для Cisco Cisco IOS Software Release 12.0(13)S7

Supported line cards

In Cisco IOS Release 12.0(31)S, the Unicast RPF for IPv6 feature is supported only on the 
10G Engine 5 SPA Interface Processor (10G SIP) in the Cisco 12000 series Internet router.

For information about the modular services cards (SIPs) and shared port adapters (SPAs) supported 
on the Cisco 12000 series Internet router, refer to the 

Cisco express forwarding

The Unicast RPF for IPv6 feature requires Cisco express forwarding (CEF) to function properly 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 for IPv6 is an input-side function that is enabled on an interface 
or subinterface that supports any type of encapsulation and operates on IPv6 packets received by the 
router. It is necessary that CEF is enabled globally in the router—Unicast RPF for IPv6 does not 
work without CEF.

For more information about CEF, refer to the

 Cisco IOS Switching Services Configuration Guide

Release 12.3.

The following restrictions apply to the Unicast RPF for IPv6 feature on the 10G SIP in the Cisco 12000 
series Internet router:

Customer-facing interfaces on PE routers

The Unicast RPF for IPv6 feature is designed to be used only on customer-facing interfaces (or 
subinterfaces) of the 10G SIP in a Cisco 12000 series Internet router, which is deployed as a 
provider edge (PE) node. We recommend that you do not enable Unicast RPF on:

A Cisco 12000 series Internet router used as a core router in a service-provider network

On a core-facing interface of a PE node

Because traffic in the core network arrives through a customer-facing interface on a PE router, 
enabling Unicast RPF on a core router or core-facing interface is redundant.

In addition, core-facing interfaces are likely to have routing asymmetry, meaning multiple routes to 
the source of a packet. Only apply Unicast RPF where there is natural or configured symmetry. If 
administrators carefully plan the interfaces on which they activate Unicast RPF, routing asymmetry 
is not a serious problem.

PE routers at the edge of a service-provider network are more likely to have symmetrical reverse 
paths than routers in the core network. Routers that are in the core network have no guarantee that 
the best forwarding path out of the router will be the path selected for packets returning to the router. 
Hence, it is not recommended that you apply Unicast RPF where there is a chance of asymmetric 
routing. It is simplest to place Unicast RPF for IPv6 only at the edge of a network or, for an Internet 
service provider (ISP), at the customer edge of the network.