Modelsim - SDF incompatibility

  Hi,

I am using Modelsim to simulate placed-and-routed Xilinx design. I
have noticed that
in some cases iopath from sdf is being treated as a transport delay
and in some other as
inertial delay, although the type of the cell is the same (for
example, x_buf_pp).
Could someone tell me why is this happening and is there a way to
change it?

Thanks in advance!

Ruzica