* The IKE (Internet Key Exchange) Phase 1 handshake error indicates a failure in negotiating a secure connection. * Option A: The IKE hashing algorithm mismatch, including key lengths, often causes such failures. Both VPN devices must agree on compatible algorithms and key lengths for the handshake to succeed. * Option B: Multiple cipher suites do not inherently cause errors; they provide flexibility during negotiation. * Option C: While using a legacy Diffie-Hellman group is less secure, it does not typically cause the handshake to fail unless explicitly rejected. * Option D: Site-to-site VPNs do not use SSL/TLS; they rely on IPSec protocols, making this irrelevant. References: * CompTIA CASP+ Exam Objective 2.2: Implement network security solutions, including VPN configurations. * CASP+ Study Guide, 5th Edition, Chapter 7, VPN Technologies and Troubleshooting.