H3C Technologies H3C S5560 Series Switches - Manual (Page 135)

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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.
NOTE:
The term "interface" in this chapter collectively refers to Layer 3 interfaces, including VLAN interfaces and
Layer 3 Ethernet interfaces. You can set an Ethernet port as a Layer 3 interface by using the port link-mode
route command (see Layer 2—LAN Switching Configuration Guide).
Overview
ICMP Router Discovery Protocol (IRDP), an extension of the ICMP, is an 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.