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Deepak Pakhale
Deepak Pakhale

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Maximizing Container Density on Amazon ECS


If you're leveraging Amazon Elastic Container Service (ECS) with EC2 instances for your containerized workloads, optimizing your resource utilization is crucial. In this article, we'll explore the importance of profiling your application and selecting the right EC2 instance type based on your app's resource requirements. Specifically, we'll delve into a game-changing feature called ENI (Elastic Network Interface) trunking, designed to enhance container density on your EC2 instances.

Understanding ENI Attachment Limits

Each EC2 instance type supports a limited number of ENI attachments. This is a critical factor to consider when provisioning your ECS cluster. Larger instance types naturally support more ENIs, allowing for a greater number of concurrently running tasks. But larger instances also costs more.

ECS Simplifies Container Orchestration

ECS abstracts away the complexities of setting up containerized environments, providing a streamlined solution for running tasks. In ECS, each task consumes one IP address, equivalent to one ENI attachment. For example, if you're running an ECS cluster on an m5.xlarge instance, supporting three ENI attachments, you may find yourself underutilizing the instance, limiting you to running only three tasks.

ENI Trunking Unleashes Container Density

ENI trunking, a feature enabled by the awsvpc network mode. ENI trunking resolves the limitation on the total number of ENIs that can be attached to an EC2 instance, allowing for a more efficient use of resources.

How ENI Trunking Works

With ENI trunking, an EC2 instance undergoes two ENI attachments: the primary ENI and the Trunk ENI. The addition of the Trunk ENI empowers the EC2 instance to host a greater number of containers, effectively maximizing container density.

Managing IP Address Exhaustion

It's important to note that even with ENI trunking, IP addresses are still allocated from the subnet. Small subnets could lead to IP exhaustion issues. To mitigate this, Amazon VPC Container Network Interface (CNI) can be configured to utilize a different IP space than the host network, preventing potential challenges related to IP address limitations.


ENI trunking is a powerful feature that enables ECS users to break free from the constraints of ENI attachment limits, unlocking the potential for higher container density on EC2 instances. By understanding and leveraging this feature, you can optimize your ECS cluster, ensuring efficient resource utilization and maximizing the benefits of containerization on AWS.


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