ICMP Internet Router Discovery Protocol (IRDP) uses Internet Control Message Protocol (ICMP) router advertisements and router solicitation messages to allow a host to discover the addresses of operational routers on the subnet.
It basically consists of 2 Message-Types (see this list) used for discovering local routers. The message type 9 is sent periodically or on request (using a message of type 10) to the local subnet from the local router(s) to propagate themselves. On boot, the client may send a ICMP-Message of type 10 to ask for local routers. When a client receives a message type 9, they add the router to their local routing table.