R3166-R3206-HP High-End Firewalls VPN Configuration Guide-6PW101

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GRE configuration
GRE overview
Generic Routing Encapsulation (GRE) is a protocol designed for encapsulating and carrying the packets
of one network layer protocol (for example, IP or IPX) 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 dep
icts the
encapsulation and de-encapsulation processes.
Figure 1 X protocol networks interconnected through a 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.
GRE encapsulation format
Figure 2 Format of an encapsulated packet
As an example, Figure 3 shows the format of an X packet encapsulated for transmission over an IP tunnel.