Unicast RPF with Routing Asymmetry
In general, we recommend that you not enable unicast RPF on interfaces that are internal to the network because internal interfaces are likely to have routing asymmetry. Routing asymmetry means that a packet's outgoing and return paths are different. Routers in the core of the network are more likely to have asymmetric reverse paths than routers at the customer or provider edge. Figure 3 shows unicast RPF in an environment with routing asymmetry.
In Figure 3, if you enable unicast RPF on interface so-0/0/0, traffic destined for Router A is not rejected. If you enable unicast RPF on interface so-1/0/1, traffic from Router A is rejected.
If you need to enable unicast RPF in an asymmetric routing environment, you can use fail filters to allow the router to accept incoming packets that are known to be arriving by specific paths. For an example of a fail filter that accepts packets with a specific source and destination address, see Example: Configure Unicast RPF.