What is a successful method for protecting a router from potential smurf attacks?

Placing the router in broadcast mode would increase its exposure to smurf amplification rather than protect it, as it would forward broadcast traffic more aggressively.

Enabling port forwarding is a NAT feature unrelated to ICMP broadcast amplification and provides no defense against smurf attacks.

Moving the router outside the firewall increases its attack surface and does not prevent it from being used as a smurf amplifier for internal hosts.

In a smurf attack, an attacker sends ICMP echo requests to a subnet's broadcast address with the victim's IP spoofed as the source, causing all hosts on that subnet to reply to the victim. Disabling directed broadcast on the router's interface (e.g., 'no ip directed-broadcast' in Cisco IOS) prevents the router from forwarding these broadcast ping messages into the network, removing the amplification mechanism entirely. This is the industry-standard mitigation for smurf attacks.