L2TP access controller
The Layer 2 Tunneling Protocol (L2TP) access concentrator (LAC) is a device that acts as one endpoint to an L2TP tunnel. The LAC sits between the corporate network L2TP server and the remote client and is responsible for relaying data between them.
The LAC is typically found as a concentrating node in an ISP network into which remote clients dial. For example, remote users for the fictional company MegaCorp have a dial-in number for their corporate virtual private network (VPN). Included with this number is an end-user user name and a password similar to the normal dial access into an ISP requires.
All of these remote users dial into a LAC, which has a tunnel set up between itself and the corporate network L2TP server. As callers dial into the LAC and receive authentication, the LAC bundles their calls into the existing tunnel.