Cross-Zone Load Balancing

5 minutes 5 Questions

Cross-Zone Load Balancing is an essential concept to ensure high availability and fault tolerance of your application deployed across multiple Availability Zones. By enabling Cross-Zone Load Balancing, the traffic is distributed evenly not just among instances in a single Availability Zone but acro…

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.

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AWS Certified Solutions Architect - Cross-Zone Load Balancing Example Questions

Test your knowledge of Cross-Zone Load Balancing

Question 1

You operate an Application Load Balancer (ALB) with cross-zone load balancing enabled across two Availability Zones. After a recent change, requests routed to one target group show increased 5xx errors and high latency, and several targets in that group are marked unhealthy by health checks. What should you do to quickly restore performance?

Change the Application Load Balancer to Network Load Balancer Create an AWS Direct Connect connection between Availability Zones Enable Amazon Route 53 latency-based or geolocation-based routing Fix the misconfiguration in the target group and verify that the load balancer can access targets in multiple Availability Zones

Correct Answer: Fix the misconfiguration in the target group and verify that the load balancer can access targets in multiple Availability Zones

The disruption is caused by a misconfigured target group. The correct resolution is to fix the target group configuration (e.g., health check path/port/protocol, target registration, listener rules, and security group/NACL rules that allow the ALB to reach targets) and verify that the ALB can route to healthy targets in multiple Availability Zones. Cross-zone load balancing does not compensate for a broken target group; the ALB will only send traffic to healthy targets. Route 53 latency or geolocation routing will not fix an unhealthy target group behind the ALB. Switching to a Network Load Balancer does not address application-layer misconfiguration. AWS Direct Connect is unrelated and cannot be used between Availability Zones.

Question 2

An application runs in a single AWS Region behind a Network Load Balancer (NLB) deployed across three Availability Zones. Cross-zone load balancing on the NLB is currently disabled. Due to an EC2 capacity shortfall, the Auto Scaling group in one Availability Zone has only half as many healthy targets as the other two. Users whose traffic is routed to that Availability Zone report higher latency. Without adding capacity immediately, what should you do to reduce the latency?

Implement Amazon Route 53 with latency-based routing Enable cross-zone load balancing on your load balancer Create a VPN tunnel between the affected Availability Zone and other Availability Zones Use AWS Direct Connect between the affected Availability Zone and other Availability Zones

Correct Answer: Enable cross-zone load balancing on your load balancer

With a Network Load Balancer (NLB) that has cross-zone load balancing disabled, each load balancer node routes traffic only to targets within its own Availability Zone. If one Availability Zone has fewer healthy targets, that zone becomes a hotspot and sees higher latency for clients whose flows land there. Enabling cross-zone load balancing allows the NLB to distribute connections across all healthy targets in all enabled Availability Zones, smoothing out imbalances and reducing latency without adding capacity. Route 53 latency-based routing operates at the DNS/region level and will not fix intra-region AZ imbalance. AWS Direct Connect and VPNs do not provide AZ-to-AZ connectivity or affect how an NLB balances traffic.

Question 3

You are seeing uneven workload distribution between instances within the same Availability Zone due to a Network Load Balancer. What should you do?

Create a Classic Load Balancer Enable cross-zone load balancing on the Network Load Balancer Create an Application Load Balancer Use Amazon Route 53 latency-based routing

Correct Answer: Enable cross-zone load balancing on the Network Load Balancer

Enabling cross-zone load balancing on the Network Load Balancer allows for the load to be distributed evenly across all instances within different Availability Zones. Application Load Balancer or Classic Load Balancer will not solve the issue since they handle different types of traffic. Route 53 latency-based routing is not related to cross-zone load balancing. Auto Scaling and Placement Groups are not relevant here, as they don't address the uneven workload distribution issue.

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