RIP is implemented by in.routed, the routing daemon, which automatically starts when the machine boots. When run on a router with the s option specified, in.routed fills the kernel routing table with a route to every reachable network and advertises "reachability" through all network interfaces.
Specify the S flag and in.routed creates a minimal kernel table, containing a single default route for each available router.
Hosts used RDISC to obtain routing information from routers. Thus, when hosts are running RDISC, routers must also run another protocol, such as RIP, in order to exchange router information among themselves.
RDISC is implemented by in.rdisc, which should run on both routers and hosts. Normally, when in.rdisc runs on a host, it enters a default route for each router that is also running in.rdisc. A host that is running in.rdisc can not discover routers that are running only RIP. Furthermore, when routers are running in.rdisc (rather than in.routed), they can be configured to have a different preference, which causes hosts to select a better router. See the rdisc(1M) man page.