Line a colander with cheesecloth or damp paper towels and place over a bowl. In a large pot, bring 2 cups water, the cranberries, sugar, port, lemon zest and aniseeds to a boil. Lower the heat, partially cover and simmer, stirring occasionally, until the cranberries burst, 12 to 15 minutes. Pour into the colander, pressing with a spoon to extract all the juice; discard the solids.
Coat a decorative gelatin mold or loaf pan with cooking spray. In a small saucepan, stir the gelatin into 1 cup cold water and let sit until softened, about 1 minute. Heat the gelatin mixture over medium heat, stirring, until dissolved, about 2 minutes. Whisk the gelatin mixture into the cranberry liquid and pour into the prepared mold. Let cool to room temperature, then cover with plastic wrap and refrigerate until set, at least 6 hours.
Run a thin knife around the perimeter of the gelatin and unmold onto a platter.