Specifications

ICMP Router Discovery Advertisements (CR00010614) 158
Version 291-18
C613-10488-00 REV Q
ICMP Router Discovery Advertisements (CR00010614)
Router discovery
The router or switch supports all RFC 1256, ICMP Router Discovery Messages as it applies to routers. If this feature is configured, the router or switch 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, it has to know the IP address of a neighbouring router that can forward it to its destination. ICMP Router Discovery
messages let routers automatically advertise themselves to hosts. Other methods either require someone to manually keep these addresses up to date, or
require DHCP to send the router address, or require the hosts to be able to eavesdrop on whatever routing protocol messages are being used on the LAN.
Router discovery process
The following table summarises what happens when Router Discovery advertisements are enabled for interfaces on the router or switch.
When... Then...
Router Discovery advertising starts on a
router or switch interface because:
- the router or switch starts up, or
- advertisements are enabled on the switch
or on an interface
the router or switch 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.
the router or switch receives a router
solicitation
the router or switch multicasts an early router
advertisement on the multicast 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 lifetime of all existing router
advertisements on a host expires
the host sends a router solicitation.