The Concept: In legacy site to site (S2S) VPN’s we are used to defining crypto maps and applying them to a physical interface. However, since these does not utilize GRE, you have no way of supporting multicast and routing protocols.
L2TPv3 – A wormhole through your L3 network.
Using L2 tunnel protocol version 3, you can accomplish what so many scientists are working on. A Wormhole! Granted, this wormhole is just through your L3 network, so it might not raise as many eyebrows, but its still a nifty tool in your toolbox.