In this paper, we study a special case of multiperiod network design (MPND) problems, multiperiod network design with incremental routing (MPNDI), as applied to the transmission layer of an optical network. We optimize simultaneously the network architecture and the link dimensioning under discrete cost functions in order to route all traffic demands in line with traffic growth over a discrete time horizon. Because of operational requirements, routing paths used at a given period to route some traffic demands have to be preserved over subsequent periods; this is called incremental routing with respect to incremental flow. To handle the high dimensionality, a compact formulation omitting explicitly written flow variables is given. We examine the dominant polyhedron of the incremental multicommodity flow and provide a polyhedral study of it. Finally, a solution approach taking advantage of the polyhedral structure of MPNDI is proposed, and computational results for a range of instances with 8 and 10 nodes are provided. © 2007 Wiley Periodicals, Inc. NETWORKS, Vol. 50(1), 109–117 2007