— We consider a ring topology with limited or full switching capability as deployed in high bandwidth metro optical networks and develop a model to estimate the probability of blocking for interconnecting links. The model is based on the lower and upper bounds of link blocking probability. We analyze the performance for a homogeneous traffic case and present simulation results for representative ring networks. We demonstrate that the bounds are very tight with an error of less than 2% when the traffic load is modest. The approach is based on partitioning the state space into subspaces and weighting the upper bound of blocking probability in each subspace with the occurrence of the states. The computational complexity of the approach is comparable to solving a degree of N polynomial equation.