What is GRE tunnels?

GRE (Generic Routing Encapsulation) is a tunneling protocol that encapsulates network layer packets inside GRE packets. This allows routing protocols to be used over network paths that would not normally support them, creating a virtual point-to-point link to network devices at remote networks. It's commonly used to create VPNs or to carry multicast traffic over a network that doesn't support it natively. It provides a simple way to transport packets between two points over an IP network.