A network administrator notices that the company's edge router is being overwhelmed by a flood of ICMP echo replies from many hosts across the internet, none of which the internal network actually pinged. Investigation reveals an attacker is sending spoofed ICMP echo requests using the company's public IP as the source address, directed to the broadcast addresses of misconfigured remote networks. Which configuration change on the company's routers BEST prevents their own network from being used as an amplifier in this type of attack?