Cisco 10000-2P2-2DC Software Guide - Page 317
Configuring Unicast RPF
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Chapter 13 Unicast Reverse Path Forwarding Configuring Unicast RPF • By default, without uRPF provision urpf drops can be seen in pxf when: - the interface is not up - there is no ip address on the interface Configuring Unicast RPF To use Unicast RPF, you must configure the router for CEF switching or CEF distributed switching. There is no need to configure the input interface for CEF switching because Unicast RPF has been implemented as a search through the 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 IP packets received by the router. It is very important that CEF be turned on globally in the router-Unicast RPF will not work without CEF. To configure Unicast RPF, use the following commands beginning in global configuration mode: Step 1 Step 2 Step 3 Command Purpose Router(config)# ip cef Enables CEF on the router. You might want to disable CEF on a particular interface if that interface is configured with a feature that CEF does not support. You can enable CEF globally, but disable CEF on a specific interface by using the no ip route-cache cef interface command that enables all but that specific interface to use express forwarding. If you have disabled CEF operation on an interface and want to reenable it, you can use the ip route-cache cef command in interface configuration mode. Router(config-if)# interface type Selects the input interface on which you want to apply Unicast RPF. This is the receiving interface, which allows Unicast RPF to verify the best return path before forwarding the packet on to the next destination. Router(config-if)# ip verify unicast source reachable-via any or Router(config-if)# ip verify unicast source reachable-via rx The interface type is specific to your router and the types of interface cards installed on the router. To display a list of available interface types, enter the interface ? command. Enables Unicast RPF on the interface. The any option enables a Loose Mode uRPF on the router. This mode allows the router to reach the source address via any interface. The rx option enables a Strict Mode uRPF on the router. This mode ensures that the router reaches the source address only via the interface on which the packet was received. Step 4 Router(config-if)# exit You can also use the allow-default option, so that the default route can match when checking source address. The allow-self-ping option allows the router to ping itself. Exits interface configuration mode. Repeat Steps 2 and 3 for each interface on which you want to apply Unicast RPF. OL-2226-23 Cisco 10000 Series Router Software Configuration Guide 13-13
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