At least one clock constraint is required for period and jitter. The following command signifies the setting of +/- 100 ps (= 0.1 nsec) peak to peak jitter on the primary clock port propagating through input port clk_in1.
create_clock -period 10.0 [get_ports clk_in1]
set_input_jitter [get_clocks -of_objects [get_ports clk_in1]] 0.1
The core-level XDC has early processing order, meaning that core-level XDC constraints are applied first and are then overridden by the user-provided constraints.