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SAA-C03 Practice Question 55: NAT Gateway per AZ

AWS Certified Solutions Architect – Associate (SAA-C03) · Test 6 · question 5 of 10

High Availabilitymedium

A VPC has private subnets in three Availability Zones. All three subnets route 0.0.0.0/0 to a single NAT gateway that lives in a public subnet in the first Availability Zone. During a recent impairment of that Availability Zone, instances in all three private subnets lost outbound internet access, even though the instances in the other two zones were healthy the whole time.

What should the architect change so that a single zone's failure can no longer take outbound access away from the other zones?

Select one answer
Show answer and explanation

Answer:

  • C. Create a NAT gateway in each Availability Zone and point each private subnet's route table at the NAT gateway in its own zone

A NAT gateway is created in one specific Availability Zone and is made redundant only *within* that zone. It does not fail over to another zone, so when the zone is impaired every subnet whose route table points at it loses internet access, which is exactly the incident described. AWS's own guidance for this case is to create a NAT gateway per Availability Zone and give each private subnet a route table that sends 0.0.0.0/0 to the gateway in its own zone. That removes the shared failure domain and, as a side benefit, keeps NAT traffic from crossing zone boundaries.

Option A addresses a different limit. Extra IP addresses raise the number of simultaneous connections a NAT gateway can hold to a single destination; they do nothing about the zone the gateway sits in.

Option B describes something that cannot exist. A subnet belongs to exactly one Availability Zone, only a VPC spans zones, so there is no multi-zone subnet to place the gateway in.

Option D trades a managed service for one you have to run. An Auto Scaling group can replace a failed NAT instance, but the replacement comes up with a new network interface, so the private route tables have to be rewritten by custom automation before traffic flows again, and a single NAT instance is capped by its instance type rather than scaling to tens of gigabits. It is more work and less available than simply having one gateway per zone.

Reference: NAT gateway basics

Practice Test 6 ›

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