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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?

Adi Dembak
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kiran01bm
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1 Answers1

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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.

Adi Dembak
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  • 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 ? – kiran01bm Jun 08 '20 at 05:19
  • 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. – Adi Dembak Jun 08 '20 at 05:47