3Com 10014299 manual GRE Protocol, Encapsulation, Packet

44 CONFIGURING GRE

This chapter covers the following topics:

■GRE Protocol Overview

■Configuring GRE

■Displaying and Debugging GRE

■GRE Configuration Example

■Troubleshooting GRE

GRE Protocol	The Generic Routing Encapsulation (GRE) protocol encapsulates datagram of
Overview	network layer protocols, such as IP and IPX, and enables these encapsulated
	datagrams to transmit in another network layer protocol, such as IP. GRE is a Layer
	3 protocol that creates Virtual Private Network (VPN) tunnels. A tunnel is a virtual
	point-to-point connection and is a virtual interface that only supports
	point-to-point connections. It is necessary to encapsulate and de-encapsulate it
	when a message is transmitted on the tunnel. The interface provides a channel
	where the encapsulated datagram can be transmitted. The interface also
	encapsulates and de-encapsulates the datagram at both ends of a tunnel.
Encapsulation	As shown in Figure 198, after receiving an IPX datagram, the interface connecting
	“Group1” first delivers it to be processed by the IPX protocol which checks the
	destination address domain in the IPX header and determines how to route the
	packet.
	Figure 198 Typical networking diagram of GRE

IPX protocol		Internet		IPX protocol
Group1	Router A	Tunnel	Router B	Group2

If it is found that the destination address of the message will route through the network with network number 1f (virtual network number of the tunnel), the message will be transmitted to the tunnel port with network number 1f. After receiving the packet, the tunnel port will perform GRE and then, the packet will be processed by the IP module. After IP header is encapsulated, the packet will be processed by the corresponding network interface according to the destination address and router table.

De-encapsulationThe de-encapsulation is the opposite of encapsulation. When an IP message is received at a tunnel interface, its destination address is checked and if the router is the destination, then the IP header is removed and processed by the GRE protocol,