3Com 3C13636 Configuration Manual page 1211

3Com Router 3000 Ethernet Family
Configuration Guide
Control messages and data messages share the same type of packet headers. Tunnel
ID and Session ID are included in L2TP packet header, to identify different tunnels and
sessions. The packets with the same Tunnel ID but different Session IDs will be
multiplexed in the same tunnel. Tunnel ID and Session ID in the packet header are
assigned by peer ends.
IV. Two typical L2TP tunnel modes
The following figure shows the tunnel modes available between remote system or LAC
clients (hosts running L2TP) and LNS:
Remote System
Figure 2-3 Two typical L2TP tunnel modes
1) Initiated by remote dial-up user. Remote system dials in LAC via PSTN/ISDN. LAC
sends tunnel setup request to LNS through the Internet. Dial-up users' addresses
are assigned by LNS. The authentication and accounting of remote dial-up users
can be accomplished either by LAC side as an agent or by LNS side directly.
2) Initiated directly by LAC users (local users who support L2TP). In this case, LAC
users can send tunnel setup request directly to LNS, without requiring an
additional LAC device. LAC users' addresses are assigned by LNS.
V. Call setup flow of L2TP tunnel
Typical L2TP application network is as follows:
LAC client
LAC
Internet
PSTN/ISDN
Frame Relay
LAC
or ATM
3Com Corporation
2-4
Chapter 2 Configuration of L2TP
LNS
Internal Server
LNS
Internal Server