Software Defined Networking (SDN) is an evolving network paradigm which promises greater interoperability, more innovation, flexible and effective solutions. Although SDN on the surface provides a simple framework for network programmability and monitoring, few has been said about security measures to make it resilient to hitherto security flaws in traditional network and the new threats the architecture is ushering in. One of the security weaknesses the architecture is ushering in due to separation of control and data plane is Denial of Service (DoS) attack. The main goal of this attack is to make network resources unavailable to legitimate users or introduce large delays. In this paper, the effect of DoS attack on SDN is presented using Mininet, OpenDaylight (ODL) controller and network performance testing tools such as iperf and ping. Internet Control Message Protocol (ICMP) flood attack is performed on a Transmission Control Protocol (TCP) server and a User Datagram Protocol (UDP) server which are both connected to OpenFlow switches. The simulation results reveal a drop in network throughput from 233 Mbps to 87.4 Mbps and the introduction of large jitter between 0.003 ms and 0.789 ms during DoS attack.