Unicast Reverse Path Forwarding (uRPF)

Table 1. Unicast Reverse Path Forwarding product support
Feature	Product	Release introduced
Unicast Reverse Path Forwarding (URPF) checking (IPv4)	5320 Series	Fabric Engine 8.6 5320-48P-8XE and 5320-48T-8XE only
	5420 Series	VOSS 8.4
	5520 Series	VOSS 8.2.5
	5720 Series	Fabric Engine 8.7
Unicast Reverse Path Forwarding (URPF) checking (IPv6)	5320 Series	Fabric Engine 8.6 5320-48P-8XE and 5320-48T-8XE only
	5420 Series	VOSS 8.4
	5520 Series	VOSS 8.2.5
	5720 Series	Fabric Engine 8.7

The Unicast Reverse Path Forwarding (uRPF) feature prevents packet forwarding for incoming unicast IP packets that have incorrect or forged (spoofed) IP addresses. The uRPF feature checks that the traffic received on an interface comes from a valid IP address, thereby preventing address spoofing. On a reverse path check, if the source IP address of the received packet at the interface is not reacheable using the FIB, the system drops the packet as the packet may have originated from a misconfigured or a malicious source.

You can configure uRPF for each IP interface or VLAN. When uRPF is enabled on an interface, the switch checks all routing packets that come through that interface. It ensures that the system displays the source address and source interface in the routing table, and that it matches the interface, on which the packet was received.

You can use one of two modes for uRPF:

Strict mode: In strict mode, uRPF checks whether the source IP address of the incoming packet exists in the FIB. If the incoming interface is not the best reverse path, the packet check fails and uRPF drops the packet. If the routing engine finds the source IP entry, uRPF further checks if the source IP interface matches the incoming interface of the packet. If they match, the system forwards the packet as usual, otherwise, the system discards the packet.

Note

The number of packets dropped due to uRPF check on the ingress interface gets incremented along with other general dropped statistics under the IN-DISCARD column in the output of the command show interfaces gigabitEthernet error <collision|verbose> {slot/port[-slot/port][,...]}.
Loose mode: In loose mode, uRPF checks whether the source IP address of the incoming packet exists in the FIB. The packet is dropped only if the source address is not reachable via any interface on that router.

uRPF can be enabled independently for IPv4 and IPv6. However, on a given interface, if uRPF is enabled for both IPv4 and IPv6, the urpf-mode can be either strict-mode or loose-mode for both IPv4 and IPv6. That means we cannot have IPv4 urpf-mode configured differently than that of IPv6.

Note

When you enable uRPF mode the MTU values for both IPv4 and IPv6 packets on the same VLAN are matched. Different Layer 3 MTU sizes on the same VLAN are not allowed in uRPF mode.

Note

uRPF check cannot detect spoofed source IP address if the source IP address belongs to a known subnet.