Aggresive mode is less secure then main mode. But still secure enough, if the preshared key is a long random string.
AFAIK you have to use aggressive mode if you want to establish a connection/accept a connection from an unknown IP Address.

The authentication of the peer is not done based on the IP of the peer, but on a string. Draytek calls this "Local ID"

I've read through these guides to setting up a VPN to a 3G router, but unfortunately they all describe a Vigor-to-Vigor VPN.

I'm trying to establish a connection from a Vigor 2110n using a 3G dongle, to a Sonicwall at head office.

The Sonicwall supports IPSec Aggressive mode, but it expects the remote firewall to provide the correct 'Remote ID' (i.e. the firewall identity of the Sonicwall) and Local ID on connection.

I can specify the Local ID in the VPN profile of the Vigor (and the corresponding profile on the Sonicwall), but there's nowhere to provide the Remote ID.

Has anyone managed to achieve this with a Vigor 2110n? If so, can you please provide some advice.

Thanks

do you missed this ?
http://www.forum.draytek.co.uk/viewtopic.php?f=8&t=16229