для Cisco Cisco IOS Software Release 12.0(13)S7

IPv4 packets

On the Cisco 12000 series Internet router, the Unicast RPF for IPv6 feature does not filter 
IPv4 traffic.

Loose versus strict checking modes

On the Cisco 12000 series Internet router, the Unicast RPF for IPv6 feature does not support loose 
checking mode as on other platforms. Only strict checking mode is supported.

Strict checking mode verifies that the source IPv6 address of an IPv6 packet exists in the routing 
table and that the source IPv6 address is reachable by a path through the input interface. To 
configure strict checking mode, use one of the following commands:

Starting in Cisco IOS Release 12.0(31)S, the Cisco 12000 series Internet router supports both 
commands to enable Unicast RPF to be compatible with the Cisco IOS Release12.3T and 12.2S 
software trains.

Loose (exist-only) checking mode only verifies that a source IPv6 address exists in the routing 
table, and is enabled using the ipv6 verify unicast source reachable-via any command.

To configure the Unicast RPF for IPv6 feature on the Cisco 12000 series Internet router, you should 
understand the following concepts:

When the Unicast RPF for IPv6 feature is enabled on an interface:

The Cisco 12000 series Internet router examines all IPv6 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 ability to “look backwards” 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.

Unicast RPF for IPv6 is an input function and is applied only on the input interface of a 
Cisco 12000 series Internet router at the upstream end of a connection.