Stir together flour, baking powder, soda and salt. Set aside.
In separate mixing bowl, mix together KELLOGG'S ALL-BRAN cereal, bananas, eggs and vanilla. Let stand about 5 minutes or until cereal softens.
In large electric mixer bowl, beat together margarine and sugar on medium speed, until thoroughly combined. Add flour alternately with cereal mixture. Mix well after each addition. Stir in walnuts. Spread evenly in 9 x 5 x 3-inch loaf pan coated with cooking spray.
Bake at 350° F about 45 minutes or until wooden pick inserted in center comes out clean. Remove from pan and cool on wire rack. Serve warm or cold. Store in airtight container. Yield: 1 loaf, 14 slices