Irdp Configuration; Irdp Overview; Background; Working Mechanism - HP A5120 EI Series Configuration Manual

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IRDP configuration

IRDP overview

As an extension of the Internet Control Message Protocol (ICMP), the ICMP Router Discovery Protocol
(IRDP) enables hosts to discover the IP addresses of their neighboring routers and set their default routes.
NOTE:
The hosts in this document support IRDP.

Background

Before a host can send packets to another network, it must know the IP address of at least one router on
the local subnet. The host can obtain this information either through manual configuration, or from routing
protocol packets sent by routers on the local subnet.
Both methods have disadvantages. The first method requires the administrator to manually configure and
maintain router address information on hosts, and cannot track dynamic changes. The second method
requires hosts to recognize various routing protocols, and will fail to work if no routing protocol runs on
the local subnet.
IRDP was introduced to solve the problem. IRDP uses two new types of ICMP messages to allow hosts to
discover neighboring routers. IRDP adapts to dynamic changes, requires less manual configuration, and
does not rely on any routing protocols.

Working mechanism

IRDP uses the following types of ICMP messages.
Router advertisement (RA): Sent by a router to advertise its IP address and preference.
Router solicitation (RS): Sent by a host to voluntarily request the IP addresses of routers on the subnet.
IRDP works with the following steps.
A router periodically broadcasts or multicasts an RA, which contains the IP addresses (including the
primary IP address and manually configured secondary IP addresses) of interfaces. Hosts listen for
RAs to obtain the IP addresses of neighboring routers.
Rather than wait for RAs, a newly attached host can voluntarily send an RS to request immediate
RAs for the IP addresses of routers on the subnet. If no response to the RS is received, the host
retransmits the RS several times. If the host still receives no RAs, it will obtain the IP addresses of
routers from periodic RAs.
Upon receiving an RA, a host adds the IP addresses in the RA to its routing table. The host selects
the IP address with the highest preference among all obtained IP addresses as the default gateway.
IRDP allows hosts to locate routers, but does not suggest the best route to a specific destination. If a host
selects a router that is not the best next hop to a specific destination, the router will send back an ICMP
redirect message to provide a better next hop.
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