A.route symmetric environment.

B.If the source address of the packet does not exist in the FIB table of the USG, and the default route is configured, the packet will be forwarded through the URPF check.
URPF strict mode is recommended in a

C.Loose mode does not check whether the interface matches. As long as the source address of the packet exists in the FIB table of the USG, the packet can pass.

D.strict mode requires not only the corresponding entry in the forwarding table, but also the interface must match to pass the URPF check.