You might use GRE in the following situations:

• Connect networks that are running non-IP protocols, such as native LAN protocols, across the public IP network. Non-IP protocols, such as Novell IPX or Appletalk, are not routable across an IP network. A GRE tunnel allows you to create a virtual point-to-point link between two such networks over the public WAN.
• Route IPv6 packets across an IPv4 network, or connect any two similar networks across an infrastructure that uses different IP addressing.
• Encrypt multicast traffic. IPsec, which is a standard mechanism for providing security on IP networks, cannot encrypt multicast packets. However, multicast packets can be encapsulated within a GRE tunnel and then routed over a VPN connection, so that the encapsulated packets are protected by the IPsec tunnel.