This read-write flag, defaulting to false, controls whether URPF is allowed to use the default route when checking the ingress port, a feature only supported in strict mode since permitting it in loose mode would effectively allow all traffic through. Admins enable it when they want strict-mode URPF to accept packets whose source matches only the default route rather than requiring a more specific route. An admin whose edge router relies heavily on a default route to reach most source networks would set this to true(1) so strict URPF doesn't reject otherwise-legitimate traffic lacking a specific route entry.