3Com S7906E Configuration Manual page 501

A GRE tunnel is a virtual point-to-point connection for transferring encapsulated packets. Packets are
encapsulated at one end of the tunnel and decapsulated at the other end.
encapsulation and decapsulation processes.
Figure 1-5 X protocol networks interconnected through the GRE tunnel
The following takes the network shown in
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, Router 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, Router 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 1-6 shows the format of an encapsulated packet.
Figure 1-6 Format of an encapsulated packet
As an example, Figure 1-7
tunnel.
Figure 1-7 Format of an X packet encapsulated for transmission over an IP tunnel
These are the terms involved:
Payload: Packet that needs to be encapsulated and transmitted.
Figure 1-5
shows the format of an X packet encapsulated for transmission over an IP
1-7
as an example to describe how an X protocol
Figure 1-5 depicts the