Supporting Generic Routing Encapsulation Using Tunnel Interfaces
You can configure a tunnel interface to support Generic Routing Encapsulation version 1 (GREv1) encapsulation. When enabled, the interface encapsulates IP packets in the tunnel in IPv4 packets using GREv1. You must specify the key parameter to append the value to outgoing packets (incoming packets must have this value too).
You can use GRE to forward multicast packets through non-multicast aware routers and devices.