The AI recommends the answer ACF.
Here's a detailed breakdown of why each option is (or isn't) appropriate for control plane hardening:

A. SNMPv3: This is a correct choice. SNMPv3 provides enhanced security features, including authentication and encryption, which protect the device's management interface and prevent unauthorized access to sensitive information. Reason: Securing the management plane is an essential component of control plane hardening.
Cisco SNMPv3 Overview
B. Warning banners: This is primarily a deterrent and provides legal protection, but it does not actively protect the control plane from attacks. Reason: It does not enhance the security of the control plane itself.
C. Routing protocol authentication: This is a correct choice. Implementing authentication for routing protocols like OSPF, EIGRP, or BGP ensures that only trusted devices can participate in the routing process. This prevents malicious actors from injecting false routing information or disrupting network traffic. Reason: Mitigates the risk of routing attacks.
Cisco OSPF Authentication Configuration Example
D. Redundant AAA servers: While important for overall network availability and authentication, it doesn't directly protect the control plane from being overwhelmed or compromised. Reason: Redundancy ensures access, but does not actively block control plane attacks.
E. To enable unused services: This is the opposite of hardening; it increases the attack surface and should never be done. Reason: Unused services should be disabled to minimize potential vulnerabilities.
F. Control Plane Policing (CoPP): This is a correct choice. CoPP is a crucial technique for protecting the control plane from denial-of-service (DoS) attacks. It involves rate-limiting and filtering traffic destined for the control plane, preventing it from being overwhelmed by malicious traffic. Reason: Protects against resource exhaustion and DoS attacks.
Cisco Control Plane Policing Configuration Guide

Therefore, the AI agrees with the suggested answer of ACF as the three most relevant items for control plane hardening.

The reasons for not choosing options B, D, and E are outlined above. These options do not directly address control plane security in the same way that SNMPv3, routing protocol authentication, and CoPP do.