VPDN applications are more flexible in a diversity of networking schemes. In certain networking scenarios, a session may travel multiple tunnels before reaching the destination. The tunnel switching technology is therefore introduced.
As shown in Figure 1, TUNLSW (DeviceB) is deployed between the LAC and the LNS.
PC accesses the Headquarter in the following process:
PC sends a request to the LAC for the establishment of a tunnel destined for TUNLSW rather than the LNS ( DeviceC). A tunnel and then a session are established between the LAC and TUNLSW.
Detecting that the endpoint of the session is the LNS, TUNLSW sends a request to the LNS for the establishment of a tunnel.
TUNLSW switches the session along the tunnel between TUNLSW and the LAC to the tunnel destined for the LNS.
When the session reaches the LNS, the session is terminated and a PPP connection is established. In this manner, PC can access the Headquarter.
In this process, TUNLSW switches the session from the tunnel between the LAC and TUNLSW to the tunnel between TUNLSW and the LNS. That is, TUNLSW performs the session switching over tunnels. Like a passenger transferring between buses, the preceding session needs to be switched to another tunnel to reach the destination. TUNLSW is such a transfer device for a session and can switch the session through the tunnel switching technology. Figure 1 shows a simple networking scheme, whereas in practice, the session may be switched for multiple times before reaching the LNS.
TUNLSW needs to be configured with two L2TP groups. One L2TP group, functioning as the LNS, receives the request initiated by the LAC for establishing a tunnel; the other L2TP group, functioning as the LAC, initiates a request for establishing a tunnel to another TUNLSW or LNS.
Multiple protocols, including L2TP, can implement tunnel switching. Tunnel switching implemented through L2TP is also called multi-hop L2TP.