When a VPN connection has been idle (no traffic) for 300 seconds, the
VPN server will check whether the client is still active. The
detection mechanism uses UDP packets on port 500.

The Windows Firewall blocks these packets. Because of this, the VPN connection will be terminated.
This problem can be solved by setting an "Exception" for these
packets.

You can do this as follows:

• In the Start menu, go to Control Panel -> Windows Firewall
• (in Vista) Click 'Change Settings'

• Go to the 'exceptions' tab

• Click 'Add Program'
• Click 'Browse' and go to c:\Program Files\Cisco Systems\VPN Client

• Select 'cvpnd.exe'
• Click 'Open'

• Click 'OK'

• Click 'OK'