Router Discovery

The ICMP Router Discovery protocol is an IETF standard protocol that allows hosts running an ICMP router discovery client to learn dynamically about the presence of a viable default router on a LAN.

It is intended to be used instead of having hosts wiretap routing protocols such as RIP.

It is used in place of, or in addition to, statically configured default routes in hosts.

Note - Only the server portion of the Router Discovery Protocol is supported.

GaiaClosed Check Point security operating system that combines the strengths of both SecurePlatform and IPSO operating systems. implements only the ICMP router discovery server portion, which means that a Check Point router can advertise itself as a candidate default router, but it will not adopt a default router using the router discovery protocol.

The ICMP Router Discovery Service provides a mechanism for hosts attached to a multicast or broadcast network to discover the IP addresses of their neighboring routers.

This section describes how you can configure a router to advertise its addresses by using ICMP Router Discovery.

How Router Discovery Works

The router discovery server runs on routers and announces their existence to hosts. It does this by periodically multicasting or broadcasting a router advertisement to each interface on which it is enabled. These advertisements contain a list of all the router addresses on a given interface and their preference for use as a default router.

Initially, these router advertisements occur every few seconds. They then fall back to every few minutes. In addition, a host can send a router solicitation, to which the router responds with a unicast router advertisement. However, if a multicast or broadcast advertisement is due in a moment, the router does not respond with a unicast advertisement.

Each router advertisement contains an advertisement lifetime field indicating the length of time that the advertised addresses are valid. This lifetime is configured such that another router advertisement is sent before the lifetime expires. A lifetime of zero (0) indicates that one or more addresses are no longer valid.

On systems that support IP multicasting, the router advertisements are sent by default to the all-hosts multicast address 224.0.0.1. However, you can specify the use of broadcast. All IP addresses configured on the physical interface are included in the router advertisement when:

  • Router advertisements are sent to the all-hosts multicast address,

    or

  • An interface is configured for the limited-broadcast address 255.255.255.255.

When the router advertisements are sent to a net or subnet broadcast, only the address associated with that net or subnet is included.