300-420 Question #12: Real Exam Question with Answer & Explanation

Question

A branch office has a primary L3VPN MPLS connection back to the main office and an IPSEC VPN tunnel that serves as backup. Which design ensures that data is sent over the backup connection only if the primary MPLS circuit is down?

Options

• AUse EIGRP to establish a neighbor relationship with the main office via L3VPN MPLS and the
• BUse BGP with the multipath feature enabled to force traffic via the primary path when available.
• CUse static routes tied to an IP SLA to prefer the primary path while a floating static route points to
• DUse OSPF with a passive-interface command on the backup connection.

Explanation

{"question_number": 8, "question": "A branch office has a primary L3VPN MPLS connection back to the main office and an IPSEC VPN tunnel that serves as backup. Which design ensures that data is sent over the backup connection only if the primary MPLS circuit is down?", "correct_answer": "C. Use static routes tied to an IP SLA to prefer the primary path while a floating static route points to the backup.", "explanation": "IP SLA continuously probes the primary MPLS path (e.g., via ICMP echo). A static route with a low administrative distance (AD) is conditionally installed based on IP SLA reachability - when the probe succeeds, this route is active and traffic takes the MPLS path. A floating static route pointing to the IPsec tunnel is configured with a higher AD (e.g., 254), so it sits dormant in the routing table while the primary route is present. When IP SLA detects the MPLS failure, the primary route is removed and the floating static route automatically becomes active, redirecting traffic to the IPsec backup - exactly the failover behavior required. EIGRP (A) or OSPF (D) with passive-interface cannot easily enforce strict primary/backup preference without complex metric manipulation, and BGP multipath (B) would load-balance rather than fail over.", "generated_by": "claude-sonnet", "llm_judge_score": 4}

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