Preheat oven to 350°. Arrange walnuts in a single layer in a shallow pan. Bake 12 minutes or until toasted and fragrant.
Sift together flour, cinnamon, and baking soda. Line bottoms of 3 lightly greased 9-inch round cake pans with parchment paper; lightly grease parchment paper.
Beat sugar and oil at medium speed with an electric mixer until smooth. Add eggs, 1 at a time, beating until blended after each addition. Add flour mixture, beating at low speed just until blended. Fold in carrots and 1 cup toasted walnuts. Spoon batter into prepared pans.
Bake at 350° for 35 to 40 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes. Remove from pans to wire racks; remove parchment paper, and let cakes cool 1 hour or until completely cool.
Spread 5-Cup Cream Cheese Frosting between layers and on top and sides of cake; press remaining 1 cup chopped walnuts halfway up sides of cake.