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Configuring IRDP
The term "router" in this chapter refers to a routing-capable device.
The term "host" in this chapter refers to the host that supports IRDP. For example, a host that runs
the Linux operating system.
Overview
ICMP Router Discovery Protocol (IRDP), an extension of the ICMP, is independent of any routing
protocol. It allows hosts to discover the IP addresses of neighboring routers that can act as default
gateways to reach devices on other IP networks.
IRDP enables hosts to track dynamic changes in router availability and requires a minimal amount of
manual configuration.
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.
Basic concepts
Preference of an IP address
Every IP address advertised in RAs has a preference value. A larger preference value represents a
higher preference. The IP address with the highest preference is selected as the default gateway
address.
You can specify the preference for IP addresses to be advertised on a router interface.
An address with the minimum preference value (-2147483648) will not be used as a default gateway
address.
Lifetime of an IP address
An RA contains a lifetime field that specifies the lifetime of advertised IP addresses. If the host does
not receive a new RA for an IP address within the address lifetime, the host removes the route entry.
All the IP addresses advertised by an interface have the same lifetime.
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