What is Cross-Zone load balancing?
Cross-Zone Load Balancing is a feature that enables a load balancer to distribute incoming traffic evenly across all registered targets in all enabled Availability Zones, rather than just within its own zone [citation:3][citation:10].
Cross-Zone Load Balancing is a feature that allows a load balancer to distribute incoming traffic evenly across all registered targets in all enabled Availability Zones (AZs). Without this feature, each load balancer node distributes traffic only to targets in its own AZ, which can lead to uneven load distribution if the number of targets or request rates vary by AZ. With cross-zone load balancing enabled, each load balancer node can route traffic to targets in any of the enabled AZs, ensuring a more balanced load [citation:3][citation:10].
Scenario Questions
- 1
If you enable cross‑zone load balancing on an Application Load Balancer in a VPC with three AZs, how does traffic get distributed when one AZ has twice the number of registered targets as another?
- 2
You have a Classic Load Balancer with cross‑zone load balancing turned off. What happens to requests from clients in us-east-1a when the only healthy instances are in us-east-1c?
- 1
During a recent deployment, you noticed that latency spiked even though you had enabled cross‑zone load balancing. Walk me through how you would debug the issue.
- 2
Explain the cost trade‑offs of turning on cross‑zone load balancing for a Network Load Balancer handling 10 Gbps traffic across four AZs.
- 1
Design a highly available web service that must handle traffic spikes in a single AZ without over‑provisioning in others. How would you configure cross‑zone load balancing and what edge cases would you guard against?
- 2
Your team is migrating from a Classic Load Balancer to an Application Load Balancer. What changes in cross‑zone behavior must you consider, and how would you validate that traffic distribution remains correct?
- 1
At a large enterprise, multiple teams own services behind shared ALBs. How would you approach a policy decision to enable or disable cross‑zone load balancing globally, considering cost, latency, and operational ownership?
- 2
When moving a legacy multi‑AZ deployment to a new region, what architectural steps would you take to ensure cross‑zone load balancing does not introduce hidden bottlenecks or violate compliance constraints?
Follow-up Questions
- ❓What impact does disabling cross‑zone have on latency for users in a low‑traffic AZ?
- ❓How would you monitor that traffic is being balanced as expected?
- ❓Can you think of a scenario where you would deliberately keep cross‑zone disabled?