Typical L2TP networking
Figure 20: L2TP network diagram
As shown in Figure 20, a typical L2TP network has the following components:
Remote system—A remote system is usually a remote user's host or a remote branch's device that needs to access the private network.
LAC—An L2TP access concentrator (LAC) is both PPP and L2TP capable. It is usually a network access server (NAS) located at a local ISP, which provides access services mainly for PPP users.
An LAC is an endpoint of an L2TP tunnel and lies between an LNS and a remote system. It encapsulates packets received from a remote system by using L2TP and then sends the encapsulated packets to the LNS. It decapsulates packets received from the LNS and then sends the decapsulated packets to the intended remote system.
LNS—An L2TP network server (LNS) is both PPP and L2TP capable. It is usually an edge device on an enterprise network.
An LNS is the other endpoint of an L2TP tunnel. It is the logical termination point of a PPP session tunneled by the LAC. L2TP extends the termination point of a PPP session from a NAS to an LNS by establishing a tunnel.