HP MSR Router Series Wan Access Configuration Manual page 82

4. The LAC sends the authentication information (username and password) to its RADIUS server
(RADIUS server A) for authentication.
5. RADIUS server A authenticates the user and returns the result.
6. The LAC initiates an L2TP tunneling request to the LNS (Device B) when the following conditions
exist:
The user passes the authentication.
The user is determined to be an L2TP user according to the username or the ISP domain to which
the user belongs.
7. If tunnel authentication is needed, the LAC and LNS send CHAP challenge messages to
authenticate each other before successfully establishing an L2TP tunnel.
8. The LAC and LNS negotiate to establish L2TP sessions.
9. The LAC sends PPP user information and PPP negotiation parameters to the LNS.
10. The LNS sends the authentication information to its RADIUS server (RADIUS server B) for
authentication.
11. RADIUS server B authenticates the user and returns the result.
12. If authentication succeeds and mandatory CHAP authentication is configured on the LNS, the LNS
authenticates the PPP user by sending a CHAP challenge. The PPP user replies with a CHAP
response.
13. The LNS sends the authentication information to RADIUS server B for authentication.
14. RADIUS server B authenticates the user and returns the result.
15. If the user passes the authentication, the LNS assigns a private IP address to the remote system
(Host A).
16. The PPP user can access internal resources of the enterprise.
In steps 12, 15, and 16, the LAC forwards packets for the remote system and LNS. Host A and LAC
exchange PPP frames, and the LAC and LNS exchange L2TP packets.
Client-initiated tunneling mode
As shown in
communicate with the LNS through the Internet. The LAC client can directly initiate a tunneling request to
the LNS without any dedicated LAC devices.
Figure 27 Client-initiated tunneling mode
A client-initiated tunnel has the following characteristics:
Figure 27, a remote system running L2TP (LAC client) has a public IP address to
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