Resource monitoring is a key task in network management. The concept of Software Defined Networking (SDN) allows taking benefits of the advantages of both active and passive monitoring techniques. However, this monitoring has a cost, hence the importance of the selection of the "key" switches to be interrogated and their polling frequency in order to reduce monitoring cost. This cost is expressed here in term of computing time. Monitoring of links can be used to determine the available bandwidth on each link, with the aim to meet the applicative QoS requirements based on appropriate routing. In this context, this paper first provides a formulation of the problem of choosing key switches as a vertex cover problem and proposes a heuristic method to solve the formulated problem. It then provides an implementation and a performance evaluation of the proposed algorithm within the Floodlight SDN controller. These performances are compared to those of the currently existing Floodlight monitoring module. Finally, we present one application of our proposed monitoring.