RPF strict/loose

RPF strict
   fib lookup of packet source address must return a match

RPF strict with allow default mode
   performs strict RPF check if source address lookup returns a regular FIB route, otherwise the default route will be used for strict RPF checking as well

RPF loose
  fib lookup of packet source address returns a "match" and the fib lookup result indicates that the source is reachable through at least one "real" interface

RPF loose with allow default mode
  performs loose RPF checking if source address lookup returns a regular FIB route, otherwise the default route will be used for loose RPF checking as well.