https://www.rfc-editor.org/rfc/rfc3931.html
RFC 3931: Layer Two Tunneling Protocol - Version 3 (L2TPv3)
tunneling protocolrfc3931layertwo
https://www.softether.org/index.php?title=4-docs/2-howto/9.L2TPIPsec_Setup_Guide_for_SoftEther_VPN_Server/6.Cisco_IOS_L2TPv3%2F%2F%2F%2FIPsec_Edge-VPN_Router_Setup&action=source
Cisco L2TPv3/IPsec Edge-VPN Router Setup - SoftEther VPN Project
vpn routerciscol2tpv3ipsecedge