By encapsulating arbitrary packets inside a transport protocol, tun-neling provides a private, secure path through an otherwise public network. Tunnels connect discontinuous subnetworks and enable encryption interfaces, virtual private networks (VPNs), and Multiprotocol Label Switching (MPLS). If you have a Tunnel PIC installed in your router, you can configure unicast and multicast tunnels.
The JUNOS software supports the following tunnel encapsulations:
Generic route encapsulation (GRE)
IP over IP (IP-IP)
Virtual Private Network (VPN)
To configure a bidirectional unicast tunnel, configure the gr interface (to use GRE encapsulation) or the ip interface (to use IP-IP encapsulation) ...