Rope Jumping? Good or Bad?

I'm thinking of adding rope jumping to my workout. I have a few problems with my knees though and I'm wondering if rope jumping is the right thing for me? Or what I can do to lessen the impact on my knees (maybe softer underground, specific technique, etc.)?

Also, what are the benefits of rope jumping? I know it's a good cardio workout, but which muscles is it going to tone the most? I assume thighs? Maybe arms? What about ass?

How often should this be done? And for how many minutes? I'm not necessarily looking to lose weight, but would like to tone my body a bit!
Rope Jumping? Good or Bad?
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