Node created in subnet with low number of IP adresses: failed to assign an IP address to container

[PeteMac88]

What happened:

Hey there,
we are currently having problems with the IP address allocation in our cluster. We initially created the EKS cluster with CIDR /16 and subnets with /24 settings. Our VPC still has enough IP addresses in the subnets but occasionally pods are stuck in creation because one of the subnet's ips is exhausted while the other subnets still have available IP addresses. We are already evaluating adding more subnets or an additional CIDR range but I don't understand why a node is created in the first place in a subnet that has a low number of available IP addresses and does not prefer the other subnets to distribute the IP allocation better.

How is the scheduling of nodes in subnets determined and can we configure it to check the amount of available IP addresses before creation? I only found one blog post describing our problem, all other posts are about the general problem of exhausted subnets (https://edwin-philip.medium.com/aws-eks-subnet-insufficient-ip-address-d0855154c596).

Does anyone has some advice on how to handle this problem or came across a good workaround?

Environment:

• Kubernetes version (use kubectl version): 1.25
• CNI Version: v1.16.3

[orsenthil]

Hello @PeteMac88 , this depends upon how you setup your EKS Cluster.

https://docs.aws.amazon.com/eks/latest/userguide/creating-a-vpc.html

The default VPC and Subnet creation templates provided for your EKS Cluster assign sufficient number of IP addresses in each subnet (around 16k)

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But the maximum number of pods that can be launched on a node depends on the max pods limit supported by the instance type, which is detailed here - https://github.com/awslabs/amazon-eks-ami/blob/master/files/eni-max-pods.txt

You might be running into this.

And in terms of organizing your work load, with various features provided by VPC CNI, you can refer to the EKS best practices guide that will provide suggestions based on your use case : https://aws.github.io/aws-eks-best-practices/networking/vpc-cni/

Let us know if this helps.

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