How subnet is choose by a ECS Fargate Task with multiple subnets assigned?

Question

Working with ECS Fargate, there are some times that some subnets don't have IPs available to run the task, causing an error on the task. But I wonder how is the subnet is chosen. There are some strategy by available IPs, balancing or some of kind or is it just random?

I tried searching for this on ECS Fargate docs, but i found nothing.

Thanks for any answer!

I tried searching for this on ECS Fargate docs, but i found nothing.

Thanks for any answer!

Riku_Kobayashi · Accepted Answer

Hello.

In the case of ECS Fargate, it is stated that the placement will be distributed within the selected subnet, so it is probably random.  
Therefore, I recommend that you set up a subnet with sufficient available IP addresses.  
https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task-placement.html#fargate-launch-type  
> Task placement strategies and constraints aren't supported for tasks using the Fargate launch type. Fargate will try its best to spread tasks across accessible Availability Zones. If the capacity provider includes both Fargate and Fargate Spot, the spread behavior is independent for each capacity provider.

re:Post Agent · Answer

When you run an Amazon ECS task on Fargate with multiple subnets specified, AWS Fargate selects one of the subnets to place the task in, but the specific selection strategy is not explicitly documented.

When you create a service or run a task with Fargate, you need to specify one or more subnets as part of the network configuration. Each Fargate task requires an elastic network interface (ENI) which is attached to the task, and this ENI needs an IP address from one of the specified subnets.

If a subnet doesn't have available IP addresses when Fargate attempts to launch a task, you'll receive an error. This is why it's a good practice to specify multiple subnets across different Availability Zones to increase the likelihood of having available IP addresses.

While AWS doesn't explicitly document the subnet selection algorithm, the behavior you're observing suggests that Fargate may not be checking for available IPs before attempting to place a task in a subnet. If one subnet is full, the task launch will fail rather than automatically trying another subnet.

To mitigate this issue, you should:
1. Monitor your subnet IP utilization
2. Ensure you have sufficient IP addresses available across all subnets
3. Consider using larger CIDR blocks for your subnets if you're running many tasks
4. Spread your tasks across multiple subnets in different Availability Zones  
**Sources**  
[Amazon ECS task definition differences for the Fargate launch type - Amazon Elastic Container Service](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/fargate-tasks-services.html)  
[Architect your solution for Amazon ECS - Amazon Elastic Container Service](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/ecs-configuration.html)  
[Creating an Amazon ECS Linux task for the Fargate launch type with the AWS CLI - Amazon Elastic Container Service](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/ECS_AWSCLI_Fargate.html)  
[Amazon ECS task networking options for the Fargate launch type - Amazon Elastic Container Service](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/fargate-task-networking.html)

How subnet is choose by a ECS Fargate Task with multiple subnets assigned?

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