H3C Technologies H3C SecPath F1000-E - Manual (Page 813)

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Figure 6 Typical L2TP network
Figure 7 depicts the setup procedure of an L2TP call in NAS-initiated mode.
Figure 7 L2TP call setup procedure
(1) Call setup
(2) PPP LCP setup
(3) PPP or CHAP
authenticaion
(4) Access request
(5) Access accept
(6) Tunnel setup
(7) CHAP authentication
(challenge/response)
(9) User CHAP response,
PPP negotiation parameter
(12) CHAP authentication twice (challenge/response)
(10) Access request
(11) Acesss accept
(13) Access request
(14) Acesss accept
(8) Authentication passes
(15) Authentication passes
Remote system
Host A
LAC
Device A
LNS
Device B
LAC
RADIUS server
LNS
RADIUS server
The setup procedure of an L2TP call is as follows:
1. The remote user (Host) makes a PPP call.
2. The remote user and the LAC (Device A) perform PPP LCP negotiation.
3. The LAC authenticates the remote user using the Password Authentication Protocol (PAP) or
Challenge Handshake Authentication Protocol (CHAP).