uRPF - Unicast Reverse Path Forwarding

Cisco Docs: Cisco Docs: Secure DATA PLANE>Security Configuration Guide: Unicast Reverse Path Forwarding
http://www.cisco.com/en/US/docs/ios-xml/ios/sec_data_urpf/configuration/12-4t/sec-data-urpf-12-4t-book.html

The Unicast RPF feature helps to mitigate problems that are caused by the introduction of malformed or forged (spoofed) IP source addresses into a network by discarding IP packets that lack a verifiable IP source address

Configure the receiving interface, which allows Unicast RPF to verify the best return path before forwarding the packet on to the next destination. For example, verify if the SOURCE IP is reachable via that exact interface:
(config-subif)#ip verify unicast source reachable-via ?
  any  Source is reachable via any interface
  rx   Source is reachable via interface on which packet was received <-EXACT INTERFACE

#sh ip int s1/0.21 | b verify
  IP verify source reachable-via RX
   0 verification drops
   0 suppressed verification drops
   0 verification drop-rate

!!!If the check fails, and this is NOT the best interface to reach the IP from which the incoming packed was sourced the packed it DROPPED.