Network virtualization is a promising technology that offers high levels of flexibility, isolation, extensibility and cost-effectiveness. In this paper, we focus on router virtualization. We evaluate the forwarding performance of virtual routers when the data plane runs in the guests. This scenario offers a high level of isolation and flexibility, however, it suffers from performance limitations due to the virtualization overhead. We show that the he I/O communication between the driver domain and the guests is the bottleneck. To overcome this limitation, we propose a new packets aggregation mechanism that transfers groups of packets between the driver domain and the guests. This enhancement makes the forwarding performance of one guest scale up to 1600 Kp/s. Furthermore, we propose a dimensioning tool in order to determine the maximum achievable throughput with regard to the container size.