Draytek Vigor2920 Series User Manual page 307

Item
IPSec Security Method
After finishing all the settings here, please click OK to save the configuration.
Description
Pre-Shared Key - Check the box of Pre-Shared Key to invoke
this function and type in the required characters (1-63) as the
pre-shared key.
Digital Signature (X.509) – Check the box of Digital
Signature to invoke this function and Select one predefined
Profiles set in the VPN and Remote Access >>IPSec Peer
Identity.
This group of fields is a must for IPSec Tunnels and L2TP
with IPSec Policy when you specify the remote node. Check
the Medium, DES, 3DES or AES box as the security method.
Medium-Authentication Header (AH) means data will be
authenticated, but not be encrypted. By default, this option is
invoked. You can uncheck it to disable it.
High-Encapsulating Security Payload (ESP) means payload
(data) will be encrypted and authenticated. You may select
encryption algorithm from Data Encryption Standard (DES),
Triple DES (3DES), and AES.
Local ID - Specify a local ID to be used for Dial-in setting in
the LAN-to-LAN Profile setup. This item is optional and can
be used only in IKE aggressive mode.
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Vigor2920 Series User's Guide