Differential mode involves a pair of traces (wires) between
the driver and receiver.s the following are the important facts to be considered while routing
>traces should be of equal length.
>route differential traces closely together.
>the separation between the two traces (of the differential pair) must remain
constant over the entire length.
>trackwidth should be same for both the traces
you can assign the trace width, physical type incontraints.
then as neena said the trace should be equal length, pair should run parallely with specified space in all the area.
As sugvivek said u can Use Constraint maneger to constrain this net. to help U in routing ( help U mean he will intrrupt u if U route this track differ from your constrain