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Consider the following scenario. You install a third-party virtual private network (VPN) adapter on a Windows Vista-based computer. Then, you establish a VPN connection by using the VPN adapter. In this scenario, the packets that are sent to the local computer are dropped. Therefore, some applications that use the VPN adapter have communication difficulties. Additionally, when other computers initiate connections, these connections may fail.
An error occurs when Windows Vista and the VPN software configure the local routing table. An incorrect route may prevent identification of the packets as packets whose destination is the local computer. Therefore, the packets are dropped.
Service pack information
To resolve this problem, obtain the latest service pack for Windows Vista. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
935791 How to obtain the latest Windows Vista service pack
A supported hotfix is available from Microsoft. However, this hotfix is intended to correct only the problem that is described in this article. Apply this hotfix only to systems that are experiencing this specific problem. This hotfix might receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next software update that contains this hotfix.
If the hotfix is available for download, there is a "Hotfix download available" section at the top of this Knowledge Base article. If this section does not appear, contact Microsoft Customer Service and Support to obtain the hotfix.
Note If additional issues occur or if any troubleshooting is required, you might have to create a separate service request. The usual support costs will apply to additional support questions and issues that do not qualify for this specific hotfix. For a complete list of Microsoft Customer Service and Support telephone numbers or to create a separate service request, visit the following Microsoft Web site:
Note The "Hotfix download available" form displays the languages for which the hotfix is available. If you do not see your language, it is because a hotfix is not available for that language.
No prerequisites are required.
You must restart the computer after you apply this hotfix.
Hotfix replacement information
This hotfix does not replace any other hotfixes.
The English version of this hotfix has the file attributes (or later file attributes) that are listed in the following table. The dates and times for these files are listed in Coordinated Universal Time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time item in Control Panel.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section. This problem was first corrected in Windows Vista Service Pack 1.
To provide loopback connectivity and to handle incoming packets, the TCP/IP protocol requires a local IP address route. This route has a netmask of 255.255.255.255, and the route points to the local address object. In Windows Vista, this route is configured by the TCP/IP protocol. Other components should not configure the route.
However, when certain VPN adapters are installed, the Routing and Remote Access service may configure the local IP address route to point to an IP address other than the local computer address. Therefore, the TCP/IP protocol cannot configure the route to point to the local address object. In this situation, the local address is not recognized. Therefore, all incoming packets that are sent to the local address are dropped. For more information about the standard terminology that is used to describe Microsoft software updates, click the following article number to view the article in the Microsoft Knowledge Base:
824684 Description of the standard terminology that is used to describe Microsoft software updates