Method and apparatus for implementing a constraint-based shortest path first (“CSPF”) technique for dynamically switched optical transport networks are described. One embodiment comprises a traffic network element (“TNE”) in a network comprising a traffic engineering network database (“TEND”) that stores network topology information for the network and bandwidth availability information for links in the network; and a routing engine that receives a request for a label switched path (“LSP”) with specified constraints, processes network topology information from the TEND to create a network graph comprising links meeting the specified constraints, and computes a primary path through the network, wherein the primary path comprises links selected from the network graph.