3Com MSR 50 Series Configuration Manual page 1605

Figure 451 Typical L2TP network
PSTN/ISDN
Host A
Figure 452 shows the setup procedure of an L2TP call.
Figure 452 L2TP call setup procedure
LAC
Client
Router A
(1) Call setup
(2) PPP LCP setup
(3) PPP or CHAP
authenticaion
(4) Access request
(5) Access accept
(12) CHAP authentication twice (challenge/response)
(15) Authentication passes
Setup procedure of an L2TP call follows:
1 The remote user makes a PPP call.
2 The remote user and the LAC (Router A) perform PPP LCP negotiation.
3 The LAC authenticates the remote user using the password authentication
protocol (PAP) or challenge handshake authentication protocol (CHAP).
4 The LAC sends the authentication information (the username and password) to its
RADIUS server for authentication.
5 The LAC RADIUS server authenticates the user.
6 If the user passes authentication, the LAC initiates a tunneling request to the LNS.
RADIUS server
IP network
WAN
Router A
LAC
LAC
RADIUS server Router B
(6) Tunnel setup
(7) PPP or CHAP authentication
(challenge/response)
(8) Authentication passes
(9) User CHAP response,
PPP negotiation parameter
L2TP Overview
RADIUS server
IP network
Router B
LNS
LNS
LNS
RADIUS server
(10) Access request
(11) Acesss accept
(13) Access request
(14) Acesss accept
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Host B
Host C