In this paper, we design, implement, and evaluate the functions and performance of a parallel running and comparing system that is built on OpenVswitch software switch. Parallel running is a very important method to change from an existing system to a new system. The conversion usually occurs when the technology of the old system is outdated and a new system needs to be installed to replace the old one. During parallel running, both the old and new systems are running side by side and their behavior are compared against each other until the users are certain that the new and old systems behave exactly the same. At that time, the old system will be removed and the new system will totally replace the old system. In this paper, we use the OpenVswitch to design and implement a comparing system for parallel running. Our system has many useful features and can be applied to real mission-critical systems.