250Configuring L2TPAbout L2TPThe Layer 2 Tunneling Protocol (L2TP) is a Virtual Private Dialup Network (VPDN) tunneling protocol.L2TP sets up point-to-point tunnels across a public network (for example, the Internet) and transmitsencapsulated PPP frames (L2TP packets) over the tunnels. With L2TP, remote users can access theprivate networks through L2TP tunnels after connecting to a public network by using PPP.As a Layer 2 VPN technology, L2TP provides a secure, cost-effective solution for remote users toaccess private networks.Typical L2TP networkingFigure 73 L2TP network diagramAs shown in Figure 73, 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 devicethat needs to access the private network.• LAC—An L2TP access concentrator (LAC) is both PPP and L2TP capable. It is usually anetwork access server (NAS) located at a local ISP, which provides access services mainly forPPP users.An LAC is an endpoint of an L2TP tunnel and lies between an LNS and a remote system. Itencapsulates packets received from a remote system by using L2TP and then sends theencapsulated packets to the LNS. It decapsulates packets received from the LNS and thensends the decapsulated packets to the intended remote system.• LNS—An L2TP network server (LNS) is both PPP and L2TP capable. It is usually an edgedevice on an enterprise network.An LNS is the other endpoint of an L2TP tunnel. It is the logical termination point of a PPPsession tunneled by the LAC. L2TP extends the termination point of a PPP session from a NASto an LNS by establishing a tunnel.L2TP message types and encapsulation structureL2TP uses the following types of messages:
