Are there mandatory bike paths in Netherlands?
Mandatory
Mandatory bike paths do exist in the Netherlands (Article 5 of RVV 1990  regarding traffic and signage rules)  and will be marked with a &quot;G11&quot; sign, a round lollipop sign with a white bicycle on a blue background.
Sometimes they will additionally be distinguished from footpaths, parking spaces and other paved surfaces by a red coloring.

Some mandatory bike paths are also mandatory for mopeds and will be marked with the &quot;G12a&quot; sign below, a round lollipop sign with both a white bicycle and a white moped on a blue background.
You will typically find these outside of city limits or along roadways with a speed limit exceeding 50 km/h.

Mandatory bike lanes are not physically separated from the the main  roadway but the lane will be marked with bike (often enclosed by a white circle) , a (semi) continuous white line separating the bike lane from the main traffic lanes and sometimes the red colored riding surface.

Optional
Optional for cyclists are bike paths marked with a square blue sign &quot;fietspad&quot;.

Forbidden for bicycles
Probably a bit redundant but cycling is forbidden where and when indicated by the explicit sign, but also by the implicit signs indicating a pedestrian only area, on bus lanes, and on motor- and expressways.
    
Absence
When no mandatory bike path is indicated and there is no sign explicitly or implicitly prohibiting access for bikes, then bicyles are allowed on the main road.