A computer that is running Microsoft Internet Security and Acceleration (ISA) Server 2006 may randomly stop routing packets from certain VPN clients or from certain VPN site-to-site networks. This problem makes network destinations unreachable for affected clients and for affected branch office networks.
Note When this problem occurs, other VPN clients and other VPN site-to-site networks do not experience this symptom.
This problem may occur because the Microsoft Firewall service makes incorrect routing decisions for VPN virtual interfaces.
To resolve this problem, apply the hotfix rollup package that is described in the following Microsoft Knowledge Base article:
947257 Description of the Internet Security and Acceleration (ISA) Server 2006 hotfix package: January 8, 2008
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.