Prerequisites for Unicast Reverse Path Forwarding for IPv6
Enable Cisco Express Forwarding switching or distributed Cisco Express Forwarding switching in the device. There is no need to configure the input interface for Cisco Express Forwarding switching. As long as Cisco Express Forwarding is running on the device, individual interfaces can be configured with other switching modes.
Cisco Express Forwarding must be configured globally in the device. uRPF will not work without Cisco Express Forwarding.
uRPF should not be used on interfaces that are internal to the network. Internal interfaces are likely to have routing asymmetry; this means that there are multiple routes to the source of a packet. uRPF should be applied only where there is natural or configured symmetry.
For example, devices at the edge of the network of an ISP are more likely to have symmetrical reverse paths than devices that are in the core of the ISP network. Devices that are in the core of the ISP network have no guarantee that the best forwarding path out of the device will be the path selected for packets returning to the device. Therefore, we do not recommend that you apply uRPF where there is a chance of asymmetric routing. Place uRPF only at the edge of a network or, for an ISP, at the customer edge of the network.