H3C Technologies H3C SecPath F1000-E - Manual (Page 810)

2
PPPoE/ISDN Internet
L2TP tunnel
Remote user
Remote branch
LAC LNS
Internal server
A VPDN built by using L2TP consists of three components:
• Remote system
A remote system is usually the host of a remote user or the routing device of a remote branch that needs
to access the VPDN network.
• LAC
An L2TP access concentrator (LAC) is a device that is attached to a packet-switched network and has a
PPP end system and the L2TP capability. An LAC is usually a NAS located at a local ISP, which provides
access services mainly for PPP users.
An LAC lies between LNSs and remote systems. Upon receiving a packet from a remote system, it
encapsulates the packet by using L2TP and sends the encapsulated packet to the LNS. Upon receiving a
packet from an LNS, it decapsulates the packet and sends it to the intended remote system.
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) is a PPP end system as well as the L2TP protocol server. It is usually an edge
device of an enterprise network.
As an end system of an L2TP tunnel, an LNS is the peer of an LAC. It is the logical termination point of
a PPP session that is tunneled by the LAC. That is, with L2TP, the PPP termination point of a remote system
is logically extended from the LAC to the LNS, which resides on the enterprise network.
Basic Concepts of L2TP
Background of L2TP
The point-to-Point Protocol (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 a NAS, the PPP session terminates
at the same physical device where the Layer 2 link terminates, that is, the NAS.
L2TP (RFC 2661) is intended to tunnel PPP packets. It extends the PPP model by allowing the Layer 2 link
and the PPP session endpoints to reside on different devices interconnected by a packet-switched network.
This makes PPP sessions be able to traverse frame relay networks or the Internet.
Combining the advantages of L2F and PPTP, L2TP has become the Layer 2 tunneling industry standard of
the Internet Engineering Task Force (IETF).