4000 Router Series Layer 2 - WAN Configuration Guide

Figure 17 NAS-initiated tunneling mode

A NAS-initiated tunnel has the following characteristics:

• The remote system only needs to support PPP, and does not need to support L2TP.

• Authentication and accounting of the remote system can be implemented on the LAC or the LNS.

Figure 18 Establishment process for NAS-initiated tunnels

As shown in Figure 18, the following workflow is used to establish a NAS-initiated tunnel:

1. A remote system (Host A) initiates a PPP connection to the LAC (Device A).

2. The remote system and LAC perform PPP LCP negotiation.

3. The LAC authenticates PPP user information of Host A by using PAP or CHAP.