What is the use of a tunnel interface on a cisco router?
Tunnel interfaces have many uses, including participating in a larger VPN configuration. A VPN setup usually has many parts, including encryption, authentication, routing, and finally, the tunneling. Tunneling is also used for IPv4/IPv6 coexistence setups, such as encapsulating IPv6 packets in IPv4 packets payloads, creating GRE tunnels, and multicast tunneling. The point is that while tunnels may be part of a VPN setup, they do not necessarily represent the entire VPN configuration, but only the traffic encapsulation between endpoints.