R3303-HP HSR6800 Routers Layer 2 - WAN Configuration Guide

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• 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.
About L2TP
Protocol overview
PPP (RFC 1661) defines an encapsulation method for transporting multiprotocol diagrams over Layer 2
point-to-point links. When PPP runs between a user and the network access server (NAS), the Layer 2
connection termination point and the PPP session endpoint reside on the same physical device (NAS).
L2TP (defined in RFC 2661) is a tunneling protocol that transmits PPP frames. It extends the PPP model by
allowing the Layer 2 endpoint (LAC) that connect to users and the PPP endpoint (LNS) to reside on
different devices interconnected by a packet-switched network, such as the Internet.
L2TP combines the benefits of L2F and PPTP, becoming a Layer 2 tunneling protocol standardized by
IETF.
Protocol structure
Figure 22 shows the relationship between the PPP frame, control channel, and data channel. PPP frames
are transferred over unreliable L2TP data channels. Control messages are transferred within reliable L2TP
control channels.
Figure 22 L2TP protocol structure
Figure 23 L2TP packet encapsulation structure
Figure 23 depicts the encapsulation structure of an L2TP data packet between the LAC and the LNS.
Usually, L2TP data is transferred as UDP packets. The well-known UDP port for L2TP is 1701, though this
is only used in the tunnel creation stage. The L2TP tunnel initiator selects an idle port (not necessarily 1701)
to send a packet to port 1701 of the receiver. After receiving the packet, the receiver also selects an idle
port (not necessarily 1701 either) to return a packet to the specified port of the initiator. Then, the two
parties use the negotiated ports to communicate until the tunnel is disconnected.
L2TP tunnel and session
The following types of connections are present between an LNS and an LAC:
• Tunnel—A tunnel corresponds to a LNS-LAC pair, and comprises a control connection and one or
more sessions.