For a wide range of features such as MPLS, MPLS-over-GRE, nexthop-groups, VPN, EVPN, BGP-LU, etc, the kernel injects CPU-generated packets into the hardware pipeline as a result of fwd*/txfwd kernel interfaces being used as next-hop devices on kernel routes.

Routing control functions (RCF) is a language that can be used to express route filtering and attribute modification logic in a powerful and programmatic fashion.