Reporting SKEW in TRACE
2.1i TRACE will automatically account for clock skew on PERIOD constraints. Use the -skew switch or environment variable. XILINX_DOSKEWCHECK=1 (or XILINX_DORACECHECK).
Avoid using environment variable with PAR if there are IBUF clocks. PAR’s timing score may oscillate.
If you use the -skew switch with TRACE, remember that the timing score may be different than the score given by PAR (it could be better or worse).