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Understanding the ICMP Protocol for Discovering Gateways to Other Networks

The ICMP Router Discovery Protocol (IRDP) enables hosts to locate routers on the local subnet and use them as a gateway to reach other networks. Junos OS supports running IRDP in server mode, meaning that router discovery packets are generated. Junos OS does not support IRDP in client mode running as a host sending router solicitation messages. IRDP is specified in RFC 1256, ICMP Router Discovery Messages.

For a host to participate on an internetwork, it needs connectivity to at least one router on the local network. One way to ensure that this is the case is to manually configure each host with the address of a local router as its default router (also called a gateway). This method is time-consuming to set up, difficult to maintain, and inflexible.

When you enable the Dynamic Host Configuration Protocol (DHCP) on a host, you do not need to configure the default router. DHCP uses a method called router discovery to automatically discover local routers, and learn other information about them.

The information provided includes the router’s address (or addresses, if it has more than one) and how long the host should retain information about the router. Router advertisement messages are sent periodically. Hosts listen for these messages. When an advertisement is received, the host processes it and adds the information about the router to its routing table. A host that has no manually configured routing information has no connectivity to routers when it first powers on. Instead of waiting for the next Router Advertisement message, the host sends a router solicitation message on its local network. This prompts any router that receives this message to immediately send an extra router advertisement message directly to that host.

By default, router discovery is disabled on Junos OS routing devices. When router discovery is enabled, the default behavior is to advertise all interfaces. If the router supports multicast, all the IPv4 Layer 3 interfaces are advertised through multicast. Otherwise, all the IPv4 Layer 3 interfaces are advertised through broadcast.