-----------                 -----------                 -----------                    
|Node: S    |               |Node: I1   |               |Node: I2   |      
|Loopback:  |---------------|Loopback:  |---------------|Loopback:  |                          
|2001:db8::a|               |2001:db8::1|               |2001:db8::2|               
 -----------                 -----------                 -----------                     
      |                                                       | 
      |                      -----------                      |
      |                     |Node: D    |                     |
       ---------------------|Loopback:  |---------------------
                            |2001:db8::b| 
                             -----------