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SAA-C03 Practice Question 39: Cross AZ Data Transfer Cost

AWS Certified Solutions Architect – Associate (SAA-C03) · Test 4 · question 9 of 10

Network Cost Optimizationhard

A data processing tier of EC2 instances runs across three Availability Zones for resilience. The instances read roughly 200 TB a month from a self-managed caching cluster that runs on EC2 instances in a single Availability Zone, over private IP addresses. The cost report shows a large and growing Region-DataTransfer-Regional-Bytes line item. The team must cut that charge while keeping the tier able to survive the loss of an Availability Zone.

Which change reduces the charge most effectively?

Select one answer
Show answer and explanation

Answer:

  • D. Run cache nodes in all three Availability Zones and configure each processing instance to read from the node in its own Availability Zone

Traffic between EC2 instances in different Availability Zones in the same Region is charged in both directions, and it is billed under the Region-DataTransfer-Regional-Bytes usage type, the exact line item in the report. Traffic between instances in the *same* Availability Zone over private IP addresses is free. Because the cache sits in one zone, two-thirds of the reads currently cross a zone boundary and are billed twice over. Placing a cache node in each zone and having clients read locally makes almost all of that traffic same-zone and therefore free, while the cluster remains spread across three zones, so the resilience requirement is untouched.

Option A does eliminate the charge, by eliminating the resilience. A single-AZ deployment fails the explicit requirement to survive the loss of an Availability Zone.

Option B makes the bill worse. Traffic sent to a public IP address is billed at higher public data transfer rates rather than the regional rate, it still leaves the Availability Zone, and it forces private workloads onto public addressing for no benefit.

Option C adds cost without moving the bytes. Cross-zone load balancing spreads each node's requests across targets in every Availability Zone, so it *increases* the proportion of traffic that crosses a zone boundary, and the load balancer adds its own hourly and capacity-unit charges on top.

Reference: Understanding data transfer charges

Practice Test 4 ›

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