IRDP operation
IRDP uses the following types of ICMP messages:
Router advertisement (RA)—Sent by a router to advertise IP addresses (including the primary and secondary IP addresses) and preference.
Router solicitation (RS)—Sent by a host to request the IP addresses of routers on the subnet.
An interface with IRDP enabled periodically broadcasts or multicasts an RA message to advertise its IP addresses. A receiving host adds the IP addresses to its routing table, and selects the IP address with the highest preference as the default gateway.
When a host attached to the subnet starts up, the host multicasts an RS message to request immediate advertisements. If the host does not receive any advertisements, it retransmits the RS several times. If the host does not discover the IP addresses of neighboring routers because of network problems, the host can still discover them from periodic RAs.
IRDP allows hosts to discover neighboring routers, but it does not suggest the best route to a destination. If a host sends a packet to a router that is not the best next hop, the host will receive an ICMP redirect message from the router.