3Com Switch 8800 Advanced Software V5 Configuration Guide

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GRE CONFIGURATION
When configuring GRE, go to these sections for information you are interested in:
■ “GRE Overview” on page 405
■ “Configuring a GRE over IPv4 Tunnel” on page 408
■ “Displaying and Maintaining GRE” on page 410
■ “GRE Tunnel Configuration Example” on page 411
■ “Troubleshooting GRE” on page 416
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■ Routers mentioned and router icons illustrated in the contents below represent
the general routers and Ethernet switches running routing protocols. To
simplify the description, this explanation will not be provided otherwise.
■ Currently, the products do not support configuring IS-IS, IPv6-IS-IS or
multicasting on tunnels.
GRE Overview
Introduction to GRE Generic routing encapsulation (GRE) is a protocol designed for performing
encapsulation of one network layer protocol (for example, IP or IPX) over another
network layer protocol (for example, IP). GRE uses the tunneling technology and
serves as a Layer 3 tunneling protocol of virtual private network (VPN).
A 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.
A packet transferred through a tunnel undergoes an encapsulation process and a
decapsulation process.
Figure 122 depicts the network used to illustrate these two
processes.
Figure 122 IPX networks interconnected through the GRE tunnel
Encapsulation process
1 After receiving an IPX packet through the interface connected to IPX network
Group1, Router A submits it to the IPX module for processing.
2 The IPX module checks the destination address field in the IPX header to
determine how to route the packet.
Novell IPX protocol
Group 1
Router A
Router B
Novell IPX protocol
Group 2
Internet
GRE tunnel