GRE
This section presents the following topics:
The Generic Routing Encapsulation (GRE) protocol provides a simple general-purpose mechanism for encapsulating a packet from a wide variety of network protocols to be forwarded over another protocol. The original packet (the “passenger” packet) can be one of many network protocols—for example, a multicast packet, an IPv6 packet, or a non-IP LAN protocol, such as AppleTalk, Banyan VINES, or Novell IPX. The delivery protocol can be one of a number of routable IP protocols.
A GRE tunnel is stateless, which means that the protocol does not automatically monitor the state or availability of other endpoints. You can, however, direct the router to monitor the far end of the tunnel by sending keep-alive messages. If the other end of the tunnel becomes unavailable, its failure to respond to the messages alerts the router.
GRE uses the IP protocol number 47.