L2TP client not able to use shared secret
Plus Target Version:
It is not possible to use Shared Secret by L2TP client,
no such field in WebGUI
set l2tp secret secret Sets the L2TP tunnel secret. Used to authenticate tunnel connection and encrypt important control packets avpairs. For server side, only one unique secret supported for every pair of listening IP (set l2tp self ...) and peer ip (set l2tp peer ...). If several secrets defined, only the first matching will be used for all incoming connections. NOTE: This options is not related with usual PPP authentication. Windows client does not support tunnel authentication.
see also #10527