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.

Figure 5-1. LAC to LNS Tunneling Model

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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.

Figure 5-2. LNS to LNS Tunneling Model

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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.

Figure 5-3. LAC to LAC Tunneling Model

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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.