Current network planning and design methods use the average delay, packet loss and throughput as metrics to optimize the network cost and performance. New multimedia applications, on the other hand, also have critical jitter requirements that are not taken into account by these methods. Here, we explore the impact on the network performance of adding these jitter constraints. We use a fast jitter calculation model to solve the optimal routing problem for flows subject to jitter or delay constraints. We find that the optimal routing is very different for the two kinds of flows: They should be routed on different paths, the jitter-constrained flows should not be split on multiple paths while the opposite conclusion is true for delay-constrained flows.