Place the beef bones and oxtails in a large stockpot, cover with water. Bring the liquid up to a boil and cook for 10 minutes. Remove from the heat and drain. Rinse the pot and bones. Return the bones to the pot and cover with 6 quarts of water. Bring the liquid up to a boil and skimming often to remove the foam. When the foam stops rising to the surface, add 3 quarts of water and return to a boil.