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If two clock domains are asynchronous and you have applied set_false_path between these two clocks, no timing checks can be performed. Also, if you have defined a clock group with asynchronous clocks using the set_clock_groups command​ with the -asynchronous option, by default the tool cannot perform a timing check. But if you use the -allow_paths option with the set_clock_groups command, timing check can be performed.