Guide: Cross-Zone Load Balancing in AWS

Cross-Zone Load Balancing(ELB) is a vital feature of AWS. It is of major importance in maintaining high availability and fault tolerance of applications. By distributing traffic evenly across all registered instances in its enabled Availability Zones, it provides better fault tolerance. This ensures that your application is not affected even if a few instances are unhealthy.

What is Cross-Zone Load Balancing?
It is a method that allows the load balancer to distribute traffic equally across all registered instances in all enabled Availability Zones, not just the one where the request was initially directed.

How does it work?
Whenever your load balancer receives a request, it routes the request to the backend instance with the smallest load in any Availability Zone. This is determined by Connection Draining, a process from the Elastic Load Balancing feature.

Exam Tips: Answering Questions on Cross-Zone Load Balancing
1. Understanding: Understand the basic working of Cross-Zone Load balancing. Know that it does not restrict traffic to the same zone. Traffic is distributed evenly across instances in all zones.
2. Comparisons: Be able to compare Cross-Zone with single-zone load balancing. Know the differences clearly and when to use each.
3. Enable/Disable: Know how to enable and disable the feature, and the situations where you would want to do so.
4. Costs: Understand the costs that apply when Cross-Zone load balancing is enabled.
5. Key Features: Be aware of key features - such as Connection Draining and Sticky Sessions that are associated with ELB and can be related to questions on Cross-Zone load balancing.