Generic Routing Encapsulation is a protocol for Point-to-Point Protocol. The encapsulation of a variety of network layer protocol packet types inside IP tunnels is done by GRE. This is done by creating virtual point-to-point link to routers which are pointed over an IP internetwork. It is completely stateless protocol based. Soon after it is configured, the GRE tunnel interface comes up and stays up until a valid tunnel resource address or interface is up.
