ICMP Router Discovery

The Windows 2000 Router includes support for the router advertisement and discovery scheme as documented in RFC 1256. To aid in the ease of configuring IP hosts with the IP addresses of local routers and to provide a way for hosts to sense downed routers, RFC 1256 describes the use of ICMP messages to send Router Advertisements and Router Solicitations.

Router Advertisements

A router sends out a periodic Router Advertisement using an ICMP message (Type 9, Code 0). The Router Advertisement can be sent to the all-hosts IP multicast address of 224.0.0.1, the local IP broadcast address, or the limited broadcast address (255.255.255.255). In practice, the Router Advertisement is sent to the multicast address. Router Advertisements are explicit notifications to the hosts on the network that the router is still available.

The Router Advertisement message contains an Advertisement Lifetime, the time after the last received Router Advertisement that the router can be considered down (default of 30 minutes), and a Preference Level, an indication of the router's preferred status as the default gateway for the network. The highest preference level router becomes the default gateway.

Router Solicitations

When a host that supports RFC 1256 needs to be configured with a default gateway (either upon initialization or because its default gateway is down), it sends out a Router Solicitation using an ICMP message (Type 10, Code 0). The Router Solicitation can be sent to the all-routers IP multicast address of 224.0.0.2, the local IP broadcast address, or the limited broadcast address (255.255.255.255). In practice, hosts send Router Solicitation messages to the multicast address. Routers on the host's network that support RFC 1256 immediately respond with a Router Advertisement, and the host chooses the router with the highest preference level as its default gateway.

Note

ICMP router discovery is not a routing protocol. Routers only advertise their existence, not the best way to reach a given destination network. If a host uses a non-optimal route, ICMP Redirect messages redirect the host to the better route.

Table 3.7 describes the settings for ICMP Router Discovery.

Table 3.7 ICMP Router Discovery Settings

Router Discovery Setting	Description
Level of Preference	The preference level of this router to be the default gateway. The default value is 0.
Advertisement Lifetime (minutes)	The time, in minutes, after which a host considers a router down (after it has received its last Router Advertisement). The default time is 30 minutes.
Advertisement interval minimum time (minutes)	The minimum amount of time between Router Advertisements sent by this router. The default is 7 minutes.
Advertisement interval maximum (minutes)	The maximum amount of time between Router Advertisements sent by this router. The default is 10 minutes. Router Advertisements are sent at a random interval between the minimum and maximum times.

To enable router discovery advertisements

1. In the Routing and Remote Access snap-in, open the IP Routing container for the desired server.

2. Open the General container, right-click the desired interface, and click Properties .

3. From the General tab, select the Enable router discovery advertisements check box.

Windows 2000, with the Enable router discovery advertisements option selected, sends Router Advertisements periodically and in response to Router Solicitations using the IP multicast address of 224.0.0.1. TCP/IP for Windows 2000 and Windows 98 supports the use of Router Solicitation messages to discover the default gateway. For information about how to disable router discovery for TCP/IP for Windows 2000, see "Windows 2000 TCP/IP" in the TCP/IP Core Networking Guide .