I would like to make an extended LAN through a WAN without using a MPLS configuration.
SO what I am going to set look like this :
LAN 1 --------WAN ------------- LAN 2
The two LANs are using the same network addresses.
Is there any solution to make this working by using for example GRE, NAT or something else?
That link is a bit out of date as far as support goes. There are a number of feature sets for the 2811 and 3825 routers that support L2TPv3 eg. Advanced IP Services, Enterprise Services.
Have a look at Feature Navigator - http://tools.cisco.com/ITDIT/CFN/jsp/index.jsp and select by feature and you will see which versions are available for your routers.