Ethernet is the hottest candidate for future in-car communication architecture, promising much higher bandwidth, flexibility and reduced costs. In the coming years, Ethernet will likely evolve from a separate communication medium for special applications like surround-view cameras and infotainment to a central communication infrastructure as a backbone technology. To make this transition, many difficult design decisions have to be made in order to make the technology suitable for the stricter time and safety requirements of todays and future cars. There are a lot of potential real-time effects that must be taken into account.To guide these design decisions, it is necessary to analyze the various architecture concepts with respect to load, performance and real-time capabilities. In this paper, we present different design space axes of Ethernet and propose a methodology of assessing and comparing them. This includes a formal worst-case timing analysis approach and a set of metrics that make the timing analysis results comprehensible and comparable across different design options. Using this foundation, we explore the design space for a typical automotive scenario. Specifically, we compare different topologies and switching strategies and their effect on the performance of the network. This provides valuable insights and design guidelines, but also shows the need for further exploration and reduction of the design space by deeper standards for the automotive industry.