If an input clock is a spread spectrum signal, the spreading function is achieved by adding jitter to the clock. For low loop BW devices, this jitter will be attenuated and effectively removed. A problem that can arise in that there can be false OOF and LOL assertions due to the frequency of the input clock changing over time. This is caused by the dithering of the clock input's spreading function. In order to avoid these false OOF and LOL assertions, the thresholds for OOF and LOL need to be loosened to accommodate the spread.
As an example, consider a 100 MHz spread spectrum clock that has a spectrum spread of ± 5 kHz. Noting that 5 kHz is a change of 50 ppm for a 100 MHz clock, the OOF and LOL assertion thresholds should be set to 50 ppm or greater.