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Hello,
Subnets are tied to an Availability Zones, reason to choose 2 subnets is to actually select 2 Availability Zones, although you create 1 ALB it actually creates ENIs in each of the subnets/AZs you select. This is for the resiliency purposes, in the event that 1 AZ goes down you would still have ALB available via the 2nd AZ.
Specific wording from the official documentation
Select two or more Availability Zones and corresponding subnets. Enabling multiple Availability Zones increases the fault tolerance of your applications.
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But when I am selecting a 'Target group', that target group will also have instances from different subnets and AZs. So if ALB is linked to the Target Group , shouldn't ALB be able to get the AZs + Subnets of the instances from the Target group ? I understand the reason why we need multiple AZ but don't understand why we need to explicitly select them while configuring ALB. The ALB should be able to get that from the Target group EC2 instances.
ALB does not inherit AZs from the Target groups, these are 2 different entities. You could technically have ALB in 2 AZs talking to Targets in 3 different AZs.
Thank you !!