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GRE

(Generic Routing Encapsulation)

GRE Overview

GRE (Generic Routing Encapsulation) is the mechanism that encapsulate one protocol in another protocol and provides connection between the nodes. This is done basically IP over IP. A Service Provider Network or Internet is used for GRE (Generic Routing Encapsulation).

GRE provides this communication over a Tunnel.This is called GRE Tunnel. This Tunnel means that there is a transport protocol and a passenger protocol. Tunnel End Points know this passenger protocol but between the end points, the tranportation is done with transport protocol.

With this tunnels, a virtual network is build over a public physical network and different branches of the same company, can cummunicate eachother.

GRE Tunnel Configuration Parameters

GRE Tunnel configuration is a very simple configuration. To configure a GRE Tunnel, we need two routers that we want to communicate. In thsese routers, firstly, we need to create a Tunnel interface and then we add Tunnel IP Address to this interface. This address will be our Tunnel’s one end IP address. This IP can also be called as passenger IP. After configuring passenger IP, we need to define physical source and destination of our Tunnel. To do this, we will give the real interface IP’s of Tunnel start and end points.


gre-tunnel-topology

As you can see above, for a GRE Tunnel, there are two important IPs and you must not mix them. These IPs are:

Tunnel IP Addresses
Physical IP Addresses

GRE Header

For Tunnel building, a GRE Header is inserted into the packet. This GRE Header is inserted with new IP Address (Tunnel IP Address) after the IP Header. As you can see below, GRE Header is between two IP Header.


gre-header

GRE Header is a 5 Bytes header, that include 1 Bytes main parts and 4 more optional parts.
For detailed information about GRE Header, you can check Cisco’s Support Forum.

Where we use GRE?

GRE is used in many networks. Generally we can divide this usage into two. These are:

• With MPLS/L3VPN
• With 6PE/6VPE

With MPLS/L3VPN usage provides using GRE Tunnels whenever an un MPLS area in the network and you need to link MPLS enabled areas over this non MPLS areas. This GRE Tunnel usage can be divided into three :

• PE to PE Tunnelling
• P to P Tunneling
• PE to P Tunnelling


gre-usage-across-non-mpls

In every usage, the network that GRE Tunnel is used, is non MPLS. This can be between PEs, between Ps or between PE and P.

With 6PE/6VPE, GRE can also used. When a Service Provider has an IPv4 backbone and want to give IPv6 Service over this IPv4 backbone, GRE Tunnels can be used.


Other GRE (Generic Routing Encapsulation) Articles…

GRE (Generic Routing Encapsulation) – Overview
GRE (Generic Routing Encapsulation) – Cisco GRE Tunnel Configuration with Packet Tracer
GRE (Generic Routing Encapsulation) – Huawei GRE Tunnel Configuration

You can Reach Our "CCNA Lab Courses with Packet Tracer" on Udemy.!!!
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About the Author
Gokhan Kosem is a telecommunation and network engineer. His ambition to IP networks and end-to-end system installation made him to prepare this web-site. By sharing his experiences about various networking protocols beside different system installation experiences and Cisco, Juniper, Alcatel-Lucent devices configurations, he is aimed to be helpful for his collegues in all over the world. He is currently lives in Istanbul, Turkey.

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