Simple definition
Layer 2 Tunneling Protocol transports Layer 2 sessions across IP networks and is often combined with IPsec when encryption is needed.
Purpose and use
L2TP, Layer Two Tunneling Protocol, carries link-layer sessions, including PPP, between tunnel endpoints.
How does it work?
L2TPv2 over IP commonly uses UDP 1701. Tunnel signaling and carried sessions have separate identifiers.
Example and verification
For L2TP/IPsec remote access, separate checks of the L2TP tunnel from IPsec establishment and transport checks.
Points to watch
L2TP does not itself encrypt payloads. Allowing only UDP 1701 does not ensure that an L2TP/IPsec VPN can operate.
In short
The tunnel organizes session transport; confidentiality depends on the protection used alongside it.