Mahmood

Just to add to Andrew's link. A GRE tunnel is basically a Point-to-point tunnel usually from one router to another eg.

router1 -> router2 -> router3 -> router4

You could create a GRE tunnel between router1 and router4 as long as router1 can reach router4 via IP and router4 can reach router1 via IP. So packets enter the tunnel on router1 and come out of the tunnel on router4.

There are a number of uses for GRE tunnels, probably the most common is to transport non-IP protocols across an IP network. So using the example above, lets say that router1 and router4 both run IPX but router2 and router3 don't and can't. You want IPX packets to be exchanged between router1 and router4 but because router2 and router3 can't run IPX you have a problem.

The solution is to create a GRE tunnel between router1 and router4. IPX packets travelling from router1 to router4 are encapsulated inside an IP header and are sent across the IP network. When they reach router4 they are de-encapsulated and they are now IPX packets again.

Hope this helps.

Jon

Also one of the uses is to make a Dynamic routing over Internet and a VPN tunnel possible.

You can have a Leased line and also an Internet backup advertised in your OSPF domain.

Mainly it is used to create a point to point link which simplifies the topology as the tunnel can cross a large network.