The present principles are directed to a transponder aggregator-based optical loopback in a multi-degree colorless, directionless, contention-less, reconfigurable optical add/drop multiplexer. The multiplexer includes a reconfigurable optical add/drop multiplexer section for performing connect operations for wavelength division multiplexing signals among all degrees. The section has a plurality of subsections. Each of the subsections corresponds to a respective one of the degrees and has an optical separator at an input side and an optical combiner at an output side. The multiplexer further includes a transponder aggregator section having a split-and-select switch-based transponder aggregator. The multiplexer also includes an optical line loopback having a connection path between the optical separator at the input side and the optical combiner at the output side of at least one of the subsections.