In this tutorial, we are going to illustrate how to successfully configure a GRE tunnel & run EIGRP over it, in order to dynamically route between two Cisco routers through a service provider cloud.
| SYNTAX | DESCRIPTION |
| router A | |
| conf t | Enter global configuration mode |
| router eigrp 20 | Turns on the EIGRP process and configures the AS number |
network 192.168.1.0 network 20.1.1.0 |
Enables EIGRP routing for the 192.168.1.0 and 30.1.1.0 networks, by allowing the exchange of EIGRP hellos & the forming of EIGRP neighbors |
| no auto-summary | Disables the default behavior of automatic summarization |
| router B | |
| conf t | Enter global configuration mode |
| router eigrp 20 | Turns on the EIGRP process & configures the AS number |
network 192.168.1.0 network 30.1.1.0 |
Enables EIGRP routing for the 192.168.1.0 and 30.1.1.0 networks, by allowing the exchange of EIGRP hellos & the forming of EIGRP neighbors |
| no auto-summary | Disables the default behavior of automatic summarization |
| SYNTAX | DESCRIPTION |
| router A | |
| conf t | Enter global configuration mode |
| interface tunnel 1 | Create GRE tunnel interface |
| ip address 192.168.1.1 255.255.255.252 | Configures IP address on tunnel interface |
| tunnel destination 12.1.2.2 | Define tunnel destination as router B’s serial IP |
| tunnel source 12.1.1.2 | Define tunnel source as router A’s serial IP |
| router B | |
| conf t | Enter global configuration mode |
| interface tunnel 1 | Create GRE tunnel interface |
| ip address 192.168.1.2 255.255.255.252 | Configures IP address on tunnel interface |
| tunnel destination 12.1.1.2 | Define tunnel destination as router B’s serial IP |
| tunnel source 12.1.2.2 | Define tunnel source as router A’s serial IP |
This should get EIGRP running and a EIGRP neighbor should form over the "directly connected" GRE tunnel interface.
In this example we need to set up a tunnel between router A & router B. Following this, we will configure the routers to run the EIGRP routing protocol & become EIGRP neighbors. The routers will then dynamically route all traffic destined to remote LAN subnet(s) via their respective EIGRP neighbor, which is also their GRE tunnel interface next hop address. This will allow the private LAN networks to be dynamically learned & advertised and allow all data packets to be encapsulated with a GRE header when traversing the WAN, thus hiding our internal source / destination addresses.
The first thing we will need to do on both routers is create the tunnel interfaces, the tunnel source, & destination addresses.
The tunnel source & destination addresses must be reachable through the service provider’s network in order for the tunnel to be established. However, the tunnel interface IPs do not need to be (probably better they are not) reachable directly from the service provider's internal routers.
Next, we will need to add the appropriate static routes to allow for the GRE tunnel to be established. This must be completed before we can establish EIGRP neighbors.
| SYNTAX | DESCRIPTION |
| router A | |
| conf t | Enter global configuration mode |
| route 12.1.2.2 255.255.255.255 12.1.1.1 | Route to get to tunnel destination (router B) |
| router B | |
| conf t | Enter global configuration mode |
| route 12.1.1.2 255.255.255.255 12.1.2.1 | Route to get to tunnel destination (router A) |
The last step is to configure EIGRP on both routers.
