Configuring Unicast RPF
The unicast RPF feature reduces problems that are caused by the introduction of malformed or forged (spoofed) IPv4 or IPv6 source addresses into a network by discarding IPv4 or IPv6 packets that lack a verifiable IP source address. For example, a number of common types of Denial-of-Service (DoS) attacks, including Smurf and Tribal Flood Network (TFN) attacks, can take advantage of forged or rapidly changing source IPv4 or IPv6 addresses to allow attackers to thwart efforts to locate or filter the attacks. Unicast RPF deflects attacks by forwarding only the packets that have source addresses that are valid and consistent with the IP routing table.
When you enable unicast RPF on an interface, the switch examines all ingress packets received on that interface to ensure that the source address and source interface appear in the routing table and match the interface on which the packet was received. This examination of source addresses relies on the Forwarding Information Base (FIB).
Note: Unicast RPF is an ingress function and is applied only on the ingress interface of a switch at the upstream end of a connection.
Unicast RPF verifies that any packet received at a switch interface arrives on the best return path (return route) to the source of the packet by doing a reverse lookup in the FIB. If the packet was received from one of the best reverse path routes, the packet is forwarded as normal. If there is no reverse path route on the same interface from which the packet was received, the source address might have been modified by the attacker. If unicast RPF does not find a reverse path for the packet, the packet is dropped.
Note: With unicast RPF, all equal-cost “best” return paths are considered valid, which means that unicast RPF works where multiple return paths exist, if each path is equal to the others in terms of the routing cost (number of hops, weights, and so on) and as long as the route is in the FIB. Unicast RPF also functions where Enhanced Interior Gateway Routing Protocol (EIGRP) variants are being used and unequal candidate paths back to the source IP address exist.
This section contains payload examples and CLIs to demonstrate how to use the NX-API REST API to configure Unicast RPF on Cisco Nexus 3000 and 9000 Series switches and to show how the REST APIs correspond to the CLI commands. For more information, see the Cisco Nexus 9000 Series NX-OS Security Configuration Guide, Release 7.x.