EUMETCast Terrestrial mainly relies on native multicast. However, in those special cases where an NREN has not yet enabled native IP-multicast support, GEANT is able to provide GRE tunnel concentrators located within their infrastructure to which the end-users can initiate a GRE tunnel connection from their EUMETCast client.
The GRE (Generic Routing Encapsulation) tunnelling protocol was developed by Cisco Systems for encapsulating a wide variety of network layer protocols inside virtual point-to-point links over an IP network. GRE tunnels are used here to encapsulate the IP-multicast streams and transport them over a routed IP-unicast connection.
GRE tunnels build point-to-point TCP connections from the GEANT concentrator directly to the reception station. GRE tunnels are also used as a backup path to address temporary loss of native multicast to end-users and overcome islands of non-multicast enabled networks. The back-up GRE tunnel is manually activated when a temporal loss of multicast is detected.