This paper contributes to understanding of how Quality of Service (QoS) can be optimised in Autonomous System (AS) networks using Open Shortest Path First (OSPF) routing. We assume that, although guaranteed QoS can not be provided, network wide performance can be optimised in a manner which significantly improves QoS. We particularly focus on devising a network performance objective, which is convenient for the network optimization process, while closely representing QoS experience for end users. Our results show that, firstly, network-wide performance can be significantly improved by optimizing OSPF weights. Secondly, the resulting optimal set of weights and network performance depends very much on the definition of the optimization objective function. We argue that min max utilisation is an appropriate function for heavily loaded network, while end to end delay is more appropriate optimization objective for lightly loaded condition, at which most of the backbones are operating.