uRPF check modes

uRPF supports strict and loose modes.

• Strict uRPF check—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 scenarios (for example, asymmetrical routing), strict uRPF might discard valid packets. Strict uRPF is often deployed between a PE and a CE.

• Loose uRPF check—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 might let go attack packets. Loose uRPF is often deployed between ISPs, especially in asymmetrical routing.