H3C SR6600 Configuration Manual page 82

Figure 4-7 L2TP call setup procedure
The setup procedure of an L2TP call is as follows:
2) A remote user on Host A places a PPP call.
3) Host A and the LAC (Router A) perform PPP LCP negotiation.
4) The LAC authenticates the remote user using the Password Authentication Protocol
(PAP) or Challenge Handshake Authentication Protocol (CHAP).
5) The LAC sends the authentication information (the username and password) to its
RADIUS server for authentication.
6) The LAC RADIUS server authenticates the user.
7) If the user passes authentication, the LAC initiates a tunneling request to the LNS.
8) If tunnel authentication is required, the LAC sends a CHAP challenge to the LNS. The
LNS returns a CHAP response and sends its CHAP challenge to the LAC. Accordingly,
the LAC returns a CHAP response to the LNS.
9) The tunnel passes authentication.
10) The LAC sends the CHAP response, response identifier, and PPP negotiation
parameters of the user to the LNS.
11) The LNS sends an access request to its RADIUS server for authentication.
12) The RADIUS server authenticates the access request and returns a response if the
user passes authentication.
13) If the LNS is configured to perform a mandatory CHAP authentication of the user, the
LNS sends a CHAP challenge to the user and the user returns a CHAP response.
14) The LNS resends the access request to its RADIUS server for authentication.
4-72