Internet Core Protocols: The Definitive Guide

Router Solicitation and Router Advertisement query messages

RFC 1256 introduced the concept of a Router Discovery protocol that allows the devices on a network to locate routers using ICMP query messages, rather than having to be configured to use static routing entries or run a full-fledged routing protocol like RIP or OSPF.

The Router Discovery protocol consists of a Router Solicitation query message, which is issued by hosts when they first become active on the network (sent to the all-routers multicast address of 224.0.0.2). Each router on the network should then respond to the Router Solicitation query messages with a unicast Router Advertisement query message, informing the querying device directly of the IP addresses that can be used for packet forwarding. In addition, routers will also issue unsolicited Router Advertisement messages on a periodic basis (sent to the all-hosts multicast address of 224.0.0.1), allowing hosts both to change their routing tables as higher-priority routers become available, and to expire old routers if they are no longer being advertised after a certain length of time.

According to RFC 1256, unsolicited Router Advertisement query messages should be sent to the all-hosts multicast address of 224.0.0.1 through every network interface in use by the router, although directed Router Advertisements that result from Router Solicitations are only unicast back to the requesting device, through the most appropriate interface. Each of these advertisement ...