3Com Switch 8800 Family IPsec Module Configuration and Command reference Guide

130 CHAPTER 9: CONFIGURATION OF GRE
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:
Figure 29 Format of encapsulated Tunnel packets
For instance, an IPX Delivery packet encapsulated in IP Tunnel is as follows:
Figure 30 Format of Delivery packets in Tunnel
Application scope
GRE mainly provides services below:
1 Allowing a multi-protocol local network to make transmission through a
single-protocol backbone
Figure 31 Multi-protocol local network that makes transmission through a single-protocol
backbone
In the above figure, Group1 and Group2 are the local networks employing the
Novell IPX protocol; Term1 and Term2 are the local networks running IP. By setting
up a GRE Tunnel between 3Com A and 3Com B, you can allow Group1 to
Delivery Header
£¨ Transport Protocol £©
GRE Header
£¨ Encapsulation Protocol £©
Payload Packet
£¨ Passenger Protocol £©
IP GRE IPX
Passenger Protocol
˄Carrier Protocol˅
Encapsulation Protocol
Transport Protocol
Internet
Novell IPX
protocol
Group1
IP protocol
Term 1
Novell IPX
protocol
Group2
IP protocol
Term 2
Tunnel
Internet
Novell IPX
protocol
Group1
IP protocol
Term 1
Novell IPX
protocol
Group2
IP protocol
Term 2
Tunnel
Switch 8800 A
Switch 8800 B