The second layer tunneling protocol (L2TP) is used to integrate multi-protocol dial-up services to existing Internet service providers. PPP defines a packaging mechanism that spans the second layer point link link. In particular, the user is connected to the Network Access Server (NAS) by using one of the many technologies (such as dial POTS, ISDN, ADSL, etc.), and then runs PPP on this connection. In such a configuration, the second layer terminal point and the PPP session end point in the same physical device (eg NAS).
The L2TP extends the PPP model, allowing the second layer and the PPP end point to be differentiated by the package switched network. With L2TP, the user is connected to an access concentrator (eg, modem pool, ADSL DSLAM, etc.) in the second layer, and then this concentrator will separate PPP frame tunnel to NAS. Thus, the actual processing of the PPP package can be separated from the end of the L2 connection.
For more information, please visit the following page: 中文 版: http://www.networkDictionary.com/chinese/protocols/l2tp.php
English: http://www.networkDictionary.com/protocols/l2tp.php
