Microsoft KB Archive/128978

= Dead Gateway Detection in TCP/IP for Windows NT =

Article ID: 128978

Article Last Modified on 11/1/2006

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APPLIES TO


 * Microsoft Windows NT Workstation 3.5
 * Microsoft Windows NT Workstation 3.51
 * Microsoft Windows NT Workstation 4.0 Developer Edition
 * Microsoft Windows NT Server 3.5
 * Microsoft Windows NT Server 3.51
 * Microsoft Windows NT Server 4.0 Standard Edition

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This article was previously published under Q128978



IMPORTANT: This article contains information about modifying the registry. Before you modify the registry, make sure to back it up and make sure that you understand how to restore the registry if a problem occurs. For information about how to back up, restore, and edit the registry, click the following article number to view the article in the Microsoft Knowledge Base:

256986 Description of the Microsoft Windows Registry



SUMMARY
Microsoft TCP/IP for Windows NT provides a method of detecting whether or not the default gateway is still functioning. Dead gateway detection is enabled by default (on NT 3.51, 4.0 and above) when you install the TCP/IP protocol. However, Microsoft TCP/IP does not provide a method to detect when the default gateway returns to operation.



MORE INFORMATION
WARNING: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.

For the dead gateway detection to work correctly, you must specify more than one default gateway address in the Advanced Options of the TCP/IP configuration dialog box in the Network portion of Control Panel. If the default gateway fails to respond to TCP requests after several tries, IP will direct packets to the address specified for a backup gateway. Here is an example of how Windows NT will behave when the current default gateway is unavailable, and you have more than one default gateway configured:

Windows NT will switch to the next default gateway on the list. When the original default gateway comes back up, it broadcasts its routes to other intelligent routers.

When Windows NT tries to access a network through the current default gateway, it will be informed of a better route (the original default gateway). Windows NT will add a host route to it's routing table, but will NOT switch back to the original default gateway. To switch back to using the original default gateway you will need to reboot the Windows NT Server.

Note: If the second default gateway becomes unavailable, it will switch to the next default gateway configured on the list. When the end of the list of default gateways is reached, the search returns to the beginning again.

For information about obtaining a RFC document, please see the following article in the Microsoft Knowledge Base:

185262: How to Obtain Request for Comments Documents from the Internet

The registry setting that enables dead gateway detection can be found in the following location of the Windows NT 3.5 Registry:

Key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters

Value Name: EnableDeadGWDetect

Value Type: REG_DWORD

Value Range: 0 or 1 (False, True)

Default setting is 1 (True)

Note: Dead gateway detection is enabled by default on Windows NT 3.51, 4.0 and above. This registry change applies to Windows NT 3.5 only.

For additional information about dead gateway detection in Windows NT, click the article number below to view the article in the Microsoft Knowledge Base:

171564 TCP/IP Dead Gateway Detection Algorithm Updated for Windows NT

