ZyXEL Communications ZYWALL USG 1000 - EDITION 2 Manual page 196

Chapter 8 L2TP VPN Example
11 Select Use this string to protect the key exchange (preshared key), type
password in the text box, and click Next.
Figure 181 IP Security Policy Properties: Authentication Method
12 Click Add.
Figure 182 IP Security Policy Properties: IP Filter List
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ZyWALL USG 1000 User's Guide