By using at least two subnets in different Availability Zones with the Cross-Zone Load Balancing feature enabled, your ELBs can distribute the traffic evenly across all backend instances. To use Cross-Zone Load Balancing at optimal level, Amazon recommends maintaining an equal EC2 capacity distribution in each of the AZs registered with the load balancer.
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Enabling Cross-Zone Load Balancing makes it easier to deploy and manage applications that run across multiple subnets in different Availability Zones. This would also guarantee better fault tolerance and more consistent traffic flow. If one of the availability zones registered with the ELB fails (as result of network outage or power loss), the load balancer with the Cross-Zone Load Balancing activated would act as a traffic guard, stopping any request being sent to the unhealthy zone and routing it to the other zone(s).
Note: no extra charges — with cross-zone load balancing implemented, Amazon will not charge for data transfer between the load balancer nodes and backend instances.
To determine if Cross-Zone Load Balancing is enabled, you need to perform the following:
Remediation / Resolution
To enable Cross-Zone Load Balancing with at least two subnets in different AZs, you need to perform the following:
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ELB Cross-Zone Load Balancing Enabled
Risk level: Medium