The transmission of packets in GRE tunnels can be divided into two processes: encapsulation and decapsulation. For example, the network shown in Figure shows the two processes.

Figure  Private IP network interconnection through GRE tunnels

• Encapsulation

  After receiving the IP packet from the interface that is connected to IP group1, the Eudemon A delivers the packet to the IP protocol module for processing.

  The IP protocol module checks the destination address field in the IP packet header, and decides how to route this packet. If the outgoing interface is the tunnel interface, the IP protocol module sends the packet to the tunnel module.

  After receiving the packet, the tunnel interface encapsulates it into a GRE packet, and then delivers GRE packet to the IP module. The IP module adds an IP header to the packet, and then delivers the packet to the corresponding network interface according to the destination IP address and the routing table.

• Decapsulation

  The process of decapsulation is opposite to the process of encapsulation. After receiving the packet from the interface that is connected to the public network, the Eudemon B analyzes the IP header. If it finds that itself is the destination of the packet, it delivers the packet to the GRE module for processing. After completing processing, the GRE module removes the IP header and the GRE header, and delivers the packet to IP protocol. IP protocol forwards the packet as the ordinary packet.

Super post. Nice way. thanks

I think it is from the documentation