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The Router Discovery Protocol, an extension to Internet Control Message Protocol (ICMP), allows hosts to discover routers on their networks and determine which router to use as the default router. When a host needs to send a packet to another network, it first sends the packet to a router that forwards the packet toward the destination. To accomplish this, the host needs to know where the routers are on its network and which one to send packets to.
When you configure the router discovery mechanism, the router advertises itself with periodic ICMP router advertisement messages. Then the host listens to this message and decides whether to use a router as the default router.
You can configure the host to solicit the router advertisement on attached networks. All participating routers then reply to the inquiry. By collecting those replies, the host discovers the routers on the network and determines which router to use.
A host might not select the best router (the router with the optimal path) to forward packets to a specific destination. When a router receives a packet from a host that is better forwarded to another router on the network, the router uses an ICMP Redirect message to notify the host of the optimal path.
NetWare routing software provides both host and router implementations of the Router Discovery Protocol. The mode of operation of the Router Discovery Protocol is determined by whether the IP Packet Forwarding parameter is enabled. If IP Packet Forwarding is enabled, the Router Discovery Protocol will send Router Advertisement messages. If IP Packet Forwarding is disabled, the Router Discovery Protocol will send Router Solicitation Messages. these messages are explained in the next section.
The two message types that are used by the Router Discovery Protocol to communicate between hosts and routers are discussed in the following sections.
The ICMP Router Advertisement Message is ICMP message type 9. This message is used by routers to advertise their presence on the network and is broadcast or multicast to all hosts on the network.
This message type carries the IP address of the router and its preference level. Hosts use the preference level to determine which router to use for forwarding. The router with the highest preference becomes the default router. A value of 0x80000000 indicates the router is not to be used. Routers with this value are used only when other routers send ICMP Redirect messages to the host.
The ICMP Router Solicitation Message is ICMP message type 10. Hosts use this message to solicit router advertisements from all participating routers on the network.
Router Discovery uses two IP multicast addresses. The IP address 224.0.0.1 is reserved to multicast the Router Advertisement Message to the hosts. The IP address 224.0.0.2 is reserved to multicast the Router Solicitation Message to the routers. If the network does not support multicast, then broadcast address 255.255.255.255 is used for both the Router Advertisement and Router Solicitation messages.
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