3Com Switch 8800 Advanced Software V5 Configuration Guide

GRE Overview 407
3 The GRE module checks the key, checksum and sequence number, and then strips
off the GRE header and submits the payload to the IPX module.
4 The IPX module performs the subsequent forwarding processing for the packet.
n
Encapsulation and decapsulation processes on both ends of the GRE tunnel and
the resulting increase in data volumes will degrade the forwarding efficiency for
the GRE-enabled device to some extent.
GRE Applications GRE supports these types of applications:
■ “Multi-protocol communications through a single-protocol backbone” on
page 407
■ “Scope enlargement of the network running a hop-limited protocol” on page
407
■ “VPN creation by connecting discontinuous subnets” on page 408
Multi-protocol communications through a single-protocol backbone
Figure 125 Multi-protocol communications through a single-protocol backbone
In the example as shown in Figure 125, Group1 and Group2 are local networks
running Novell IPX, while Team1 and Team2 are local networks running IP.
Through the GRE tunnel between Router A and Router B, Group1 can
communicate with Group2 and Team1 can communicate with Team2. They will
not interfere with each other.
Scope enlargement of the network running a hop-limited protocol
Figure 126 Scope enlargement of the network
Novell IPX protocol
Group 1
Internet
IP protocol
Team 1
Novell IPX protocol
Group 2
IP protocol
Team 2
Router A Router B
GRE tunnel
IP network
IP network
IP network
Host A Host B
Router A
Router B Router C
Router D
GRE tunnel