Configuring Unicast Reverse Path Forwarding

18 Mar

Configuring Unicast Reverse Path Forwarding
Enterprise networks should use Unicast RPF as an ingress filter to protect themselves from untrusted networks. Although most enterprises use access lists for ingress filtering, Unicast RPF provides many advantages over the traditional access list approach. The following section will provide some examples of how Unicast RPF can provide valuable protection options for networks connected to the Internet.

Note Unicast RPF should not be configured on any internal network device where asymmetric routing is taking place. This will cause Unicast RPF to drop legitimate return traffic.

When Unicast RPF is enabled on an interface, the router examines all packets received on that interface. The router checks to make sure that the source address appears in the routing table and matches the interface on which the packet was received. To configure Unicast RPF for ingress filtering, follow these steps:

1.Use the ip cef or ip cef distributed command to enable CEF switching or distributed CEF switching.

2.Use the following command to select the input interface on which to apply Unicast RPF:

interface <interface name> <interface number>

The input interface is the receiving interface, which allows Unicast RPF to verify the best return path before forwarding the packet to the destination.

3.Use the following command to enable Unicast RPF on the interface:

ip verify unicast reverse−path <access list number>

The access list number option identifies an optional access list. If the access list denies network access, packets with changed headers are dropped at the interface. If the access list permits network access, packets with changed headers are forwarded to the destination address.

4.Use the following command to define an extended access list and its parameters:

access−list <access−list−number> {deny|permit} <protocol> −
<source> <source−wildcard> <destination> <destination>−

A deny statement configures the router to drop the packet and a permit statement allows the packet to forward out the egress interface toward its destination.

Figure 3.3 displays a network in which Unicast RPF is enabled on both interfaces of Router 1.

Figure 3.3: Unicast RPF.
The objective is to use Unicast RPF for filtering traffic at the ingress interfaces of Router 1 to provide protection from malformed packets arriving from the Internet or from the internal network. The following commands configure Router 1 for Unicast RPF:

ip cef distributed
interface Serial1/0
ip verify unicast reverse−path
interface Ethernet0/0
ip verify unicast reverse−path

The preceding configuration is all that is needed to have Unicast RPF running on the router. It is very important to remember that CEF must be enabled on the router prior to configuring Unicast RPF. In fact, the router will not allow Unicast RPF to be configured until CEF is enabled, as shown in the following display:

Router−1(config−if)#ip verify unicast reverse−path
% CEF not enabled. Enable first

As you can see, the router will display a prompt that demands that you enable CEF on the router prior to configuring Unicast RPF. To verify that Unicast is operational, use the show cef interface <interface name> <interface number> command. The output should verify that Unicast RPF is in fact operational. Listing 3.3 displays the output.

Listing 3.3: An example of the show cef interface command.

Router−1#sh cef interface serial1/0 detail
Serial1/0 is up (if_number 3)
Internet address is
ICMP redirects are always sent
Per packet loadbalancing is disabled
IP unicast RPF check is enabled
Inbound access list is not set
Outbound access list is not set
IP policy routing is disabled
Hardware idb is serial1/0
Fast switching type 1, interface type 18
IP CEF switching enabled
IP CEF Feature Fast switching turbo vector
Input fast flags 0×4000, Output fast flags 0×0
ifindex 2(2)
Slot 1 Slot unit 0 VC −1

Transmit limit accumulator 0×0 (0×0)
IP MTU 1500

Unicast RPF also allows for the configuration of an optional access list to control the exact behavior when the received packet fails the source IP address check. The access list can be defined as a standard access list or as an extended access list. If an access list is defined, then after a packet fails a Unicast RPF check, the access list is checked to see if the packet should be dropped or forwarded. Unicast RPF events can also be logged by specifying the logging option for the access list entries used by Unicast RPF.

The following example configures Router 1 in Figure 3.3 to use access lists and logging with Unicast RPF. In the example in Listing 3.4, the extended access list 114 contains entries that should permit or deny network traffic for specific address ranges received on interface serial1/0. Unicast RPF is configured on interface serial1/0 to check packets arriving at that interface.

Listing 3.4: An example Unicast RPF logging configuration.

ip cef distributed
int serial1/0
ip verify unicast reverse−path 114
int ethernet0/0
ip verify unicast reverse−path
access−list 114 deny ip any log−input
access−list 114 deny ip any log−input
access−list 114 deny ip any log−input
access−list 114 permit ip any log−input

The configuration in Listing 3.4 denies packets with a source address of,, or from arriving at interface serial1/0 because of the deny statement in access list 114. The access lists also logs any packet that is matched by the access list. Packets with a source address within the subnet arriving at interface serial1/0 are forwarded if the source cannot be verified against interface serial1/0 because of the permit statement in access list 114. To verify that logging of the access list entries are taking place, use the show access−lists command:

Router−1# show access−lists
Extended IP access list 114\
deny ip any log−input (87 match)
deny ip any log−input (32 match)
deny ip any log−input (76 match)
permit ip any log−input (63 match)

Each time a packet is dropped at an interface, information is not only logging globally on the router but also at each interface configured for Unicast RPF. Global statistics about packets that have been dropped provide information about potential attacks. To view the global drop statistics, use the show ip traffic command. Here is the output:

Router−1#show ip traffic
IP statistics:
Rcvd: 1290449399 total, 75488293 local destination
0 format errors, 183 checksum errors, 8684 bad hop count

62 unknown protocol, 0 not a gateway
0 security failures, 0 bad options, 1147 with options
Drop: 1468583 encap failed, 325 unresolved, 0 no adjacency
7805049 no route, 41 unicast RPF, 1428682 forced drop

Interface statistics help to provide information about which interface is the source of the attack. Statistics for each interface can be viewed using the show ip interface command. Interface statistics display two separate types of RPF drops: Unicast RPF drops and Unicast RPF suppressed drops. The display for Unicast RPF drops shows the number of drops at the interface, and the display for Unicast suppressed drops shows the number of packets that failed the Unicast RPF reverse lookup check but were forwarded because of a permit statement configured within the access list that is applied to Unicast RPF. The following output is from the show ip interface command:

Router−1#show ip interface serial1/0

Unicast RPF ACL 114
37 unicast RPF drops
12 unicast RPF suppressed drops

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