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.

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.