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JSI Tip 1314. Router Discover added in SP5.

Per RFC 1256:

   The ICMP router discovery messages are called "Router Advertisements" and
   "Router Solicitations. " Each router periodically multicasts a Router
   Advertisement from each of its multicast interfaces, announcing the IP
   address(es) of that interface. Hosts discover the addresses of their
   neighboring routers simply by listening for advertisements.
Using Router Discovery, clients dynamically discover routers and can switch to a backup router when a network failure occurs or the network is reconfigured. Router discovery is made up of Router Solicitations packets sent by hosts, and Router Advertisements sent by routers. Windows NT 4.0 supports router discovery as a host only.

To enable a Windows NT 4.0, SP5 clients to perform Router Discovery, use Regedt32 to navigate to:


On the Edit menu, Add Value name PerformRouterDiscovery as a type REG_DWORD and set the data value to 1. The default is 0. Set PerformRouterDiscovery for each <adaptername> required.

You may also Add Value name SolicitationAddressBcast, a type REG_DWORD, and set the data value to 1 to send Router Discovery as limited broadcasts. The default is 0, which sends to the all routers multicast group.

The IRDP client will listen to both multicast and broadcast router advertisements.

When a host that supports router discovery initializes, it joins the "all-systems" IP multicast group (, and then listens for the router advertisements sent to that group.

Hosts send ICMP Router Solicitation messages to the all routers IP multicast address ( The router will respond by sending a router advertisement. This ensures the client receives a default gateway immediately without waiting for a periodic router advertisement. Windows NT sends a maximum of 3 solicitations at intervals of approximately 600 milliseconds.

Windows 98 supports IRDP, Windows 95 requires Windows Socket Update 2.

See Q183902 - Dead Gateway Detection Is Not Triggered During Logon.

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