Multi-Availability Zone Distribution
If you register multiple subnets on a single load balancer, the load can be distributed across the availability zones.
If you specify multiple subnets with different availability zones on a load balancer, the traffic will be allocated across the availability zones.
Figure: Load Distribution when Connecting Multiple Availability Zones
Resolving the name of the FQDN of a load balancer that can handle multi-availability zone distribution returns the IP addresses of all load balancers in both of the availability zones.
When a failure occurs in one of the availability zones, it will not be possible to connect to the load balancer of the availability zone where the failure has occurred. It is necessary to take preventative action such as using a library on the client side that processes retrying of http connections, so that when a failure occurs the connection destination is switched to the load balancer of the unaffected availability zone.