Internet Protocol (IP) Addressing and Protocols
©2008 Allied Telesis Inc. All rights reserved.
Software Version 5.2.1
AlliedWare Plus
TM
Operating System Software Reference C613-50003-00 REV E
22.9
ICMP Router Discovery Protocol
(IRDP)
Router discovery
Your device supports the router specification sections of RFC 1256,
ICMP Router Discovery
Messages
. If this feature is configured, your device sends router advertisements periodically and
in response to router solicitations. It does not support the Host Specification section of this
RFC.
Benefits
Before an IP host can send an IP packet, the host has to know the IP address of a neighboring
router that can forward the packet to its destination. ICMP Router Discovery messages let
routers automatically advertise themselves to hosts. Other methods either require someone to
manually keep these addresses current, or require DHCP to send router addresses.
Router discovery
process
The following table summarizes what happens when Router Discovery advertisements are
enabled on an interface.
Advertisement
messages
A router advertisement is an ICMP (type 10) message that contains the following:
■
in the destination address field of the IP header, the interface's configured advertisement
address, either 224.0.0.1 or 255.255.255.255.
■
in the lifetime field, the interface's configured advertisement lifetime.
■
in the Router Address and Preference Level fields, the addresses and preference levels of
all the logical interfaces that are set to advertise.
Your device does not send router advertisements by default.
When...
Then...
Router Discovery advertising starts on an
interface because:
- your device starts up, or
- you enable advertisements on your device
or on an interface
your device multicasts a router advertisement
and continues to multicast them periodically
until router advertising is disabled.
a host starts up
the host may send a router solicitation
message.
your device receives a router solicitation
your device multicasts an early router
advertisement from the interface on which it
received the router solicitation.
a host receives a router advertisement
the host stores the IP address and preference
level for the advertisement lifetime.
the lifetimes of all existing router
advertisements on a host expire
the host sends a router solicitation.
a host does not receive a router
advertisement after sending a small number of
router solicitations
the host waits for the next unsolicited router
advertisement.
a host needs a default router address
the host uses the IP address of the router or
L3 switch with the highest preference level.
Router Discovery advertising is deleted from
the interface
your device multicasts a router advertisement
with the IP address(es) that stopped
advertising, and a lifetime of zero. It continues
to periodically multicast router advertisements
for other interfaces, if configured to.
the router receives a router advertisement
from another router
the router does nothing but silently discards
the message
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