automatic-metric

SUMMARY

This article describes the Automatic Metric feature that is used in Windows XP for Internet Protocol (IP) routes.

MORE INFORMATION

A metric is a value that is assigned to an IP route for a particular network interface that identifies the cost that is associated with using that route. For example, the metric can be valued in terms of link speed, hop count, or time delay. Automatic Metric is a new feature in Windows XP that automatically configures the metric for the local routes that are based on link speed. The Automatic Metric feature is enabled by default, and it can also be manually configured to assign a specific metric.

The Automatic Metric feature can be useful when the routing table contains multiple routes for the same destination. For example, if you have a computer with a 10 megabit (Mb) network interface and a 100 Mb network interface, and the computer has a default gateway that is configured on both network interfaces, the Automatic Metric feature assigns a higher metric to the slower network interface. This feature can force all of the traffic that is destined for the Internet, for example, to use the fastest network interface that is available.

NOTE: Typically, Microsoft does not recommend that you add default gateways across disjoint networks. For example, edge servers, such as, Network Address Translation (NAT) and proxy servers, are typically configured to connect two or more disjoint networks: The public Internet and one or more private intranets. In this situation, you should not assign the default gateways on the private interfaces, as doing so may result in improper routing on your network.

The following table outlines the criteria that is used to assign metrics for routes that are bound to network interfaces of various speeds.

The following table lists the link speeds and metrics for computers that run Windows XP Service Pack 2.

The Automatic Metric feature is configured independently for each network interface in the network. This feature is useful in situations where you have more than one network interface of the same speed, for example, when each network interface has been assigned a default gateway. In this situation, you may want to manually configure the metric on one network interface, and enable the Automatic Metric feature to configure the metric of the other network interface. This setup can enable you to control the network interface that is used first in the routing of IP traffic.

In addition, the metric that is assigned to specific default gateways can be configured independently for each gateway. This setup enables a further level of control over the metric that is used for the local routes. For example, it is possible to enable the Automatic Metric feature to configure the routes that are assigned to the network interface, and at the same time to manually configure the metric that is assigned to the default gateways.

NOTE: If a metric is specified at the network interface level, but a gateway is added and configured for the Automatic Metric feature, the gateway can inherit the metric that is assigned to the network interface. For example, if you assign a metric of five at the network interface level, and then you add a gateway and leave the Automatic Metric feature checked for the gateway, the gateway is also assigned a metric of five.

The Automatic Metric feature is different from the Dead Gateway Detection feature that can force the network to switch the default gateways that are based on Transmission Control Protocol (TCP) retransmissions. Also, the Routing and Remote Access feature does not activate the Dead Gateway Detection feature. This activation is performed by the TCP/IP stack on the computer that initiates the TCP session.

For additional information, click the article number below to view the article in the Microsoft Knowledge Base:

205027 (http://support.microsoft.com/kb/205027/EN-US/) Dead Gateway Detection with RRAS and Demand Dial Connections

To configure the Automatic Metric feature:

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