URPF check modes

URPF supports two check modes:

• Strict URPF—To pass strict URPF check, the source address of a packet and the receiving interface must match the destination address and output interface of a FIB entry. In some cases such as asymmetrical routing, strict URPF may discard valid packets. Strict URPF is often deployed between a PE device and a CE device.

• Loose URPF—To pass loose URPF check, the source address of a packet must match the destination address of a FIB entry. Loose URPF can avoid discarding valid packets, but may let attack packets pass. Loose URPF is often deployed between ISPs, especially in asymmetrical routing.