What is Cross-Zone Load Balancing?

As per the 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.”

In simpler terms, in a Multi-AZ setup, if cross-zone load balancing is not enabled, the load balancer only forwards traffic to targets within the same AZ where the request started.

Image Ref: AWS Docs

In the above image, you can see:

• Each target in Availability Zone A is serving traffic 25% of the traffic, while each target in Availability Zone B is serving only 6.25% of it.

However, with cross-zone load balancing, traffic is distributed across all registered targets, regardless of the availability zones they are in.

And if you consider the same target setup with a cross-zone load balancing setup, you can see that each of the 10 targets receives 10% of the traffic.

Image Ref: AWS Docs

If you are using Application Load Balancer, cross-zone load balancing is enabled for you by default.

In case you are using Network Load Balancer, You need to enable the cross-zone load balancing as it comes as disabled by default.

But why need CrossZone Loadbalancing?

• It helps in utilizing the registered targets more efficiently.

• It helps in improved fault tolerance, if one zone fails, the other zone is already ready to serve traffic.