ZyXEL Communications ZyWALL USG Series Application Note page 146

Click IPSec Settings.
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Select the Use pre-shared key for authentication check box and enter the pre-shared key used
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in the VPN gateway configuration that the ZyWALL is using for L2TP VPN. Click OK.
Click Networking. Select L2TP IPSec VPN as the Type of VPN. Click OK.
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