Internet Core Protocols: The Definitive Guide

Name

Router Advertisement

Synopsis

When a device that supports the Router Discovery protocol first comes onto the network, it issues a Router Solicitation query message. Routers on the network can then respond with the Router Advertisement query message, informing the device of the available routers.

In addition, routers will send Router Advertisement query messages every seven to ten minutes, allowing the local devices to refresh their lists of available routers.

ICMP Message Type

The Message Type for Router Advertisement is 9.

ICMP Message Codes

Router Advertisement does not use any Codes, and this field must be zero.

Defined In

RFC 1256.

Additional Fields

The Router Advertisement query message uses several additional fields. Table 5.15 lists these fields, their size (in bytes), and their usage.

Table 5.15. Additional Fields in Router Advertisement Query Messages

Field	Bytes	Description
Number of Addresses	1	The number of addresses in this advertisement
Address Size	1	The number of 32-bit words used to describe each entry
Lifetime	2	The number of seconds that devices should consider this information to be valid
Address Entry N	4	The IP address of interface N (this field will appear N number of times, as defined by the Number of Addresses field above)
Preference for N	4	The preference that devices should assign this interface

These fields are used as follows:

• The Number of Addresses field indicates the number of IP address entries in use on this interface. Since the Router Advertisement messages are sent ...