How to Use Multiple Gateways with Windows NT TCP/IP (104660)

SUMMARY

If the default gateway of a Windows NT computer becomes unavailable, the computer cannot communicate with systems outside its own subnet (aside from those it has established connections with on remote subnets prior to failure).

For example, if Router A, the default gateway to Server2, which is on a different subnet, is out of commission when Client1 starts, Client1 is not able to connect to Server2. This is because by default, Windows NT uses only the default gateway specified during setup. So, even though there may be an alternate Router available to connect to Server2, Windows NT will not use it unless it is configured to use multiple default gateways.

MORE INFORMATION

Use the following procedure to configure Windows NT to use multiple gateways by default:

Open the Registry Editor (REGEDT32.EXE).

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

Use the following path:

      HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\ 
      Tcpip\Parameters

From the Edit menu, choose Add Value to create the following entry: AdditionalGateways
Set the Data Type to REG_MULTI_SZ.
The value should be set to a string representing the default gateway, either the dotted decimal format or a fully qualified domain name that can be resolved by either the HOSTS file or by Domain Name Service. For example:
```
      130.25.000.000
						
```
-or-
```
      rhino.microsoft.com
						
```
With the Data Type set to REG_MULTI_SZ, you can make multiple entries to represent several other default gateways.

NOTE: These entries should be added in order of preference.

When a Windows NT system is configured this way, retransmission problems at the TCP layer cause the IP routing software to try the routers in the AdditionalGateways list. In this example, if Client1 and Server2 had a session established and Router A went down, TCP would send a message to IP telling it to try one of the additional routers. IP would then try Router B, and use the double-hop route to continue exchanging data between Client1 and Server2. When Router A becomes available, the interrouting protocol forces Router B (the current first-hop router in the path from Client1 to Server2) to redirect Client1 to use Router A for traffic destined for Server2.

The ROUTE utility can be used to display the currently active default (or smart) gateway; the ROUTE -S PRINT command prints the currently active default gateway, and the ROUTE PRINT command prints all current routes in the routing tables, identifying all default gateways in the table in order of preference.

The ROUTE -S ADD command can be used to add additional default gateways after the system has initialized. Any gateways added in this way are appended to the existing list. The ROUTE command does not provide a way to order the gateways, so you must use ROUTE -F to flush all the routing table entries and then add entries in the order of preference. But any changes to the routing tables using the ROUTE utility are not added to the Registry, so to permanently affect the default gateway routing tables, you must add the AdditionalGateways value entry to the Registry.

NOTE: The registry value entry only applies to TCP applications. UDP applications cannot use alternate gateways because it is a connectionless datagram.