A domain-level gradient-based routing (DLR) algorithm for heterogeneous optical networks with synchronous digital hierarchy and optical transport network domains is proposed and experimentally validated. This algorithm classifies domains into groups with incremental levels on the basis of domain-level partitioning, and guides paths level by level along a gradient on the basis of interdomain routing tree evolution. The proposed algorithm is implemented in the hierarchical path computation element-based control architecture for connection provisioning. Testbeds with commercial and emulated nodes are established to verify the feasibility and performance of the algorithm. Experimental and emulation results show that DLR effectively performs in terms of network blocking probability, real time characteristics, and scalability.