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User Manual - Configuration Guide (Volume 3)
Versatile Routing Platform
Chapter 3
Configuration of GRE
3-1
Chapter 3 Configuration of GRE
3.1 Brief Introduction to GRE Protocol
I. Brief introduction to the protocol
GRE (Generic Routing Encapsulation) protocol can encapsulate the datagram of some
network layer protocols (e.g. IP and IPX) and enable these encapsulated datagrams to
transmit in another network layer protocol (e.g. IP). GRE is the layer 3 tunnel protocol of
VPN (Virtual Private Network), that is, a technique called as Tunnel is adopted between
protocol layers. The tunnel is a virtual point-to-point connection and can be regarded as
virtual interface only supporting point-to-point connection in actual situation. The
interface provides a channel where the encapsulated datagram can be transmitted.
And it can also encapsulate and de-encapsulate the datagram at both ends of a tunnel.
It's necessary to encapsulate and de-encapsulate it when a message is transmitted on
the tunnel.
1) Encapsulation
As shown in figure VPN-3-6, after receiving IPX datagram, the interface connecting
“Novell group1” first delivers it to be processed by IPX protocol which checks the
destination address domain in IPX header and determines how to route the packet. 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, tunnel
port will perform GRE and then, the packet will be processed by IP module. After IP
header is encapsulated, the packet will be processed by the corresponding network
interface according to destination address and router table.
2) De-encapsulation
The de-encapsulation is opposite to the encapsulation. When an IP message is
received at Tunnel interface, its destination address is checked and the destination is
found to be this router, then the IP header will be removed and processed by GRE
protocol (examine the key, check sum or message serial number). Then after GRE
header is removed, it will be processed by IPX protocol in the same way as processing
an ordinary datagram.
The system receives a datagram to be encapsulated and routed, which is called a
payload. The payload is first encapsulated in the form of GRE to become a GRE
message. Then it is encapsulated in IP message. Thus the IP layer is in full charge of
forwarding the message. The IP protocol which is in charge of the forwarded is often
called delivery protocol or transport protocol.
The form of an encapsulated message is shown in the following figure: