This paper presents a SNMP-based Monitoring Agents for Multi-Constrain Resource Scheduling in Grids (SBLOMARS) as an effective solution for resource usage monitoring in virtual network environments. SBLOMARS is different to current large-scale distributed monitoring systems in three essential aspects: Firstly, it reaches a high level of generality by the integration of the SNMP protocol and thus, facilitates to handle heterogeneous operating platforms. Secondly, it is able to self-configure the polling periods of the resources to be monitored depending of network context and finally, it makes use of dynamic software structures to interface with third parties, allowing to be deployed in a wide range of devices, from simple mobile access devices to robust multiprocessor systems or clusters with even multiple hard disks and storage partitions. SBLOMARS has been deployed in EmanicsLab, a virtual laboratory constituted by fourteen nodes distributed in seven European Universities. Although the research is not yet concluded, available results confirm its suitability to deal with the challenges of monitoring virtual networks.