EC2 instance network bandwidth through IGW

Question

Hello,

according to the docs "Bandwidth for multi-flow traffic is limited to 50% of the available bandwidth for traffic that goes through an internet gateway".

This is clear to me if we look at an EC2 with a EIP assigned.

But what if the EC2 DOES NOT have an EIP assigned but is just in a target group of a public NLB/ALB. Does the limitation still apply or will it be able to consume 100% of its bandwidth because the traffic now comes "from NLB/ALB"? Will it make a difference if NLB is doing source-ip-preservation, will it then be "from IGW"?

re:Post Agent · Answer

The bandwidth limitation for multi-flow traffic through an Internet Gateway (IGW) applies regardless of whether the EC2 instance has an Elastic IP (EIP) directly assigned or is behind a load balancer.

When an EC2 instance is in a target group of a public Network Load Balancer (NLB) or Application Load Balancer (ALB), the traffic still ultimately passes through the IGW to reach the instance. Therefore, the 50% bandwidth limitation for multi-flow traffic going through an IGW still applies in this scenario.

In the case of an NLB with source IP preservation enabled, the situation remains the same. Even though the original client IP is preserved, the traffic still traverses the IGW to reach the EC2 instance in the private subnet. The source IP preservation feature doesn't change the network path or bandwidth limitations imposed by AWS.

It's important to note that this limitation applies specifically to multi-flow traffic. For single-flow traffic, different rules apply. Instances can achieve up to 10 Gbps for single-flow traffic when they are in the same cluster placement group, use Multipath TCP, or configure ENA Express for instances within the same Availability Zone.

Also, keep in mind that the actual available bandwidth depends on the instance type and size. Larger instances generally have more network bandwidth available. For instances with 32 or more vCPUs, the multi-flow traffic bandwidth through an IGW is limited to 50% of the available bandwidth or 5 Gbps, whichever is larger. For instances with fewer than 32 vCPUs, the limit is 5 Gbps.

In summary, whether your EC2 instance has a direct EIP or is behind a load balancer, the multi-flow traffic bandwidth limitation through an IGW still applies, and you should plan your architecture accordingly if you need to maximize network throughput.  
**Sources**  
[Amazon EC2 instance network bandwidth - Amazon Elastic Compute Cloud](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-network-bandwidth.html)  
[Single flow traffic speed on 10GBps direct connect | AWS re:Post](https://repost.aws/questions/QUsLd2ARweSS-GTvtB4oz3RQ/single-flow-traffic-speed-on-10gbps-direct-connect)

Thilo · Answer

What if the EC2 is in a different account and VPC than the NLB/ALB (IP Targets). How would it know that it originally came through IGW?

EC2 instance network bandwidth through IGW

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