3Com 3C13636 Configuration Manual page 1245

3Com Router 3000 Ethernet Family
Configuration Guide
When receiving a datagram needed encapsulating and routing, called payload, the
system first add a GRE header to the datagram to form a GRE packet. This GRE
packet is then encapsulated into an IP packet, thus allowing the IP layer to take full
charge of the forwarding of the packet. The IP protocol in this particular case is called
Delivery Protocol or Transport Protocol.
The format of an encapsulated tunnel packet is shown as follows:
Delivery Header
（Transport Protocol）
GRE Header
（Encapsulation Protocol）
Payload Packet
（Passenger Protocol）
Figure 3-2 Format of encapsulated tunnel packets
For instance, an IPX Delivery packet encapsulated in IP tunnel is as follows:
IP GRE
Figure 3-3 Format of Delivery packets in Tunnel
II. Application scope
GRE mainly provides services below:
1) Allowing a multi-protocol local network to make transmission through a
single-protocol backbone
Novell IPX
Protocol
Group1
IP
Term 1
Figure 3-4 Multi-protocol local network that makes transmission through a
single-protocol backbone
IPX
Passenger Protocol
Encapsulation Protocol
（Carrier Protocol）
Transport Protocol
Router A
Internet
Tunnel
3Com Corporation
3-2
Chapter 3 Configuration of GRE
Novell IPX
Protocol
Router B
Group2
IP
Term 2