+------------+  routes for P1, P2   +------------+
          | AS2(ISP-a) |<-------------------->| AS3(ISP-b) |
          +------------+        (P2P)         +------------+
                    /\                            /\
                     \                            /
               P1[AS1]\                          /P2[AS1]
                       \                        /
         P2[AS1 AS1 AS1]\                      /P1[AS1 AS1 AS1]
                         \                    /
                        +-----------------------+
                        |      AS1(customer)    |
                        +-----------------------+
                          P1, P2 (prefixes originated)

        Consider data packets received at AS2 via AS3
        that originated from AS1 and have a source address in P1:
        * Feasible-path uRPF works (if the customer route to P1
          is preferred at AS3 over the shorter path)
        * Feasible-path uRPF fails (if the shorter path to P1
          is preferred at AS3 over the customer route)
        * Loose uRPF works (but not desirable)
        * Enhanced feasible-path uRPF works best