Routing protocols are a critical component in IP networks. Among these, the Open Shortest Path First (OSPF) has been a widely used routing protocol in IP networks for some years. Beside dedicated hardware, a great interest on routing systems based on open software is raising among Internet Service Providers. Many open source implementations of this protocol have been developed, among which GNU Zebra is one of the most complete. In this paper we perform a study of the performances of the Shortest Path First computation in GNU Zebra, as prescribed by the Internet Engineering Task Force, and we provide a comparison between a Cisco 2621 access router and a PC-based router equipped with routing software GNU Zebra. Moreover we describe a set of modifications made on the GNU Zebra code in order to optimize some processes, whose algorithms were not efficient and whose experimental measures had showed a lack of optimization, thus finally obtaining performances better than the one measured on commercial systems.