Hi,
I have been able to successfully configure it (although I have to say this is not a good design). Observe:
Router R1:
interface Loopback0
ip address 10.255.255.1 255.255.255.255
!
interface Tunnel1
ip address 192.168.12.1 255.255.255.0
tunnel source Loopback0
tunnel destination 10.255.255.2
!
interface Tunnel11
ip address 200.20.2.1 255.255.255.0
tunnel source Tunnel1
tunnel destination 192.168.12.2
!
interface FastEthernet0/0
ip address 10.1.12.1 255.255.255.0
!
ip route 10.255.255.2 255.255.255.255 10.1.12.2
Router R2:
interface Loopback0
ip address 10.255.255.2 255.255.255.255
!
interface Tunnel1
ip address 192.168.12.2 255.255.255.0
tunnel source Loopback0
tunnel destination 10.255.255.1
!
interface Tunnel11
ip address 200.20.2.2 255.255.255.0
tunnel source Tunnel1
tunnel destination 192.168.12.1
!
interface FastEthernet0/1
ip address 10.1.12.2 255.255.255.0
!
ip route 10.255.255.1 255.255.25.5255 10.1.12.1
These two routers are, for this lab purposes, directly connected via FastEthernet links. The first tunnel is Tun1, the second tunnel passing through the first one is Tun11. Notice Tun11 uses the Tun1's IP as its source, and the opposite Tun1's IP address as its destination, thereby existing exclusively in Tun1's namespace.
Best regards,
Peter