Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring Routing Information (Rev. 03)
ICMP Router Discovery Protocol (IRDP)
© 2008 Foundry Networks, Inc.
Page 39 of 48
ICMP Router Discovery Protocol (IRDP)
The ICMP Router Discovery Protocol (IRDP) provides an improved method of configuring and
detecting default gateways. Instead of manually configuring default gateways or using DHCP to set
them, hosts can dynamically discover routers on their subnet, and can automatically device to a
backup router if the primary router fails or the network administrators change router preferences.
When using DHCP or manual default gateway configuration, there is no way to adjust to network
changes. Using IRDP, clients dynamically discover routers and can device to backup routers if a
network failure or administrative change is needed.
Overview
The ICMP (Internet Control Message Protocol) provides a series of router-information messages
that can be used to discover and advertise the presence of a default router on a network. Routers
advertise themselves as offering default routes with a specific preference weight (assigned by the
user), and hosts can pick the best router based on the preference value (higher values are
preferable).
A host uses IRDP to locate routers. The host sends router discovery packets and an IRDP-enabled
router receives these packets. The server/client implementation of IRDP does not store full routing
tables but keeps track of which routers are sending the routing information. IRDP can also listen in
the routing protocol updates to find routers.
IRDP presents a router discovery method using ICMP messages. This method is independent of
any specific routing protocol and does not require manual configuration of router addresses.
Table 18
describes the conventional terms used for IRDP.
Table 18: Conventional IRDP Terms
Term
Refers to
IRDP Hosts
Devices on the network that send queries (router solicitations) to
discover default gateways.
IRDP Routers
Devices on the network that advertise themselves as routers
(gateways).
Router Advertisements
ICMP discovery messages that announce the IP address of a potential
gateway.
Router Solicitations
ICMP discovery messages that ask for immediate advertisements.
An IRDP Router sends router advertisements periodically from its IP interface to announce the IP
address. When the host starts up, it discovers the address by waiting for the periodic
advertisements to arrive. When an IRDP Host receives these packets, (if it has no default gateway)
it sets the IRDP Router IP address as its default gateway. This setting is valid throughout the
lifetime specified in the router advertisement message. The IRDP Host can be configured to send
router solicitation messages to the network and to ask for immediate advertisements. If an IRDP
Router receives such a message, it sends back router advertisements. The IRDP Host retransmits
the solicitation a number of times only as long as the IRDP router does not send any advertisement.
Figure 4
shows how an IRDP Host discovers the default gateway. The assigned preference value
20 for IRDP Router B is higher than the preference value of Router A and the IRDP Host chooses
the Router B advertisement for default gateway.