I passed mine just over 5 weeks ago now. You will have to know pretty much everything that's in the book, 291 is supposed to be about the hardest exam to pass with a lot of people failing it the first time around (I did, I failed on RRAS).
Make sure you know how to configure DHCP scopes, scope options, RRAS, RADIUS etc. Make sure you know about DNS like it's the back of your hand as well.