(core networks)
                          |     |     |
                          |     |     |
                         R1    R2     R3
                          |     |     |
                       +=gi1===gi2===gi3=+
                       +                 +
                       +      switch     +
                       +                 +
                       +=gi4===gi5===gi6=+
                          |     |     |
                         H1    H2     H3