GRE

Generic Routing Encapsulation (GRE) is a tunneling protocol that was first developed by Cisco. It can encapsulate a wide variety of network layer protocols within virtual point-to-point or point to multipoint links over an IP network.

Some characteristics of a GRE tunnel include:

Links:

https://networklessons.com/cisco/ccie-routing-switching/how-to-configure-gre-tunnel-on-cisco-ios-router

https://datatracker.ietf.org/doc/html/rfc2784

https://datatracker.ietf.org/doc/html/rfc2890