ZyXEL Communications ZyWall USG 300 User Manual page 210

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 204 IP Security Policy Properties: Authentication Method
12 Click Add.
Figure 205 IP Security Policy Properties: IP Filter List
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ZyWALL USG 300 User's Guide