Problem:

How to setup a GRE tunnel on a Cisco Router ?



Solution:


So, let’s warm up the new year with an easy tutorial on how to setup a GRE tunnel on a Cisco router.

Consider this scenario:
Router1 = 172.16.1.1
Router2 = 192.168.0.1

The routing between these routers are fixed so that they can reach each other, like on the internet.
Router2 will have the network 10.0.10.0/24 routed to it via a GRE tunnel.
The address on the tunnel interfaces will be 10.0.0.1 and 10.0.0.2 for Router1 and Router2 respectively.

Router1 configuration:

Router1(config)#interface Tunnel 0
Router1(config-if)#tunnel source 172.16.1.1
Router1(config-if)#tunnel destination 192.168.0.1
Router1(config-if)#tunnel mode gre ip
Router1(config-if)#ip address 10.0.0.1 255.255.255.252
Router1(config-if)#no shutdown
Router1(config-if)#exit
Router1(config)#ip route 10.0.10.0 255.255.255.0 10.0.0.2

Router1(config)#interface Tunnel 0
Router1(config-if)#tunnel source 192.168.0.1
Router1(config-if)#tunnel destination 172.16.1.1
Router1(config-if)#tunnel mode gre ip
Router1(config-if)#ip address 10.0.0.2 255.255.255.252
Router1(config-if)#no shutdown
Router1(config-if)#exit
Router1(config)#ip route 10.0.10.0 255.255.255.0 Null 0

You can now setup addresses within 10.0.10.0/24 on any interface you want and use them like as they were routed to your router directly.
The traceroute from Router2 to Router1 should look something like this:

Router2#traceroute 10.0.0.1

Type escape sequence to abort.
Tracing the route to 10.0.0.1

1 10.0.0.1 8 msec 8 msec 8 msec

Voila, we got routing over GRE!