BFR1(P) BFR2(P)             BFR1(P)  BFR2(P)
          |  \ /  |                    |       |
          |   X   |                    |       |
          |  / \  |                    |       |
     BFER1(PE)  BFER2(PE)        BFER1(PE)----BFER2(PE)
                                        
                                           ^ U-turn link

         Leaf BFER /               Non-leaf BFER /
          PE router                  PE router