81 GRE CONFIGURATIONWhen configuring GRE, go to these sections for information you are interested in:■ “GRE Overview” on page 1589■ “Configuring a GRE over IPv4 Tunnel” on page 1593■ “Configuring a GRE over IPv6 Tunnel” on page 1594■ “Displaying and Maintaining GRE” on page 1596■ “GRE over IPv4 Tunnel Configuration Example” on page 1596■ “GRE over IPv6 Tunnel Configuration Example” on page 1598■ “Troubleshooting GRE” on page 1600GRE OverviewIntroduction to GRE Generic routing encapsulation (GRE) is a protocol designed for performingencapsulation of one network layer protocol (for example, IP or IPX) over anothernetwork layer protocol (for example, IP). GRE uses the tunneling technology andserves as a Layer 3 tunneling protocol of virtual private network (VPN).A tunnel is a virtual point-to-point connection for transferring encapsulatedpackets. Packets are encapsulated at one end of the tunnel and decapsulated atthe other end.A packet transferred through a tunnel undergoes an encapsulation process and adecapsulation process. Figure 437 depicts the network used to illustrate these twoprocesses.Figure 437 IPX networks interconnected through the GRE tunnelEncapsulation process1 After receiving an IPX packet through the interface connected to IPX networkGroup1, Router A submits it to the IPX module for processing.2 The IPX module checks the destination address field in the IPX header todetermine how to route the packet.3 If the packet must be tunneled to reach its destination, Router A sends it to thetunnel interface.Novell IPX protocolGroup 1Router A Router BNovell IPX protocolGroup 2InternetGRE tunnel
