TCP/IP Tutorial and Technical Overview - IBM Redbooks

Figure 3-34 ICMP: Router Solicitation
Where:
Number The number of entries in the message.
Entry length The length of an entry in 32-bit units. This is 2 (32 bits for
the IP address and 32 bits for the preference value).
TTL The number of seconds that an entry will be considered
valid.
Router address One of the sender's IP addresses.
Preference level A signed 32-bit level indicating the preference to be
assigned to this address when selecting a default router.
Each router on a subnet is responsible for advertising its
own preference level. Larger values imply higher
preference; smaller values imply lower. The default is
zero, which is in the middle of the possible range. A value
of X'80000000' (-231) indicates the router should never
be used as a default router.
The ICMP header code field is zero for both of these messages.
These two messages are used if a host or a router supports the router discovery
protocol. Routers periodically advertise their IP addresses on those subnets
where they are configured to do so. Advertisements are made on the all-systems
multicast address (224.0.0.1) or the limited broadcast address
(255.255.255.255). The default behavior is to send advertisements every 10
minutes with a TTL value of 1800 (30 minutes). Routers also reply to solicitation
messages they receive. They might reply directly to the soliciting host, or they
might wait a short random interval and reply with a multicast.
Hosts can send solicitation messages. Solicitation messages are sent to the
all-routers multicast address (224.0.0.2) or the limited broadcast address
(255.255.255.255). Typically, three solicitation messages are sent at 3-second
intervals. Alternatively, a host can wait for periodic advertisements. Each time a
host receives an advertisement with a higher preference value, it updates its
default router. The host also sets the TTL timer for the new entry to match the
value in the advertisement. When the host receives a new advertisement for its
current default router, it resets the TTL value to that in the new advertisement.
114 TCP/IP Tutorial and Technical Overview