paper, we show how Network Calculus can be used to determine whether a switched network may satisfy the time constraints of a real-time application. If switched architecture are interesting in the sense that they offer flexible design and may eliminate collisions in Ethernet-based network, they are not guaranteeing end-to-end performances (in particular in terms of delay), especially when cross-traffic are present. We illustrate Network Calculus usefulness by showing how the internal switching structure of an Ethernet switch simplify the analysis and which kind of traffic inter-dependencies are problematic.