It's not mandatory....but it's certainly required if we wish to meet the spending appropriations that congress already approved. It's odd that the Republican controlled House approved spending appropriations earlier this year, but they don't want to face the reality of the debt incurred as a result of those choices.
In fairness this is mostly due to being forced into non-realistic budget reduction last time around, so now they're looking to get all they can.....just not in a very productive manor IMO.