Dead Gateway Detection in TCP/IP for Windows NT

The information in this article applies to:

• Microsoft Windows NT Workstation version 3.5, 3.51, 4.0
• Microsoft Windows NT Server version 3.5, 3.51, 4.0

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

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. Microsoft TCP/IP would have to implement polling of the routing protocol, such as the Routing Information

Here is an example of how NT will behave when the current default gateway is unavailable, and you have more than one default gateway configured:

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 NT tries to access a network through the current default gateway, it will be informed of a better route (the original default gateway). 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 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.

NOTE: Specifications for dead gateway detection are found in RFC 1122 and RFC 816. Microsoft's TCP/IP stack implements the dead gateway detection described in RFC 816.

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

WARNING: Using Registry Editor incorrectly can cause serious, system wide problems that my require you to reinstall Windows NT to correct them. Microsoft cannot garantee that any problems resulting from the use of Registry Editor can be solved. Use this tool at your own risk.

   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)

For more information about dead gateway detection in Windows NT, refer to the following article in the Microsoft Knoledge Base:

TCP/IP Dead Gateway Detection Algorithm Updated for Windows NT[winnt]

ID: Q171564    CREATED: 16-JUL-1997   MODIFIED: 23-JUL-1997

REFERENCES

"Inter-Networking with Microsoft TCP/IP Using Microsoft Windows NT", Participant's Workbook, Microsoft WWT, 1993, pg. 72; Appendix A, pg. 217.