As mentioned in Chapter 8, "Static Routing Concepts," dynamic routing protocol implementations use administrative distance (preference) for internal comparison and weighting. Based on these criteria, the UNIX Forwarding Information Base (FIB) is populated with the result of the internal comparison. The UNIX FIB has no knowledge of administrative distances; only metrics are relevant for forwarding table lookups. Depending on the configuration, these routes are removed or retained after shutdown of the routing daemon(s). From this point of view, the kernel interface resembles a routing protocol by itself and is treated similarly, especially during system startup and redistribution of kernel routes.