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The L2TP VPN connection may randomly disconnect when you change or apply an access rule on a VPN server that is running Forefront TMG 2010
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Consider the following scenario:
You create a Layer 2 Tunneling Protocol (L2TP) virtual private network (VPN) connection to a server. The server has Microsoft Forefront Threat Management Gateway (TMG) 2010 installed.
You change or apply an access rule by using the Forefront TMG Microsoft Management Console (MMC) snap-in window.
In this scenario, the L2TP VPN connection may randomly disconnect.
This issue occurs because the Routing and Remote Access service (RRAS) invalidates the L2TP VPN connection.
Forefront TMG uses RRAS to set the L2TP shared secret value. However, RRAS invalidates the L2TP VPN connection. Therefore, the L2TP VPN connection is disconnected every time that the configuration is reloaded.
To resolve this issue, install the software update that is described in the following Microsoft Knowledge Base (KB) article:
2433623 Software Update 2 for Microsoft Forefront Threat Management Gateway (TMG) 2010 Service Pack 1
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about software update terminology, 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