In a 5-to-6 quart crock pot, place carrots, potatoes and onion in bottom of pot. Place brisket, fat side up, on top of vegetables, add beer (if using) and spices from packet. Add enough water to just cover the brisket. Cover and cook on low 8 hours (4 1/2 hours on high) until meat and vegetables are tender.
After initial cooking time, arrange cabbage on top and sides of brisket. Pot may seem overcrowded. Cover and cook until cabbage is tender, 1 1/2 hours on low (45 minutes to 1 hour on high).