ICMP Router Discovery Protocol
Encyclopedia
ICMP Internet Router Discovery Protocol (IRDP) uses Internet Control Message Protocol
Internet Control Message Protocol
The Internet Control Message Protocol is one of the core protocols of the Internet Protocol Suite. It is chiefly used by the operating systems of networked computers to send error messages indicating, for example, that a requested service is not available or that a host or router could not be...

 (ICMP) router advertisements and router solicitation messages to allow a host to discover the addresses of operational routers on the subnet
Subnetwork
A subnetwork, or subnet, is a logically visible subdivision of an IP network. The practice of dividing a network into subnetworks is called subnetting....

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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.
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