The Bridge core requires a clock period constraint for the reference clock input that agrees with the REF_CLK_FREQ parameter setting. In addition, pin-placement (LOC) constraints are needed that are board/part/package specific.
See Placement Constraints for more details on the constraint paths for FPGA architectures.
Additional information on clocking can be found in the Xilinx® Solution Center for PCI Express (see Solution Centers).
refclkin Virtex®-7 devices and
sys_clk_gtin UltraScale devices.