Cross-zone load balancing is a critical feature in AWS Elastic Load Balancing that ensures even distribution of incoming traffic across all registered targets in multiple Availability Zones, enhancing reliability and business continuity for your applications.
By default, each load balancer node di…Cross-zone load balancing is a critical feature in AWS Elastic Load Balancing that ensures even distribution of incoming traffic across all registered targets in multiple Availability Zones, enhancing reliability and business continuity for your applications.
By default, each load balancer node distributes traffic only to targets registered in its own Availability Zone. This can lead to uneven traffic distribution when the number of targets varies across zones. Cross-zone load balancing solves this problem by allowing each load balancer node to distribute requests across all registered targets in all enabled Availability Zones.
For example, if you have 10 targets in Availability Zone A and 2 targets in Availability Zone B, enabling cross-zone load balancing ensures that all 12 targets receive approximately equal traffic. Each target would handle roughly 8.33% of the total requests, rather than Zone A targets handling 5% each and Zone B targets handling 25% each.
The behavior varies by load balancer type. For Application Load Balancers, cross-zone load balancing is always enabled at the load balancer level and can be configured at the target group level. For Network Load Balancers, cross-zone load balancing is disabled by default but can be enabled at either the load balancer or target group level. For Classic Load Balancers, it depends on how the load balancer was created.
From a reliability perspective, cross-zone load balancing improves fault tolerance by preventing any single target from becoming overwhelmed. It also supports business continuity by ensuring consistent application performance even when target distribution across zones is unbalanced.
Considerations include potential data transfer charges between Availability Zones when cross-zone load balancing is enabled, particularly for Network Load Balancers. SysOps Administrators should evaluate the trade-off between optimal load distribution and associated costs when designing highly available architectures on AWS.
Cross-zone load balancing is a feature of Elastic Load Balancing (ELB) that enables load balancers to distribute incoming traffic evenly across all registered targets in all enabled Availability Zones, regardless of which Availability Zone the load balancer node received the traffic.
Why is Cross-Zone Load Balancing Important?
Cross-zone load balancing is crucial for several reasons:
• Even Traffic Distribution: Ensures that all instances receive a proportional share of traffic, preventing some instances from being overloaded while others remain underutilized • Improved Resource Utilization: Maximizes the efficiency of your compute resources across all Availability Zones • Enhanced Application Performance: Prevents bottlenecks that can occur when traffic is concentrated on specific instances • Better Fault Tolerance: If one Availability Zone has fewer instances, traffic is still distributed evenly across all healthy targets
How Cross-Zone Load Balancing Works
Scenario 1: Cross-Zone Load Balancing Disabled When disabled, each load balancer node distributes traffic only to targets within its own Availability Zone. If AZ-A has 2 instances and AZ-B has 8 instances, each AZ receives 50% of traffic. This means AZ-A instances each handle 25% of total traffic, while AZ-B instances each handle only 6.25%.
Scenario 2: Cross-Zone Load Balancing Enabled When enabled, each load balancer node distributes traffic across all registered targets in all enabled Availability Zones. Using the same example, all 10 instances would each receive 10% of the traffic, regardless of their Availability Zone.
Default Settings by Load Balancer Type
• Application Load Balancer (ALB): Enabled by default at the target group level, no additional charges • Network Load Balancer (NLB): Disabled by default, charges apply for inter-AZ data transfer when enabled • Gateway Load Balancer (GWLB): Disabled by default, charges apply when enabled • Classic Load Balancer (CLB): Disabled by default, no additional charges when enabled
How to Enable Cross-Zone Load Balancing
For ALB: Navigate to Target Groups, select your target group, edit attributes, and toggle cross-zone load balancing For NLB/GWLB: Navigate to Load Balancer attributes and enable cross-zone load balancing For CLB: Navigate to Load Balancer attributes in the EC2 console and enable the feature
Exam Tips: Answering Questions on Cross-Zone Load Balancing
• Remember the defaults: ALB has it enabled by default; NLB, GWLB, and CLB have it disabled by default • Cost considerations: NLB and GWLB incur inter-AZ data transfer charges when cross-zone load balancing is enabled; ALB and CLB do not • Uneven instance distribution scenarios: When questions describe uneven traffic distribution with instances spread across multiple AZs, cross-zone load balancing is typically the solution • Look for keywords: Questions mentioning unequal instance counts per AZ, traffic imbalance, or instance overutilization often point to cross-zone load balancing • Target group level for ALB: Remember that ALB configures cross-zone load balancing at the target group level, not the load balancer level • Performance vs. Cost tradeoff: For NLB scenarios asking about minimizing costs, consider that disabling cross-zone load balancing reduces inter-AZ data transfer costs • Health and availability questions: Cross-zone load balancing helps maintain availability when targets fail in one AZ by routing traffic to healthy targets in other AZs