Article ID: 171564 - View products that this article applies to.
This article was previously published under Q171564
You may experience a loss of connectivity to computers across the router when your primary gateway fails, even though your computer running Windows NT is configured with multiple default gateways for redundancy.
Windows NT 4.0 supports dead gateway detection using the Triggered Reselection method described in RFC 816. This implementation can cause the current default gateway to switch to the second or third configured gateway very easily; however, it does not test the gateway being switched to prior to making the change.
To resolve this problem, obtain the latest service pack for Windows NT 4.0 or Windows NT Server 4.0, Terminal Server Edition. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:
(http://support.microsoft.com/kb/152734/EN-US/ )How to Obtain the Latest Windows NT 4.0 Service Pack
Microsoft has confirmed that this is a problem in Windows NT 4.0 and Windows NT Server 4.0, Terminal Server Edition. This problem was first corrected in Windows NT 4.0 Service Pack 4.0 and Windows NT Server 4.0, Terminal Server Edition Service Pack 4.
The description and explanation of the Windows NT 4.0 dead gateway detection algorithm follows:
Example of the New AlgorithmAssume that there are currently TCP connections to 11 different IP addresses that are being routed through the default gateway. Now assume that the default gateway fails, that there is a second default gateway configured, and that the value for TcpMaxDataRetransmissions is at the default of 5.
When the first TCP connection tries to send data, it will not receive any acknowledgments. After the third retransmission, the RCE for that remote IP address will be switched to use the next default gateway in the list. At this point, any TCP connections to that one remote IP address will have switched over, but the remaining connections will still try to use the original default gateway.
When the second TCP connection tries to send data, the same thing will happen. Now, two of the 11 RCEs will point to the new gateway.
When the third TCP connection tries to send data, after the third retransmission, three of 11 RCEs will have been switched to the second default gateway. Since, at this point, over 25 percent of the RCEs have been moved, the default gateway for the whole computer will be moved to the new one.
At this point, that default gateway remains the primary one for the computer until it experiences problems (causing the dead gateway algorithm to try the next one in the list again) or until the computer is restarted.
When the end of the list of default gateways is reached, the search returns to the beginning again.
Article ID: 171564 - Last Review: November 1, 2006 - Revision: 2.2