What is L2TP?
L2TP (Layer 2 Tunneling Protocol) is a tunneling protocol that enables transmission of Layer 2 packets over IP networks. It is used for building VPNs, encapsulating PPP, and in provider access schemes.
Where is L2TP used?
- In corporate VPNs (often with IPsec)
- By ISPs for remote PPP access
- In BRAS/BNG scenarios
- In DSL/FTTH networks as an alternative to PPPoE
How L2TP works
Architecture
- LAC (L2TP Access Concentrator) — device on the client or provider side
- LNS (L2TP Network Server) — server on the centralized processing side
Session stages
- Establish TCP/IP connection between LAC and LNS
- Create and authenticate the L2TP tunnel
- Transmit PPP traffic inside UDP
- Remote access or subscriber authentication
Advantages of L2TP
- Low overhead
- Broad hardware support
- Tunneling without encryption possible (useful for CGNAT and DPI)
Disadvantages
- No encryption without IPsec
- Vulnerable to spoofing attacks
- Performance may degrade with NAT
VPN Protocol Comparison
| Protocol | Encryption | Use Case | Features |
|---|---|---|---|
| L2TP | No (Yes with IPsec) | VPN, BNG | Easy to deploy |
| PPTP | Weak | Deprecated | Insecure, unsupported |
| OpenVPN | Strong | Corporate VPN | Flexible but complex to configure |
FAQ
Is L2TP a VPN?
Yes, but it does not encrypt data on its own. It is typically used with IPsec (then called L2TP/IPsec).
Does L2TP work behind NAT?
It’s tricky. Requires NAT-T support (UDP 1701/500/4500).
Why is L2TP better than PPPoE?
Lower overhead, easier to scale, supports session aggregation.
Conclusion
L2TP remains a relevant protocol in networks where Layer 2 tunneling over IP is required. It is easy to set up, scales well, and is widely supported, especially in ISP infrastructures.
