access-A                                     access-B
           <----->    <-------- tunnel ---------->     <----->

              +---------+        ___  _        +---------+
End system    |  +----+ |       /   \/ \_      | +----+  | End system
    "A" -------F1+    | |      /         \     | |    +F2----- "B"
              |  |    +========+ IP/MPLS +=======+    |  |
              |  |SI-1| |      \__  Net._/     | |SI-2|  |
              |  +----+ |         \____/       | +----+  |
              |PE1      |                      |      PE2|
              +---------+                      +---------+