ZyXEL Communications ZYWALL USG 1000 - EDITION 2 Manual page 202

Chapter 8 L2TP VPN Example
Click Security and select Advanced (custom settings) and click Settings.
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Figure 195 Connect L2TP to ZyWALL: Security
Select Optional encryption allowed (connect even if no encryption) and the
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Allow these protocols radio button. Select Unencrypted password (PAP) and
clear all of the other check boxes. Click OK. Click Yes if a screen pops up.
Figure 196 Connect L2TP to ZyWALL: Security > Advanced
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ZyWALL USG 1000 User's Guide