4.1.g Describe basic layer 2 VPN —wireline

4.1.g [i] L2TPv3 general principals

The Layer 2 Tunneling Protocol version 3 (L2TPv3) feature employs L2TPv3 and pseudowire (PW) technology to provide tunneling service to Ethernet traffic. A L2TPv3 tunnel is a control connection between two PE routers. One L2TPv3 tunnel can have multiple data connections , and each data connection is termed as an L2TPv3 session. The control connection is used to establish, maintain, and release sessions. Each session is identified by a session ID which is unique across the entire router. L2TPv3 carries frames inside IP packets.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4718-4719).  . Kindle Edition.

http://www.cisco.com/en/US/docs/ios/12_0s/feature/guide/l2tpv30s.html