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Configuring IRDP
About IRDP
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. It does not require you to
configure the default gateways on hosts or configure hosts to listen to routing protocols to obtain IP
addresses of routers.
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 and their preferences. 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.
Protocols and standards
RFC 1256: ICMP Router Discovery Messages
IRDP tasks at a glance
To configure IRDP, perform the following tasks:
1. Enabling IRDP
2. (Optional.) Specifying the preference of advertised IP addresses
3. (Optional.) Setting the lifetime of adverti
sed IP addresses
4. (Optional.) Setting the advertising intervals
5. (Optional.) Specifying the multicast address as the destination address for RAs
6. (Optional.) Specifying a proxy-advertised IP address
Restrictions and guidelines: IRDP configuration
• IRDP takes effect on any Layer 3 interface (except the loopback interface) if the interface has
an IP address.
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