Question

I want to understand how ELB load balances between multiple availability zones. For example, if I have 4 instances (a1, a2, a3, a4) in zone us-east-1a and a single instance d1 in us-east-1d behind an ELB, how is the traffic distributed between the two availability zones? i.e., would d1 get nearly 50% of all the traffic or 1/5th of the traffic?

Answer 1

If you enable ELB Cross-Zone Load Balancing, d1 will get 20% of the traffic.

Here’s what happen without enabling Cross-Zone Load Balancing:
D1 would get nearly 50% of the traffic. This is why Amazon recommends adding the same amount of instances from each AZ to your ELB.

The following excerpt is extracted from Overview of Elastic Load Balancing:

Incoming traffic is load balanced equally across all Availability Zones enabled for your load balancer, so it is important to have approximately equivalent numbers of instances in each zone. For example, if you have ten instances in Availability Zone us-east-1a and two instances in us-east-1b, the traffic will still be equally distributed between the two Availability Zones. As a result, the two instances in us-east-1b will have to serve the same amount of traffic as the ten instances in us-east-1a. As a best practice, we recommend you keep an equivalent or nearly equivalent number of instances in each of your Availability Zones. So in the example, rather than having ten instances in us-east-1a and two in us-east-1b, you could distribute your instances so that you have six instances in each Availability Zone.