An IP router MUST support the router part of the ICMP Router Discovery Protocol [INTERNET:13] on all connected networks on which the router supports either IP multicast or IP broadcast addressing. The implementation MUST include all the configuration variables specified for routers, with the specified defaults.
Routers are not required to implement the host part of the ICMP Router Discovery Protocol, but might find it useful for operation while IP forwarding is disabled (i.e., when operating as a host).
We note that it is quite common for hosts to use RIP Version 1 as the router discovery protocol. Such hosts listen to RIP traffic and use and use information extracted from that traffic to discover routers and to make decisions as to which router to use as a first-hop router for a given destination. While this behavior is discouraged, it is still common and implementors should be aware of it.