GRE tunnels between Cisco and Linux routers using iproute

This article was first written in August 2004 for the BeezNest technical
website (http://glasnost.beeznest.org/articles/167).

GRE tunnels between Cisco and Linux (2.4 and up) routers are possible.

Given the following two routers, each one gateway for its network:

linux

Public IP: 82.72.101.226

Private network: 192.168.4.0/24

IP on that private network: 192.168.4.4

cisco

Public IP: 123.45.67.8

Private network: 192.168.10.0/24

IP on that private network: 192.168.10.4

See IP in IP or GRE tunnel using iproute for the Linux configuration part.

For the Cisco part, it is quite similar, as the needed portion of configuration is something like this:

interface Tunnel1
 description connection to Linux Network
 ip address 192.168.10.4 255.255.255.0
 tunnel source Dialer1
 tunnel destination 87.65.43.21
 tunnel mode gre ip

Where Dialer1 is the name of the interface of the Cisco router connected to Internet (it’s used to always match the actual and current public IP of the router).


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