Support for IPv4 Multicast Routing Over GRE Tunnels

PIM-DM and PIM-SM Layer 3 multicast protocols and multicast data traffic are supported over GRE tunnels. When a multicast protocol is enabled on both ends of a GRE tunnel, multicast packets can be sent from one tunnel endpoint to another. To accomplish this, the packets are encapsulated using the GRE unicast tunneling mechanism and forwarded like any other IPv4 unicast packet to the destination endpoint of the tunnel. The router that terminates the tunnel (that is, the router where the tunnel endpoint is an ingress interface) de-encapsulates the GRE tunneled packet to retrieve the native multicast data packets. After de-encapsulation, data packets are forwarded in the direction of its receivers, and control packets may be consumed. This creates a PIM-enabled virtual or logical link between the two GRE tunnel endpoints.