There is a growing interest of using switched Ethernet to support time constrained communication required by process control, factory automation and other real-time applications. Although many special-purposed industrial solutions have been proposed, it still lacks of some general quantitative evaluation of the real-time handling capability of a switched Ethernet. This paper first gives a comprehensive analysis of the interesting features offered by a switched Ethernet for supporting time constrained communication and then evaluates the performance of an Ethernet switch. The obtained analytic results can be considered as a basic knowledge for further verifying the total end to end response time guarantees (both hard real-time and soft real-time ones).