Make the punch in a saucepan, boil the orange juice, zest and cinnamon sticks over moderately high heat until reduced to 1 cup, 8 minutes. Strain and let cool.
fresh orange juice
Strips of zest from 2 navel oranges
Three 3-inch-long cinnamon sticks, broken into pieces
In a large glass pitcher, combine the orange reduction with the orange and grapefruit juices and the grenadine. Stir well and refrigerate until chilled, about 1 hour. Just before serving, pour in the grapefruit soda. Dust the orange slices with cinnamon and serve in the punch.