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ICMP Router Discovery Protocol (IRDP) is an alternative router discovery protocol using ICMP messages on multicast links. ICMP uses router discovery messages, known as router advertisements and router solicitations.
Each router periodically multicasts a router advertisement from each of its multicast interfaces, announcing the IP address of that interface, and other router IP addresses. Hosts discover the addresses of their neighbor routers by listening for the advertisements. When a host attached to a link starts up, it may multicast a router solicitation to ask for immediate advertisements, rather than waiting for the next periodic one to arrive.
If no advertisements arrive, the host re-transmits the solicitation, but does not send additional solicitations. Routers that subsequently start up, or were not discovered because of packet loss or temporary link partitioning, are eventually discovered by reception of their periodic (unsolicited) advertisements.
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