Changing the local DNS server cache settings to refuse responses from unauthorized or unknown external sources is fundamental in mitigating a DNS cache poisoning attack. It prevents the server from accepting malicious redirect information. Implementing DNSSEC adds a layer of validation that can help protect against this attack, but it's an additional step following the primary action of securing the cache. Flushing the DNS cache will temporarily remove the malicious entries, but without addressing the cache settings, the server remains susceptible to another attack. Switching DNS providers may resolve the issue temporarily but doesn't address the vulnerability in the configuration.