https://www.juniper.net/documentation/us/en/software/junos/network-access-protocols/topics/topic-map/l2tp-configuring.html
Layer 2 Tunneling Protocol (L2TP) | Junos OS | Juniper Networks
Layer 2 Tunneling Protocol (L2TP) is a protocol for tunneling Layer 2 traffic over a Layer 3 network. You can use L2TP to enable Point-to-Point Protocol (PPP)...
layer 2 tunneling protocoljunos osl2tpjunipernetworks
https://www.rfc-editor.org/rfc/rfc5571.html
RFC 5571: Softwire Hub and Spoke Deployment Framework with Layer Two Tunneling Protocol Version 2...