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Router Discovery

The Router Discovery Protocol


The Router Discovery Protocol is an IETF standard protocol used to inform hosts of the existence of routers. 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.

The protocol is split into two portions, the server portion which runs on routers, and the client portion that runs on hosts. GateD treats these much like two separate protocols, only one of which may be enabled at a time.

The Router Discovery Server


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 Router Advertisements contain a list of all the router's addresses on a given interface and their preference for use as a default router.

Initially these Router Advertisements occur every few seconds, then fall back to every few minutes. In addition, a host may send a Router Solicitation to which the router will respond with a unicast Router Advertisement (unless a multicast or broadcast advertisement is due momentarily).

Each Router Advertisement contains a Advertisement Lifetime field indicating for how long the advertised addresses are valid. This lifetime is configured such that another Router Advertisement will be sent before the lifetime has expired. A lifetime of zero is used to indicate that one or more addresses are no longer valid.

On systems supporting IP multicasting, the Router Advertisements are by default sent to the all-hosts multicast address However, the use of broadcast may be specified. When Router Advertisements are being sent to the all-hosts multicast address, or an interface is configured for the limited-broadcast address, all IP addresses configured on the physical interface are included in the Router Advertisement. When the Router advertisements are being sent to a net or subnet broadcast, only the address associated with that net or subnet is included.

The Router Discovery Server Statement

routerdiscovery server yes | no | on | off [ { traceoptions trace_options ; interface interface_list [ minadvinterval time ] [ maxadvinterval time ] [ lifetime time ] ; address interface_list [ advertise ] | [ ignore ] [ broadcast ] | [ multicast ] [ ineligible ] | [ preference preference ] ; } ] ;

The Router Discovery Client


A host listens for Router Advertisements via the all-hosts multicast address ( if IP multicasting is available and enabled, or on the interface's broadcast address. When starting up, or when reconfigured, a host may send a few Router Solicitations to the all-routers multicast address,, or the interface's broadcast address.

When a Router Advertisement with non-zero lifetime is received, the host installs a default route to each of the advertised addresses. If the preference ineligible, or the address is not on an attached interface, the route is marked unusable but retained. If the preference is usable, the metric is set as a function of the preference such that the route with the best preference is used. If more than one address with the same preference is received, the one with the lowest IP address will be used. These default routes are not exportable to other protocols.

When a Router Advertisement with a zero lifetime is received, the host deletes all routes with next-hop addresses learned from that router. In addition, any router learned from ICMP redirects pointing to these addresses will be deleted. The same will happen when a Router Advertisement is not received to refresh these routes before the lifetime expires.

The Router Discovery Client Statement

routerdiscovery client yes | no | on | off [ { traceoptions trace_options ; preference preference ; interface interface_list [ enable ] | [ disable ] [ broadcast ] | [ multicast ] [ quiet ] | [ solicit ] ; } ] ;

Tracing options

The Router Discovery Client and Server support the state trace flag which traces various protocol occurrences.

State transitions

The Router Discovery Client and Server do not directly support any packet tracing options, tracing of router discovery packets is enabled via the ICMP Statement.

next up previous contents index
Next: SNMP Statement Up: Configuration Guide Previous: Redirect

Laurent Joncheray
Wed Jun 12 15:35:22 EDT 1996