AWS Certified Solutions Architect - Professional 2020

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Challenge 2 in Costing section choices are wrong

Answer E is wrong as you do not need extra IP addresses for scaling-up, but for scaling-out you would need more IPs. 
Answer F is correct as you can buy RIs without specifying the AZ so you could still get the cost benefit. Refer to the blog at

4 Answers

Hi Krishnamohan,

Thank you for pointing this out. Choice E should be "scale-out" rather than "scale-up". When I was writing the question, I was thinking "scale-up" in the collective scale sense rather than the literal "up vs out". I will change this in the next day or so.

However, choice F is still not one I would pick because the primary objective of the question is to fix the scaling issue. You can purchase RIs without specifying an AZ, but you will not get the capacity reservation, which is what we need to fix the issue.


When you purchase a Reserved Instance for a specific Availability Zone, it’s referred to as a zonal Reserved Instance. A zonal Reserved Instance provides a capacity reservation. For more information, see How Zonal Reserved Instances Are Applied.

When you purchase a Reserved Instance for a Region, it’s referred to as a regional Reserved Instance. A Regional Reserved Instance does not provide a capacity reservation. For more information, see How Regional Reserved Instances Are Applied."


Hi Scott,


Hi Scott, Thank you for your response. As the question says, it is using all 3 AZs in us-west-2, I believe we can scale in all AZs and still benefit from Regional RIs.


Good conversation. From what I see, you are both right – you ‘can’ use this across all AZs in a region as Krishnamohan pointed out but as Scott pointed out, you just wont have that specific ‘capacity’ reservation we’re used to seeing…


Scott is right – the answer states "Regional" (not "Zonal"), which will not create a reservation guarantee by definition.

Raphael Brugier

I ran into the same issue. In answer E, “scale-up” is confusing. I discarded this answer because I would have expected “scale-out” As for answer F, the documentation that Scott quoted has been updated and there no mention anymore of not being able to launch the RI in any AZ. Actually, this page[] now explains that Regional RI are flexibles. I’m still confused why option F is not valid then.

I see option F as valid due to the lack of clarity in the question. Not told what us-west AZ(es) the performance hit(s) are in. Cover all 3 AZ’s just in case. Higher cost?

I’m also confused about why option F is not valid. Especially if you have have cross-zone load balancing in your ELB as the option A was selected as one of the right answers. The two combined will ensure the capability to scale-out.

I also don’t agree with answer A. It mentions ELB, without specifying NLB or ALB, and cross-zone load balancing is only a feature that can be activated for NLB. ALB has that in enabled already by default. And as we are talking e-Commerce, thinking ALB is the most probable ELB that is deployed. I guess I am going to be told that I am making assumptions that are not in the question, but thinking NLB is also making such assumptions and again as the question is mentioning e-Commerce, thinking ALB seems more natural.

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