In this paper we show how Network Calculus can be used to compute the optimal route for a flow (w.r.t. end-to-end guarantees on the delay or the backlog) in a network in the presence of cross-traffic. When cross-traffic is independent, the computation is shown to boild down to a functional shortest path problem. When cross-traffic perturbates the main flow over more than one node, then the ``Pay Multiplexing Only Once'' phenomenon makes the computation more involved. We provide an efficient algorithm to compute the service curve available for the main flow and show how to adapt the shortest path algorithm in this case.