Troubleshooting Load Balancer Connectivity Issues

Common Reasons for Failed Requests to an Application Load Balancer in AWS

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Question

You have created a VPC with an application load balancer and selected two EC2 instances as targets.

However, when you are trying to make a request to the internet-facing load balancer, the request fails.

What could be the reason?

Answers

Explanations

Click on the arrows to vote for the correct answer

A. B. C. D.

Answer: A.

Option A is correct because there must be a route in a route table to the internet gateway for internet connectivity

Option B is incorrect because this does not result in the failure mentioned in the question.

Option C is incorrect because any instances in the VPC must either have a public IP address or an attached Elastic IP address.

And for the internet-facing load balancer, AWS manages the underlying IP addresses.

So no need for an Elastic IP address.

Option D is incorrect because cross-zone load balancing is not a reason to cause the failure.

https://aws.amazon.com/premiumsupport/knowledge-center/create-attach-igw-vpc/
Clients cannot connect to an Internet-facing load balancer If the load balancer is not responding to requests, check for the following: Your Internet-facing load balancer is attached to a private subnet Verify that you specified public subnets for your load balancer. A public subnet has a route to the Internet Gateway for your virtual private cloud (VPC). Assecurity group or network ACL does not allow traffic The security group for the load balancer and any network ACLs for the load balancer subnets must allow inbound traffic from the clients and outbound traffic to the clients on the listener ports.

The most likely reason for the request to fail when accessing the internet-facing application load balancer (ALB) is that the associated subnet's route table does not have a route to the internet gateway.

A route to the internet gateway is necessary for internet traffic to reach the EC2 instances through the ALB. Therefore, to resolve this issue, you should add a route to the subnet's route table that points to the internet gateway.

Option B (Target EC2 instances are in a public subnet) is not a reason for the request to fail because instances in a public subnet can be accessed from the internet.

Option C (No Elastic IP address attached to the load balancer) is not a requirement for an ALB. However, if you need to provide a static IP address for the ALB, then you can associate an Elastic IP address with it.

Option D (Cross-zone load balancing is not enabled) affects the distribution of traffic across the target instances within the same Availability Zone. However, it is not a cause of the request to fail.

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