# L2TP Overview

L2TP encapsulates layer 2 packets, such as PPP, for transmission across a network. An L2TP access concentrator (LAC), configured on an access device, such as an E Series router, receives packets from a remote client and forwards them to an L2TP network server (LNS), on a remote network.

You can configure your router to act as an LAC in pass-through mode in which the LAC receives packets from a remote client and then forwards them at layer 2 directly to the LNS.

The E Series router creates tunnels dynamically by using authentication, authorization, and accounting (AAA) authentication parameters and transmits L2TP packets to the LNS via IP/User Datagram Protocol (UDP). Traffic travels in an L2TP session. A tunnel is an aggregation of one or more sessions. Figure 7 and Figure 8 show the E Series router in typical LAC and LNS arrangements.

Figure 7: Using the E Series Router as an LAC

.

Figure 8: Using the E Series Router as an LNS

| Note: The E Series router does not support terminating both ends of a tunnel or session in the same router.