Which mode of uRPF causes a router interface to accept a packet, if the network towhich the packet’s source IP address belongs is found in the router’s FIB?

Which mode of uRPF causes a router interface to accept a packet, if the network towhich the packet’s source IP address belongs is found in the router’s FIB?
A . Strict mode
B . Loose mode
C . Auto mode
D . Desirable mode

View Answer

Answer: B

Explanation:

A number of common types of DoS attacks take advantage of forged or rapidly changing source IP addresses, allowing attackers to thwart efforts by ISPs to locate or filter these attacks. Unicast RPF was originally created to help mitigate such attacks by providing an automated, scalable mechanism to implement the Internet Engineering Task Force (IETF) Best Common Practices 38/Request for Comments 2827 (BCP 38/RFC 2827) anti-spoofing filtering on the customer-to-ISP network edge. By taking advantage of the information stored in the Forwarding Information Base (FIB) that is created by the CEF switching process, Unicast RPF can determine whether IP packets are spoofed or malformed by matching the IP source address and ingress interface against the FIB entry that reaches Dbackǁ to this source (a so-called Dreverse lookupǁ). Packets that are received from one of the best reverse path routes back out of the same interface are forwarded as normal. If there is no reverse path route on the same interface from which the packet was received, it might mean that the source address was modified, and the packet is dropped (by default). .

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/sec_data_urpf/configuration/xe-3s/sec-data-urpf­xe-3s-book/sec-unicast-rpf-loose-mode.html?referring_site=RE&pos=1&page=http://www.cisco.com/c/en/us/td/docs/ios/12_0s/feature/guide/srpf_gsr.html#GUID-FFFA94D5-EEFB­4215-9EE1-DB37CD01C2CA