SAA-C03 practice question: DNS failover with Route 53

Design Resilient Architectures A free question from our original SAA-C03 practice exam. Try it, then open the answer to see why each option is right or wrong.

A company runs its customer portal behind an Application Load Balancer in eu-west-2, with a smaller standby copy behind another load balancer in eu-west-1. The standby must receive no user traffic while the primary is working. If every target behind the primary load balancer fails its health checks, users must be sent to the standby automatically, without anyone changing DNS records. Which Amazon Route 53 configuration meets these requirements?

Choose one answer.

A. Latency-based alias records for the two load balancers, with Evaluate Target Health set to Yes so unhealthy records are skipped.
B. Failover alias records, with the eu-west-2 load balancer as primary with Evaluate Target Health set to No, and eu-west-1 as secondary.
C. Failover alias records, with the eu-west-2 load balancer as primary with Evaluate Target Health set to Yes, and eu-west-1 as secondary.
D. Weighted alias records with a 90/10 split between the eu-west-2 and eu-west-1 load balancers, with Evaluate Target Health on both.

Show the answer and explanation

Answer: C

A. Wrong: Latency routing is active-active: each user is sent to the healthy load balancer with the lowest latency, so some users would reach the standby during normal running. That breaks the rule that the standby receives no traffic. Failover routing keeps it idle. Learn more: Active-active and active-passive failover (opens in a new tab)
B. Wrong: Failover routing is right, but with no health check and Evaluate Target Health off, Route 53 always treats the primary record as healthy. It would keep sending users to eu-west-2 even when every target is down. Evaluate Target Health must be Yes. Learn more: How Route 53 chooses records when health checking is configured (opens in a new tab)
C. Right: Correct. Route 53 answers with the primary while it is healthy, so the standby gets no traffic. With Evaluate Target Health on, the load balancer counts as unhealthy when a target group has no healthy targets, and Route 53 then answers with the secondary. Learn more: Values specific for failover alias records (opens in a new tab)
D. Wrong: Weighted routing would fail away from an unhealthy primary, but the 10% weight sends part of normal traffic to the standby. That breaks the rule that the standby stays idle. Failover routing keeps all users on the primary until it is unhealthy. Learn more: Active-active and active-passive failover (opens in a new tab)

Explanation

Active-passive failover uses failover records: Route 53 returns the primary while it is healthy and the secondary only when it is not. For an alias to an Application Load Balancer, setting Evaluate Target Health to Yes makes Route 53 use the health of the load balancer's target groups. A record with no health check and no target health evaluation is always treated as healthy, so it never fails over.

SAA-C03 practice question: DNS failover with Route 53

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