Layer 2 Tunneling Protocol - A Feature in Cisco IOS Software

A Key Building Block for an Access Virtual Private Network
Overview
Cisco is a leader in delivering technologies that transform Access Virtual Private Networks (VPNs) from a promising idea into a
practical reality. With the availability of Layer 2 Tunneling Protocol (L2TP) to Cisco IOS® software, Cisco offers a standard way
to provide Access VPN connectivity. Cisco’s end-to-end hardware and Cisco IOS software networking products provide
sophisticated security for sensitive private transmissions over the public infrastructure; quality of service through traffic
differentiation; reliability for mission-critical applications; scalability for supporting large bandwidth of data; and comprehensive
network management to enable a complete Access VPN solution.
VPNs enable today’s increasingly mobile workforce to connect to their corporate intranets or extranets whenever, wherever, or
however they require, improving productivity and flexibility while reducing access costs.
To provide a low-cost, easily accessible pathway to a corporate intranet or extranet, Access VPNs simulate a private network
—but over a shared infrastructure, such as the Internet. They offer access for mobile users, telecommuters, and small offices through
a range of technologies, including dial, ISDN, xDSL, mobile IP, and cable.
A key building block for Access VPNs is L2TP (Layer 2 Tunneling Protocol), an extension to the Point-to-point (PPP) protocol
and a fundamental building block for VPNs. L2TP merges the best features of two other tunneling protocols: Layer 2 Forwarding
(L2F) from Cisco Systems and Point-to-Point Tunneling (PPTP) from Microsoft. L2TP is an Internet Engineering Task Force (IETF)
emerging standard, currently under codevelopment and endorsed by Cisco Systems, Microsoft, Ascend, 3Com, and other
networking industry leaders.
Key L2TP Terms
L2TP Access Concentrator (LAC). A LAC device is attached to the switched network fabric, such as public switched telephone
network (PSTN) or ISDN or colocated with a PPP end system capable of handling the L2TP protocol. A LAC only needs to
implement the media, over which L2TP operates in order to pass traffic to one or more LNSs. It may tunnel any protocol carried
within PPP. LAC is the initiator of incoming calls and the receiver of outgoing calls. It is also known as the network access server
in Layer 2 Forwarding (L2F).