How to configure GRE tunnel between Cisco IOS and Linux

Let’s have the following scenario:

Both nodes are connected through the internet, here in my example, using private IP.

On the Cisco device, Fa0/0 is the WAN interface with the following configuration:

interface Tunnel1  description TO-LINUX-SERVER  ip address 192.168.10.2 255.255.255.252  ip mtu 1436  tunnel source Fastethernet 0/0  tunnel destination 172.16.0.2

Where tunnel source is the IP of the Cisco router, tunnel destination is the IP of the Linux server and 192.168.10.2 is the IP of the tunnel.

On the Linux server ETH0 is the WAN interface which connected to the internet

modprobe ip_gre ip tunnel add gre_tun0 mode gre remote 10.0.0.2 local 172.16.0.2 ttl 255 ip tunnel ls ip link ls dev gre_tun0 ip link set gre_tun0 up ip link ls dev gre_tun0 ip addr add 192.168.10.1/30 dev gre_tun0 ip addr ls dev gre_tun0 ifconfig gre_tun0 mtu 1380

Modeprobe runs the module for GRE; ip tunnel creates the tunnel with gre_tun0 as the name for the tunnel. Remote and local are the same as source and destination.

In order to configure the GRE tunnel permanently and to make sure it will be configured after reload of the server use the following:

Vi /etc/rc.local

And add the configuration to the file:

#!/bin/sh # # This script will be executed *after* all the other init scripts. # You can put your own initialization stuff in here if you don't # want to do the full Sys V style init stuff. touch /var/lock/subsys/local modprobe ip_gre #GRE Tunnel CONFIG modprobe ip_gre ip tunnel add gre_tun0 mode gre remote 10.0.0.2 local 172.16.0.2 ttl 255 ip tunnel ls ip link ls dev gre_tun0 ip link set gre_tun0 up ip link ls dev gre_tun0 ip addr add 192.168.10.1/30 dev gre_tun0 ip addr ls dev gre_tun0 ifconfig gre_tun0 mtu 1380 ~                                                                                                        ~                                                                                                        ~                                                                                                         ~                                                                                                        ~                                                                                                         "/etc/rc.local" 32L, 1051C

Don’t forget to type wq! to save the file.

Remember that GRE stands for General Routing Encapsulation and It’s not encrypting or protecting the data from eavesdrop eyes.  The encapsulation itself adds 24 bytes to the IP packet and (4 bytes for the GRE protocol and 20 bytes for one more IP header)