This article was previously published under Q171564
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You may experience a loss of connectivity to computers across the routerwhen your primary gateway fails, even though your computer running WindowsNT is configured with multiple default gateways for redundancy.
Windows NT 4.0 supports dead gateway detection using the TriggeredReselection method described in RFC 816. This implementation can cause thecurrent default gateway to switch to the second or third configured gatewayvery easily; however, it does not test the gateway being switched to priorto 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 theMicrosoft Knowledge Base:
152734 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 gatewaydetection algorithm follows:
When any TCP connection that is routed through the default gatewayhas attempted to send a TCP packet to the destination a numberof times equal to one-half of the registry valueTcpMaxDataRetransmissions, but receives no response, the algorithmadvises IP to switch to the next gateway in the list.
The new algorithm, however, is more cautious about advising IP to switchgateways:
When any TCP connection that is routed through the default gateway hasattempted to send a TCP packet to the destination a number of timesequal to one-half of the registry value TcpMaxDataRetransmissions, butreceives no response, the algorithm changes the Route Cache Entry (RCE)for that one remote IP address to use the next default gateway in thelist. When 25 percent of the TCP connections have moved to the nextdefault gateway, the algorithm advises IP to change the default gatewayfor the whole computer to the one that the connections are now using.
Example of the New Algorithm
Assume that there are currently TCP connections to 11 different IPaddresses that are being routed through the default gateway. Now assumethat the default gateway fails, that there is a second default gatewayconfigured, and that the value for TcpMaxDataRetransmissions is at thedefault of 5.
When the first TCP connection tries to send data, it will not receive anyacknowledgments. After the third retransmission, the RCE for that remote IPaddress will be switched to use the next default gateway in the list. Atthis point, any TCP connections to that one remote IP address will haveswitched over, but the remaining connections will still try to use theoriginal default gateway.
When the second TCP connection tries to send data, the same thing willhappen. Now, two of the 11 RCEs will point to the new gateway.
When the third TCP connection tries to send data, after the thirdretransmission, three of 11 RCEs will have been switched to the seconddefault gateway. Since, at this point, over 25 percent of the RCEs havebeen moved, the default gateway for the whole computer will be moved to thenew one.
At this point, that default gateway remains the primary one for thecomputer until it experiences problems (causing the dead gateway algorithmto 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 returnsto the beginning again.