Terminology Q. What is L2TP? A. L2TP, which stands for Layer 2 Tunneling Protocol, is an IETF standard emerging that combines Layer 2 Forwarding protocol (L2F) and Point-to-Point Tunneling protocol (PPTP). As of July 1998, it was nearing final-draft status on the way to becoming a standard. We expect L2TP to have a standard RFC soon. L2TP is a Cisco IOS Software feature that is a key building block of the Access virtual private (VPN). Access VPNs enable mobile workforces to connect to their corporate intranets or extranets wherever and whenever they require, improving productivity and flexibility while reducing costs. Q. What are LAC and LNS? A. L2TP Access Concentrator (LAC)—LAC is a device attached to a switched network fabric (such as, PSTN or ISDN) or colocated with a PPP end system capable of handling the L2TP protocol. A LAC device implements the media, over which L2TP passes traffic to one or more LNSs. The LAC may tunnel any protocol carried within PPP. LAC is the initiator of incoming calls and the receiver of outgoing calls. LAC is also known as NAS in Layer 2 Forwarding (L2F) terminology. L2TP Network Server (LNS)—LNS operates on any platform capable of PPP termination. It handles the server side of the L2TP protocol. Since L2TP relies on the single media over which L2TP tunnels arrive, an LNS may have only a single LAN or WAN interface, yet still be able to terminate calls arriving at any of the LACs’ full range of PPP interfaces (async ISDN). LNS is the initiator of outgoing calls and the receiver of incoming calls. Network Access Server (NAS)—NAS is a device providing temporary, on-demand network access to users. This access is point-to-point, typically using PSTN or ISDN lines. In the Cisco implementation a NAS serves as a LAC. Q. What is a client-initiated tunnel? A. Client-initiated tunnel is a tunnel initiated by client software on the PC. If a client initiated tunnel is used, PC is the LAC. Q. What is a NAS or SP initiated tunnel? A. NAS or SP initiated tunnel is a tunnel initiated from a NAS or a device from a SP location. For a NAS initiated tunnel, NAS is the LAC