Multimedia traffic is characterized by its sensitivity to both network delay and packet losses. In high-load nondedicated networks, Quality of Service (QoS) must be guaranteed to multimedia traffic to achieve a minimum acceptable quality. This paper presents QoSApp, an OpenFlow-based application that dynamically manages the network-bandwidth to provide QoS guarantees to multimedia traffic, while allowing its coexistence with different traffic types. QoSApp, which was implemented modifying Floodlight's API, classifies network traffic and dynamically computes network routes satisfying QoS demands. Results successfully show video transmission over a test network.