Cannot attach EBS to EC2 when instance is in specific subnet

Question

Hello gurus!
I have two private subnets A and B in the same AZ. I have also created an EBS volume in the same AZ. When I create an EC2 instance in subnet A I am able to use aws-cli to attach the volume. However, when creating an instance in subnet B, the "aws ec2 attach-volume..." results in a "Connect timeout..." error message. Note: The EBS volume is not attached to the instance in A when trying to attach it to the instance in B.

I am using Terraform for provisioning of the EC2 instance, including security group creation and instance profile. The only thing I am altering when trying subnet A vs B is the subnet. Nothing else is different.

When looking at the two subnets in AWS Console, they are identical, except for the CIDR, They are connected to separate NAT GWs and routing tables "of their own"

What am I missing?

Best regards
Vidar Evenrud Seeberg

Accepted Answer

Problem solved; I NATed to the same private subnet... :D. Don't ask me why... Sorry for troubling you @Tom-B and @kentrad. I was not thorough enough when I inspected the NAT first time. Have a nice weekend!

Answer

The "Connection timeout ...." message leads me to believe that the routing for subnet B is not quite right. That is assuming you are running the `aws ec2 attach-volume...` command from the instance in subnet B. The CLI can't communicate with the EC2 service.

Answer

@gary: yes, they are both in eu-west-1a

Answer

If you're getting a connection timeout error, I would start by checking that you can reach the ec2 API endpoint from your host by running:

`ping ec2.eu-west-1.amazonaws.com`

If the ping times out. I'd suggest reviewing your routing tables, security groups and Network ACL's.

Answer

Thank you for good suggestions about routing etc @Tom-B and @kentrad. I have looked thoroughly at the routing tables, and to me, they look the same, but I may have missed something. I will have another look tonight and report back. Time for cooking dinner now :)

Cannot attach EBS to EC2 when instance is in specific subnet

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