Due to challenging conditions of the HL-LHC, the CMS detectors are undergoing a system-wide upgrade, and specifically the complete redesign of the end cap sub-detectors.
To reach the aimed resolution of 30 ps RMS on events timing information, a precision clock distribution system providing a readout clock with a sub 15 ps RMS jitter is necessary.
This talk will present the state of the art of the precision clock distribution system and then focus on the R&D dedicated to the low-jitter clock distribution.