Chapter 5. Troubleshooting L2TPv3 Based VPNs
The Layer Two Tunneling Protocol Version 3 (L2TPv3) is described in Internet Draft draft-ietf-l2tpext-l2tp-base. L2TPv3 provides a mechanism for tunneling Layer 2 frames such as PPP, HDLC, Frame Relay, and Ethernet over a packet-switched network. This is a key enhancement over L2TPv2 (as described in RFC 2661), which allows only the tunneling of PPP frames.
Internet Draft draft-ietf-l2tpext-l2tp-base discusses three L2TPv3 tunneling models:
An L2TP Access Concentrator (LAC) to L2TP Network Server (LNS) tunneling model
An LNS to LNS tunneling model
A LAC to LAC tunneling model
The LAC to LNS tunneling model (also described in RFC 2661) allows the tunneling of frames from a remote access client or system via a LAC to an LNS. Figure 5-1 illustrates this tunneling model.
The LNS to LNS tunneling model allows the tunneling of frames over a packet switched network, with termination of the connection on the LNSs themselves.
Figure 5-2 illustrates the LNS to LNS tunneling model implemented over an IP backbone.
In the LAC to LAC tunneling model, the LAC acts as cross connect forwarding frames received on an attachment circuit from a remote system over a packet switched network to a remote LAC.
The LAC to LAC tunneling model implemented over an IP backbone is shown in Figure 5-3.
The most common application of the LAC to LAC tunneling model is to provide pseudowire connections over a service provider backbone. These pseudowires can be used to build Layer 2 VPNs.
Note that a pseudowire is an emulated Layer 2 circuit that crosses a packet switched network. In this environment, the LACs can also be provider edge (PE) routers. Remote systems can be customer edge (CE) devices, such as routers.