Le cycle magazine once called this possibly the most difficult climb in France. Maybe an exaggeration, but wow, this is difficult stuff, especially in the heat/humidity.
This is a great, quiet loop. Returning via Col du Chat, while only 638 metres, leads to fantastic views high above Lac du Bourget.
This post describes the east side of Mont du Chat. In 2017, the Tour de France and Dauphiné will climb the equally ferocious west side – see here.