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

Placing the router in broadcast mode would increase its participation in broadcast traffic, worsening exposure to smurf-style amplification attacks rather than mitigating them.

Enabling port forwarding on the router is unrelated to smurf attacks and does nothing to block ICMP directed broadcast amplification.

Installing the router outside the firewall changes its network position but does not address the core mechanism of smurf attacks, which rely on directed broadcast processing regardless of placement.

In a smurf attack, the attacker sends ICMP echo requests to a network broadcast address with the victim's IP spoofed as the source, causing all hosts to reply to the victim. Disabling the router from processing directed broadcast ICMP messages - via commands like 'no ip directed-broadcast' on Cisco IOS - prevents the router from forwarding these amplified replies, neutralizing the attack at its source.