AWS ALB - You must specify subnets from at least two Availability Zones

Question

For an ALB - "You must specify subnets from at least two Availability Zones."

For an NLB - "You can specify subnets from one or more Availability Zones."

What is the reason for enforcing 2 AZs as a MUST requirement for ALB specifically and not for NLB?

Segmentation

Answer 1

The Network Load Balancer is designed to honor zonal isolation requirements, and thus is suitable for application architectures encapuslated in a single zone. Although, if the Availability Zone fails, there is the option to automatically fail-over to other healthy AZs.

In their guidelines AWS recommend customers configure the NLB targets in multiple AZs for high availability, but it is not a must.

Vereen

thanks @Adi Dembak. The part Im still not clear about is - An ALB can load-balance an app which is homed in a single az - So why is there a mandate to have an ALB in at least 2 different AZs ? – Segmentation

There is no stated reason by AWS. It must be a design choice/limitation. Some users work around this and setup a 'dummy' subnet in a second AZ, which does not actually accept traffic, or routes it back to a FW. – Vereen