A.examine all packets received to make sure that the source addresses and source interfaces appear in the routing table and match the interfaces where the packets were received
B.check to see if any packet received at a router interface arrives on the best return path
C.combine with a configured ACL
D.log its events，if you specify the logging options for the ACL entries used by the unicast rpf command
E.inspect IP packets encapsulated in tunnels， such as GRE