Exam AWS-SysOps topic 1 question 16 discussion

Í have to agree with One Public for ELB and 2 for Web and 2 for DB..do not see why you need two ELB

The main reason of C is . The entire Infrastructure must be distributed over 2 availability zones.

I would go for B. Why do u require 2 ELB's , it defeats the whole purpose of ELB.

B is the right answer. ELB is HA by default.

For me, letter C is correct. This question is very tricky, first of all it says the solution should be restricted to two availability zones. It might mislead you to choose letter A, where you have two subnets. But subnet is one thing, and availability zone is another. You can have multiple subnets on one AZ. Moreover, ELB requires at least two subnets, so letter A and B are not possible. Letter D is incorrect because it is not best practice to put web servers and RDS in public subnets.

I also think it is B. Only one ELB can serve both AZ where the EC2 and DBs are in private subnet.

Answer is C ALB requires to reside at a public subnet and can span through AZs but, remember subnets do not span AZs, so 2 public subnets are required, 1 for each AZ Then you will need 2 private subnets for EC2 and 2 private subnets for RDS

Subnets for your load balancer To ensure that your load balancer can scale properly, verify that each subnet for your load balancer has a CIDR block with at least a /27 bitmask (for example, 10.0.0.0/27) and has at least 8 free IP addresses. Your load balancer uses these IP addresses to establish connections with the instances. Create a subnet in each Availability Zone where you want to launch instances. Depending on your application, you can launch your instances in public subnets, private subnets, or a combination of public and private subnets. A public subnet has a route to an internet gateway. Note that default VPCs have one public subnet per Availability Zone by default. When you create a load balancer, you must add one or more public subnets to the load balancer. If your instances are in private subnets, create public subnets in the same Availability Zones as the subnets with your instances; you will add these public subnets to the load balancer. https://docs.aws.amazon.com/elasticloadbalancing/latest/classic/elb-backend-instances.html

I think the correct answer is B. The ELB is already in HA and is managed and guaranteed by AWS.

For load balancers in a VPC, we recommend that you add one subnet per Availability Zone for at least two Availability Zones. This improves the availability of your load balancer. Note that you can modify the subnets for your load balancer at any time. https://docs.aws.amazon.com/elasticloadbalancing/latest/classic/elb-manage-subnets.html

C. Two public subnets for ELB two private subnets for the web-servers and two private subnets for RDS

one ALB may lead to single point of failure. better to have two ALB each located in different AZ, although one ALB can serve both AZ but there is a risk of in case that AZ fails

ELB can span multiple AZ - so one public subnet for ELB and two private for APP and DB

Sorry, you guys understood question wrong, question said "The entire Infrastructure must be distributed over 2 availability zones" not each subnet distributed over 2 AZs and RDS must be in Private subnet only. Correct answer is "A".

C. Two public subnets for ELB two private subnets for the web-servers and two private subnets for RDS

Remember never to put your DB in a public subnet, only in private ones.