H3C Technologies H3C SecPath F1000-E - Manual (Page 35)

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Figure 1 VPDN built by using L2TP
PPPoE/ISDN Internet
L2TP tunnel
Remote user
Remote branch
LAC LNS
Internal server
A VPDN built by using L2TP comprises three components:
• Remote system
A remote system is usually a remote user’s host or a remote branch’s routing device that needs to access
the VPDN network.
• LAC
An L2TP access concentrator (LAC) is a device that has PPP and L2TP capabilities. An LAC 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 using L2TP and then sends the resulting packets to the LNS. It
de-encapsulates packets received from the LNS and then sends the resulting packets to the intended
remote system.
The connection between an LAC and a remote system is a local connection or a PPP link. Usually, a PPP
link is used in a VPDN application.
• LNS
An L2TP network server (LNS) functions as both the L2TP server and the PPP end system. It is usually an
edge device on an enterprise network.
An LNS is the other endpoint of an L2TP tunnel and is a peer to the LAC. It is the logical termination point
of a PPP session tunneled by the LAC. The L2TP extends the termination point of a PPP session from a NAS
to an LNS, logically.
Basic Concepts of L2TP
Background of L2TP
PPP defines an encapsulation mechanism that allows a point-to-point link to carry packets of various
protocols. When PPP runs between a user and an NAS, the PPP session and the Layer 2 link both
terminates at the same physical device, that is, the NAS.
L2TP tunnels PPP packets and extends the PPP model by enabling the Layer 2 link endpoint and the PPP
session endpoint to reside on different devices that are interconnected by a packet-switched network. This
allows PPP sessions to cross a Frame Relay network or the Internet.
Combining the advantages of L2F and PPTP, L2TP is now the Layer 2 tunneling industry standard of the
Internet Engineering Task Force (IETF).