Optical packet switching allows very efficient bandwidth use in the metropolitan ring networks, in comparison to optical circuit switching. However, when dimensioning an optical packet switching network, the stability of the network needs to be planned carefully, as these networks encounter the same stability issues like the electronic packet networks (e.g. Ethernet or Resilient Packet Ring). In this work, we show which stability conditions need to be satisfied by inserting nodes, in an optical packet switching network. On an example, we illustrate the impact that the stability has on the network QoS performance and incite the introduction of the stability constraints in network design. Next, we explain a network design method and the corresponding CAPEX cost assumptions. Finally, we study the stability in a small metropolitan ring with uniform and symmetric traffic matrix, in order to evaluate the impact of stability on the network cost and capacity. Numerical results show that the stability constraints induce the average increase in network CAPEX cost of around 9%.