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

          Consider data packets received at AS2
          (1) from AS1 with a source address (SA) in P2, or
          (2) from AS3 that originated from AS1 with a SA in P1:
                    * Strict uRPF fails
                    * Feasible-path uRPF fails
                    * Loose uRPF works (but not desirable)
                    * Enhanced feasible-path uRPF works best