H3C Technologies H3C SecPath F1000-E - Manual (Page 812)

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same tunnel ID but different session IDs are multiplexed to the same tunnel. The tunnel ID and session ID
in a header are those of the intended receiver, not the sender.
L2TP Tunnel Modes and Tunnel Establishment Process
Two typical L2TP tunnel modes
There are two typical L2TP tunnel modes: NAS-initiated and client-initiated.
• NAS-initiated
In this mode, a remote system dials in the LAC through a PPPoE/ISDN network, and the LAC initiates a
tunneling request to the LNS over the Internet, as shown in Figure 4 . The LNS will assign the remote system
a private IP address. Authentication and accounting of the remote system can be implemented on the
LAC by an agent or on the LNS.
Figure 4 NAS-initiated tunnel mode
• Client-initiated
In this mode, after obtaining the access right to the Internet, a remote system running the L2TP client
software (LAC client) initiates a tunneling request to the LNS directly without requiring a separate LAC.
The LNS will assign the LAC client a private IP address. An LAC client needs a public IP address to
communicate with the LNS directly through the Internet.
Figure 5 Client-initiated tunnel mode
L2TP tunnel establishment process
Figure 6 shows a typical L2TP network.