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Exam AWS Certified Solutions Architect - Professional topic 1 question 71 discussion

Exam question from Amazon's AWS Certified Solutions Architect - Professional
Question #: 71
Topic #: 1
[All AWS Certified Solutions Architect - Professional Questions]

You have deployed a three-tier web application in a VPC with a CIDR block of 10.0.0.0/28. You initially deploy two web servers, two application servers, two database servers and one NAT instance tor a total of seven EC2 instances. The web, application and database servers are deployed across two availability zones
(AZs). You also deploy an ELB in front of the two web servers, and use Route53 for DNS Web (raffle gradually increases in the first few days following the deployment, so you attempt to double the number of instances in each tier of the application to handle the new load unfortunately some of these new instances fail to launch.
Which of the following could be the root caused? (Choose two.)

  • A. AWS reserves the first and the last private IP address in each subnet's CIDR block so you do not have enough addresses left to launch all of the new EC2 instances
  • B. The Internet Gateway (IGW) of your VPC has scaled-up, adding more instances to handle the traffic spike, reducing the number of available private IP addresses for new instance launches
  • C. The ELB has scaled-up, adding more instances to handle the traffic spike, reducing the number of available private IP addresses for new instance launches
  • D. AWS reserves one IP address in each subnet's CIDR block for Route53 so you do not have enough addresses left to launch all of the new EC2 instances
  • E. AWS reserves the first four and the last IP address in each subnet's CIDR block so you do not have enough addresses left to launch all of the new EC2 instances
Show Suggested Answer Hide Answer
Suggested Answer: CE 🗳️
by alfa2 at May 15, 2021, 6:41 p.m.
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amministrazione
3 months ago
C. The ELB has scaled-up, adding more instances to handle the traffic spike, reducing the number of available private IP addresses for new instance launches E. AWS reserves the first four and the last IP address in each subnet's CIDR block so you do not have enough addresses left to launch all of the new EC2 instances
upvoted 1 times
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a6a3d55
5 months, 3 weeks ago
Why C? ELB don’t scale up it is the auto scaling group that do…
upvoted 1 times
Zoro483
2 weeks, 1 day ago
Availability Zone subnets You must select at least two Availability Zone subnets. The following restrictions apply: Each subnet must be from a different Availability Zone. To ensure that your load balancer can scale properly, verify that each Availability Zone subnet for your load balancer has a CIDR block with at least a /27 bitmask (for example, 10.0.0.0/27) and at least eight free IP addresses per subnet. These eight IP addresses are required to allow the load balancer to scale out if needed. Your load balancer uses these IP addresses to establish connections with the targets. Without them your Application Load Balancer could experience difficulties with node replacement attempts, causing it to enter a failed state. Note: If an Application Load Balancers subnet runs out of usable IP addresses while attempting to scale, the Application Load Balancer will run with insufficient capacity. During this time old nodes will continue to serve traffic, but the stalled scaling attempt may cause 5xx errors or timeouts when attempting to establish a connection.
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JPA210
9 months, 1 week ago
Selected Answer: CE
There is very good explanations below for this choice.
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HassanYoussef
1 year, 10 months ago
The right answer i recommend to be ( C& E): The Route 53 is not scaling itself to handle the traffic it act for routing the traffic like AWS routing tables in the VPC but for the DNS, so the best answer would be C not D. AWS docs: https://docs.aws.amazon.com/vpc/latest/userguide/configure-subnets.html https://docs.aws.amazon.com/elasticloadbalancing/latest/userguide/what-is-load-balancing.html https://aws.amazon.com/route53/faqs/
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TigerInTheCloud
1 year, 10 months ago
Selected Answer: CE
The question has some issues. Possibly some memory mistake. The /28 is the smallest CIDR size for the AWS VPC subnet, if VPC with /28 CIDR, it cannot be deployed into two AZs, which means at least two subnets (not to mention three-tier structure) Read the answers, only C and E are the right statements. Others are wrong: A. AWS reserves more IPs in a subnet than the first and last IP, +1 IP for Route, +2 for DNS, and +3 reserved for future use (as stated in answer E); B: There is no IGW scaling up (at least not a concern for users), and IGW does not consume IP in VPC IP space. D: Only mentioned reserving +2 IP which is for DNS, but miss others. Same mistake as A Someone in this discussion has doubts about C. When ELB scales up, it does take more IPs. If you cannot deploy a load balancer into a subnet with available IPs less than 8, as AWS expects the scaleup will consume more IPs.
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aandc
2 years, 4 months ago
Selected Answer: CE
it's CE
upvoted 3 times
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Alexey79
2 years, 7 months ago
Selected Answer: DE
DE as it’s only correct answers!!! WRONG Question!!! x2 AZs usage require x2 Subnets, x1 for each AZ. But, Subnet can’t be /29!!! “IPv4 block sizes must be between a /16 netmask and /28 netmask” So only one Subnet and AZ is used. https://docs.aws.amazon.com/vpc/latest/userguide/configure-subnets.html
upvoted 3 times
Alexey79
2 years, 7 months ago
https://docs.aws.amazon.com/vpc/latest/userguide/configure-subnets.html “ For example, in a subnet with CIDR block 10.0.0.0/24, the following five IP addresses are reserved: 10.0.0.0: Network address. 10.0.0.1: Reserved by AWS for the VPC router. 10.0.0.2: Reserved by AWS. The IP address of the DNS server is the base of the VPC network range plus two. For VPCs with multiple CIDR blocks, the IP address of the DNS server is located in the primary CIDR. We also reserve the base of each subnet range plus two for all CIDR blocks in the VPC. Amazon DNS server = is an Amazon Route 53 Resolver server. 10.0.0.3: Reserved by AWS for future use. 10.0.0.255: Network broadcast address. We do not support broadcast in a VPC, therefore we reserve this address. “ https://docs.aws.amazon.com/elasticloadbalancing/latest/network/network-load-balancers.html “ The load balancer has one IP address per enabled Availability Zone. “
upvoted 2 times
Alexey79
2 years, 7 months ago
/28 is x16 IPs - x5 Reserved IPs = 11 available 11 - x2 Web Servers - x2 Application Servers - x2 db - x1 NAT Intance = 4 available 4 - x1 ELB IPs per x1 AZ = 3 available Why NOT B: IGW has no IP and scaling up will not consume IP Address. Why NOT C: “ELB … scaled-up” Scaling EC2 instances in Subnet doesn’t take more IP Addresses of the ELB in AZ.
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tartarus23
2 years, 7 months ago
CE The first four IP addresses and the last IP address in each subnet CIDR block are not available for your use, and they cannot be assigned to a resource, such as an EC2 instance.
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jyrajan69
2 years, 8 months ago
10.0.0.0/28, means that 28 IP reserved for Network and 4 IP for the host, hence 2^4 which is 16. So from that 5 are reserved by AWS, hence only 11 IP's available for use. E is one definite answer. 7 of the IP's are in use, so only 4 are available now. B is IGW which is your access to the Internet. The IGW does not have any IP Address assigned to it, but can do NAT. So based on tht only possible is C. Answer C and E.
upvoted 3 times
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HellGate
2 years, 9 months ago
B or C… Both of them have feature of scaling out but does ELB consume more IPs than IGW when spike loads?
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lucesarano
2 years, 10 months ago
C,E are correct indeed, but please specify the CIDR notation for the people that may not be used to it. 10.0.0.0/28 has 14 ips available. Precisely, from 10.0.0.1 to 10.0.0.14 4 ips are reserved so availables ips are from 10.0.0.1 to 10.0.0.10 known fixed ips by reqs are 7 ips variable ips are dictated by the Balancers, at least 1. so at least 8 ips are busy, making only 2 ips for scaling up. Hence, C, E.
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nwk
3 years, 1 month ago
https://docs.aws.amazon.com/vpc/latest/userguide/VPC_Subnets.html#vpc-sizing-ipv4
upvoted 4 times
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01037
3 years, 1 month ago
Yes CE
upvoted 2 times
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alfa2
3 years, 1 month ago
CE is correct
upvoted 3 times
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