Method and system for implementing an inter-domain constraint-based shortest path first (“IrD-CSPF”) technique for supporting hierarchical routing in interconnected multi-domain OTNs are described. In one embodiment, the invention is a method for calculating a network path in an interconnected multi-domain network. The method comprises receiving a path setup request message for a new traffic flow in the network identifying a source node in one domain of the network and a destination node in a second domain of the network; determining a common ancestor hierarchical routing domain that includes ancestor nodes of both the source and destination nodes; calculating an inter-domain path from one ancestor node to the other ancestor node that determines, for each lower-level domain, border nodes in the domain from the source node to the destination node; and for each bottom-level domain, calculating an intra-domain path between the border nodes that were determined for the domain.