Draytek Vigor2920 Series User Manual page 301

Available settings are explained as follows:
Item
IKE Authentication
Method
IPSec Security Method
After finishing all the settings here, please click OK to save the configuration.
Description
This usually applies to those are remote dial-in user or node
(LAN-to-LAN) which uses dynamic IP address and
IPSec-related VPN connections such as L2TP over IPSec and
IPSec tunnel.
Pre-Shared Key -Currently only support Pre-Shared Key
authentication.
Pre-Shared Key- Specify a key for IKE authentication
Confirm Pre-Shared Key- Retype the characters to confirm
the pre-shared key.
Medium - Authentication Header (AH) means data will be
authenticated, but not be encrypted. By default, this option is
active.
High (ESP) - 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.
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Vigor2920 Series User's Guide