pfSense

Shared secret on vpn_l2tp.php page is never used,
I don't see any code that uses it, and there is no "set l2tp secret" entry in mpd.conf

http://mpd.sourceforge.net/doc5/mpd48.html#48:

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.