According to RFC2661 section 4.4.5 can we have optional support for Proxy LCP authentication allowing PPP to continue without renegotiation of LCP on LNS?
That could be an option at [l2tp] section which would allow using attributes
Proxy Authen Id
Proxy Authen Challenge
Proxy Authen Response
for authentication if they are present in ICCN message from LAC.