Unicast Reverse Path Forwarding (Unicast RPF) in Strict Mode is a network security tool designed for use at the edge of a service-provider network.
Strict Mode vs Loose Mode
Strict mode verifies that the source address of an IPv4 packet both exists in the routing table and is reachable by a path through the input interface before forwarding a packet.
Loose mode verifies only that the source IPv4 address exists in the routing table.
More info here:
http://www.cisco.com/en/US/docs/ios/12_0s/feature/guide/srpf_gsr.html
http://www.cisco.com/web/about/security/intelligence/unicast-rpf.html
