Westermo MRD-3x0 - Manual (Page 139)

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6.3.2 Layer 2 Tunnel Protocol
The Layer 2 Tunnel Protocol (L2TP) is an Internet Engineering
Task Force (IETF) standard which combines the best features
of two existing tunneling protocols, Layer 2 Forwarding (L2F)
developed by Cisco and the Point-to-Point Tunneling Protocol
(PPTP). L2TP can be viewed as an extension to the Point-
to-Point Protocol (PPP). One endpoint of an L2TP tunnel is
called the L2TP Network Server (LNS), the LNS waits for
new tunnels to be established. The other endpoint is called
the L2TP Access Concentrator (LAC), the LAC initiates tunnel
connections to the LNS, the MRD-3xx implements an L2TP
LAC. Once the L2TP tunnel has been established the traffic
over the tunnel is bidirectional.