In the compulsory tunnel model - incoming call, a tunnel is created without any action from the user and without allowing the user any choice.
As a result the user sends PPP packets to the ISP (LAC), which encapsulates them in L2TP and sends them in a tunnel to the LNS. In the compulsory tunneling cases, the ISP must be L2TP capable. In this model, the tunnel only extends across the segment of the PPP session between the ISP and the LNS.