MPLS domain
               <----------------------------------------------->

    +=======+                                  +=======+
    |MPLS-ID|                                  |MPLS-ID|
    +=======+  +-----+                 +-----+ +=======+ +-----+
               |     |   Forward as    |     |           |     |
               |PE-1 |   per ETH-ID    | P-2 +-----------+ PE-2|
Push   ----->  +-+---+        |        +---+-+           +-----+
ETH-ID           |      +-----+----+       |  \ Recognize
                 |      v          v       |   +-- ETH-ID
                 |   +-----+    +-----+    |
                 +---+     |    |     +----+
        +.......+    |ETH-1+----+ETH-2|   +=======+
        .MPLS-ID.    +-----+    +-----+   |MPLS-ID|
        +=======+                         +=======+
        |ETH-ID |         +.......+       |ETH-ID |
        +=======+         .MPLS-ID.       +-------+
                          +=======+
                          |ETH-ID |
                          +=======+
                       Ethernet domain
                     <---------------->