As part of Laboratory 1 of the NLPQT Project, a test connection was set up allowing the distribution of the reference optical frequency in the PIONIER network on the Toruń-Poznań-Toruń route using dedicated optical fibers. In the evaluated link, spectrally ultranarrow laser light with a wavelength of 1542.14 nm was transmitted as a reference signal. Thanks to the direct beatnote of the transmitted and received signal in the tested loop, it was possible to perform an accurate measurement of the stabilization efficiency of the transmission system. The tested 600 km-long  line achieved long-term stability of the distributed signal at the level of 10^-20 (for the observation time of 10^5 s).This result shows that the proposed system meets the requirements of phase stabilized distribution of the ultrastable optical frequency reference. The evaluation of the developed devices in real environmental conditions confirmed the effective operation of the implemented mechanisms for automatic transmission control and resilience to unexpected network events (including failures). The mechanisms of automatic signal relocking and polarization control allowed human-less operation of the system while at the same time guaranteeing its high quality.