RFC 3193 Securing L2TP using IPsec November 2001 standardize end-to-end security. When end-to-end security is required, it is recommended that additional security mechanisms (such as IPsec or TLS []) be used inside the tunnel, in addition to L2TP tunnel security.
VPDN Configuration Guide, Cisco IOS XE Everest 16.6 - L2TP Apr 02, 2020 MikroTik VPN Comparison – RFC Apr 08, 2015 rfc:rfc4087 [GENWiki] Thaler Standards Track [Page 1] RFC 4087 IP Tunnel MIB June 2005 Extension MIB modules may be designed for managing protocol-specific objects. Likewise, extension MIB modules may be designed for managing security-specific objects (e.g., IPsec [RFC2401]), and traffic conditioner [RFC2474] objects. RFC 3438 - Layer Two Tunneling Protocol (L2TP) Internet
RFC 3931 - Layer Two Tunneling Protocol - Version 3 (L2TPv3)
Layer 2 Tunneling Protocol - Wikipedia
L2TPv3 (Layer Two Tunneling Protocol Version 3) is a point-to-point layer two over IP tunnel. This means you can tunnel L2 protocols like Ethernet, Frame-relay, ATM, HDLC, PPP, etc. over an IP network.
M Series,MX Series,T Series. The L2TP network server (LNS) sends RADIUS authentication requests or accounting requests. Authentication requests are sent out to the authentication server port.