Cross Zone Load Balancing is one of the less known and most confusing options of the different load balancers on AWS. Until 2013 the choice was simple, Amazon offered only one load balancer as a service – the Classic Load Balancer – and there was no option to perform Cross Zone Load Balancing. No feature, no doubts, no extra costs.

In 2019, with three different load balancers (Classic Load Balancer, Application Load Balancer and Network Load Balancer), there is much more flexibility but as well a higher risk of an incorrect configuration or unexpected charges.

What is Cross Zone Load Balancing?

As for AWS documentation:

“With cross-zone load balancing, (…) each load balancer node distributes requests evenly across the registered instances in all enabled Availability Zones. If cross-zone load balancing is disabled, each load balancer node distributes requests evenly across the registered instances in its Availability Zone only.”

What is the default for the Cross Zone Load Balancing?

Unfortunately the default is different on every load balancer and not very intuitive. As for AWS documentation:

Classic Load Balancer: with the API or CLI, cross-zone load balancing is disabled by default. With the AWS Management Console, the option to enable cross-zone load balancing is selected by default.

Application Load Balancer: cross-zone load balancing is always enabled

Network Load Balancer: cross-zone load balancing is disabled by default. You can enable or disable cross-zone load balancing at any time.

Should I always enable it?

There are many documents and posts on the benefits of enabling cross-zone load balancing. And if you have only one target in every Availability Zone, it is usually an easy choice . But what are the main reasons to disable it or keep it disabled?

Maybe you want to minimise the latency between your load balancer and the application nodes and have all the traffic in the subnet. Or you take advantage of the SSL termination on the load balancer and you do not want to manage not encrypted traffic across data centres and different subnets. Or maybe you want simply to save a few dollars.

Do I pay extra for Cross Zone Load Balancing?

You do not pay for the the feature itself but you might pay for the generated regional data transfer. The voice that in your billing ends up under

$0.010 per GB - regional data transfer - in/out/between EC2 AZs or using elastic IPs or ELB

and that can end in significant charges if you manage large binaries on your load balancers. According to the AWS FAQ, the cost varies according to the specific service.

Q: Am I charged for regional AWS data-transfer for cross-zone load balancing in Application Load Balancer?
A: No. Since cross-zone load balancing is always on with Application Load Balancer, you are not charged for this type of regional data transfer.

Q: Am I charged for regional AWS data-transfer when I enable cross-zone load balancing in Network Load Balancer?
A: Yes, you will be charged for regional data transfer between Availability Zones with Network Load Balancer when cross-zone load balancing is enabled

Q: Am I charged for regional AWS data-transfer when I enable cross-zone load balancing in Classic Load Balancer?
A: No, you are not charged for regional data transfer between Availability Zones when you enable cross-zone load balancing for your Classic Load Balancer.

To summarize…

Cross Zone Load Balancing is a very useful feature and you likely end up enabling it in many common scenarios. But it is vital to understand the default values and the implications according to the specific AWS service you choose.