AWS ELB - select at least two Subnets in different Availability Zones - doesn't seem to be required

Question

I'm creating an ELB and it says:

Please select at least two Subnets in different Availability Zones to provide higher availability for your load balancer.

However, it doesn't seem to require this.

E.g. if I add just one subnet then I can go ahead and create the LB.

Is this just an optional message?

Solution 1

It is a recommendation for the old classic load balancer, as a best practice for better availability, but it is a requirement for application load balancer.

Solution 2

The common and best practice is to have everything doubled in AWS. The same holds for AWS ELB load balancer service.

Having an ELB in multiple availability zones protects you from a zone sudden outage. The load balancer can continue to route traffic to the healthy registered instances in another availability zone then.

Keep in mind that if you register an instance in an availability zone but do not enable the availability zone, the registered instance do not receive traffic.

Anyway, it is a recommendation for classic load ballancer but a requirement for application load ballancer.