Assigning network resources to Virtual Links (VLs) efficiently and on-demand is a challenging problem for any network virtualization solution. Known as the Virtual Link Mapping (VLM) problem, its objective is to compute the appropriate network paths with the required network resources that meet the quality of service expectations of arriving VLs while spreading the load over all nodes to maximise the admissibility of forthcoming VLs. Despite the efficiency of existing VL mapping algorithms, when resources are allocated and released over time due to the arrivals and departures of VLs, the network inevitably drift into a fragmented state (with nodes with very different loads) often causing a VLs request rejection that could have been avoided with a different resource allocation. In practice, defrag-mentation algorithms are used in complement to VL mapping algorithms to proactively or reactively (on the event of a VLs request refusal) trigger some VLs reallocation (or migration). In this paper, we propose an Integer-Linear program (ILP) based reactive defragmentation and VLs mapping algorithm for an SDN/OpenFlow network. In addition to selecting the VLs that should be migrated to reduce network defragmentation, our algorithm also computes the paths (and the associated resources) that support the previously rejected VLs. Our solution was evaluated on a real network topology and the experiments showed that our proposal outperforms existing approaches from the literature by about 12% in terms of acceptance rate with a gain on migration costs around 40%.