GRE Tunnel used as a OSPF Virtual-Link
Share this page:GRE - Generic Routing Encapsulation, is a method of tunneling data from one router to another. Packets of one routing protocol - PASSANGER PROTOCOL are being encapsulated into another routing protocol - TRANSPORT PROTOCOL.
In this example there is a need to establish the connectivity between some OSPF Areas that are not connected to the Area 0, and we do not want to use the Virtual Links.
GRE is a pretty simple concept, you need to configure the following on the BOTH ENDS of the tunnel:
1. Create a Tunnel Interface and assign the IP Address
** (config)#int tunnel 1**
** (config-if)#ip add 172.25.185.3 255.255.255.0**
2. Define the SOURCE and the DESTINATION of the tunnel
** (config-if)#tunnel source 100.10.34.3**
** (config-if)#tunnel destination 100.10.34.4**
If we are using OSPF then the Tunnel subnet needs to be advertised with the “network” command on both ends of the tunnel.
** (config-router)#network 172.25.185.0 0.0.0.255 area 0**
*The IP Address of the Tunnel MUST be advertised into Area 0!!!
You will see that the OSPF Neighbor will be formed on the Tunnel 1 interface.
**Cisqueros_R4#show ip ospf neighbor **
Neighbor ID Pri State Dead Time Address Interface
3.3.3.3 0 FULL/ - 00:00:38 172.25.185.3 Tunnel1 <— GRE-Neighbor
3.3.3.3 0 FULL/ - 00:00:38 100.10.34.3 Serial1/0.43
5.5.5.5 1 FULL/DR 00:00:38 100.10.45.5 GigabitEthernet5/12