+----------+  
                 | AS4(ISP4)|                       
                 +----------+       
                     /\   /\                        
                     /     \  P1[AS3 AS1]                                  
      P1 and P2 not /       \ P2[AS3 AS1]           
        propagated /         \ (C2P)                
          (C2P)   /           \                  
           +----------+    +----------+         
           | AS2(ISP2)|    | AS3(ISP3)|                        
           +----------+    +----------+       
                    /\           /\          
                     \           / P1[AS1]          
    P1[AS1] NO_EXPORT \         / P2[AS1]              
    P2[AS1] NO_EXPORT  \       / (C2P)     
                 (C2P)  \     /          
                     +----------------+ 
                     |  AS1(customer) |
                     +----------------+
                          P1, P2 (prefixes originated)
         
       Consider that data packets (sourced from AS1) 
       may be received at AS4 with a source address 
       in P1 or P2 via AS2:       
       * Feasible-path uRPF fails
       * Loose uRPF works (but not desirable)
       * Enhanced feasible-path uRPF with Algorithm A fails
       * Enhanced feasible-path uRPF with Algorithm B works best