In this paper, we investigate the performance issue of PNNI routing for ATM networks. A brief introduction to PNNI routing protocol is given. We extend the reduced load approximation algorithm to ATM networks with PNNI routing. A hierarchical reduced load approximation scheme is proposed. The scheme includes two steps: the reduced load approximation algorithm is first applied on the aggregated network at upper level; then the approximation is refined at the lower level as needed using the results from the upper level. This hierarchical scheme follows the PNNI routing framework and results a fast and scaleable algorithm.