Dead Gateway Detection in TCP/IP for Windows NT

This article was previously published under Q128978
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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 ornot the default gateway is still functioning. Dead gateway detection isenabled by default (on NT 3.51, 4.0 and above) when you install the TCP/IPprotocol. However, Microsoft TCP/IP does not provide a method to detectwhen the default gateway returns to operation.
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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 morethan one default gateway address in the Advanced Options of the TCP/IPconfiguration dialog box in the Network portion of Control Panel. If thedefault gateway fails to respond to TCP requests after several tries, IPwill direct packets to the address specified for a backup gateway.Here is an example of how Windows NT will behave when the current defaultgateway is unavailable, and you have more than one default gatewayconfigured:

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

When Windows NT tries to access a network through the current defaultgateway, it will be informed of a better route (the original defaultgateway). Windows NT will add a host route to it's routing table, but willNOT switch back to the original default gateway. To switch back to usingthe original default gateway you will need to reboot the Windows NT Server.

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

For information about obtaining a RFC document, please see the followingarticle 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 inthe 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, 4.0and 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
REFERENCES
"Inter-Networking with Microsoft TCP/IP Using Microsoft Windows NT",Participant's Workbook, Microsoft WWT, 1993, pg. 72; Appendix A, pg. 217.
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Article ID: 128978 - Last Review: 12/04/2015 11:15:29 - Revision: 2.1

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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