Unicast Reverse Path Forwarding


Is Unicast Reverse Path Forwarding applied on the input interface or the output interface of a router?

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vikrantambhoreConnect With a Mentor Commented:

Unicast RPF is an input function and is applied only on the input interface of a router at
the upstream end of a connection.

When Unicast RPF is enabled on an interface, the router examines all packets received as input on that
interface to make sure that the source address and source interface appear in the routing table and match
the interface on which the packet was received. This “look backwards” ability is available only when
Cisco express forwarding (CEF) is enabled on the router, because the lookup relies on the presence of
the Forwarding Information Base (FIB). CEF generates the FIB as part of its operation.


shubhanshu_jaiswalConnect With a Mentor Commented:
Normally, the security appliance only looks at the destination address when determining where to forward the packet. Unicast RPF instructs the security appliance to also look at the source address; this is why it is called Reverse Path Forwarding. For any traffic that you want to allow through the security appliance, the security appliance routing table must include a route back to the source address. See RFC 2267 for more information.

For outside traffic, for example, the security appliance can use the default route to satisfy the Unicast RPF protection. If traffic enters from an outside interface, and the source address is not known to the routing table, the security appliance uses the default route to correctly identify the outside interface as the source interface.

If traffic enters the outside interface from an address that is known to the routing table, but is associated with the inside interface, then the security appliance drops the packet. Similarly, if traffic enters the inside interface from an unknown source address, the security appliance drops the packet because the matching route (the default route) indicates the outside interface.
QlemoBatchelor, Developer and EE Topic AdvisorCommented:
This question has been classified as abandoned and is closed as part of the Cleanup Program. See the recommendation for more details.
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