Advertise a more specific route for the backup traffic via the secondary circuit.. The inexpensive way to prevent backup traffic from saturating the primary MPLS circuit is to steer that backup traffic over the secondary circuit. Advertising a more specific route for the backup destination through the secondary path influences routing without buying a new dedicated circuit or merely moving servers between switches or VLANs. A dedicated switch or VLAN does not prevent WAN saturation if the same WAN path is still used.
Cisco CCNA 200-301 v1.1 IP Connectivity expects engineers to apply route specificity and path selection to meet traffic-engineering goals. The corrected answer is advertising a more specific route for the backup traffic via the secondary circuit. Longest-prefix matching can then direct the backup flows to the intended path while leaving other traffic on the primary circuit.