Cisco Cisco IOS Software Release 12.0(13)S7

Note the following perquisites when you configure the Unicast RPF in Strict Mode feature on the 
Cisco 12000 series Internet router.

Cisco IOS Release 12.0(33)S introduces support for the Unicast RPF in Strict Mode feature on all 
IP Services Engine (ISE/Engine 3) and Engine 5 line cards in the Cisco 12000 series Internet router. For 
information about the ISE and Engine 5 service interface processors (SIPs) and shared port adapters 
(SPAs) supported on the Cisco 12000 series Internet router, refer to the 

ISE and Engine 5 line cards already support Unicast RPF in loose 

On the Cisco 12000 series Internet router, Unicast RPF is supported on other Engine types as follows:

Engine 4 line cards do not support Unicast RPF.

Engine 4+ line cards support Unicast RPF only in loose mode, but only on main interfaces. 
Subinterfaces are not supported.

Engine 6 line cards support Unicast RPF only in loose mode.

For information on the differences between loose and strict checking modes, see 

.

The Unicast RPF in Strict Mode feature requires Cisco express forwarding (CEF) to function properly. 
CEF is enabled, by default, on the Cisco 12000 series Internet router.

It is not necessary to configure an input interface for CEF switching because Unicast RPF has been 
implemented as a search through the Forwarding Information Base (FIB) using the source IP address. 
As long as CEF is running on the router, individual interfaces can be configured with other switching 
modes. Unicast RPF  is an input-side function that is enabled on an interface or subinterface that supports 
any type of encapsulation and operates on IPv4 packets received by the router. CEF must be enabled 
globally in the router—Unicast RPF  does not work without CEF.