H3C Technologies H3C SecPath F1000-E - Manual (Page 737)

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GRE Configuration
GRE Overview
Introduction to GRE
Generic Routing Encapsulation (GRE) is a protocol designed for encapsulating and carrying the packets
of one network layer protocol (for example, IP) over another network layer protocol (for example, IP).
GRE is a tunneling technology and serves as a Layer 3 tunneling protocol.
A GRE tunnel is a virtual point-to-point connection for transferring encapsulated packets. Packets are
encapsulated at one end of the tunnel and de-encapsulated at the other end.
Figure 1 depicts the
encapsulation and de-encapsulation processes.
Figure 1 X protocol networks interconnected through the GRE tunnel
The following takes the network shown in Figure 1 as an example to describe how an X protocol packet
traverses the IP network through a GRE tunnel.
Encapsulation process
1. After receiving an X protocol packet through the interface connected to Group 1, Device A submits
it to the X protocol for processing.
2. The X protocol checks the destination address field in the packet header to determine how to route
the packet.
3. If the packet must be tunneled to reach its destination, Device A sends it to the tunnel interface.
4. Upon receipt of the packet, the tunnel interface encapsulates it in a GRE packet. Then, the system
encapsulates the packet in an IP packet and forwards the IP packet based on its destination
address and the routing table.
Format of an encapsulated packet
Figure 2 shows the format of an encapsulated packet.