Cross-Zone Load Balancing

Cross-Zone Load Balancing

When Enabled

• Each Load Balancer node can distribute traffic evenly across all registered targets in all Availability Zones.

• Instance count differences between AZs do not affect the distribution.

• Example:
• You have 10 instances total: 2 in AZ1, 8 in AZ2.
• 100 incoming requests → each instance gets 10 requests, regardless of which AZ the request enters.
• This keeps load perfectly balanced even if AZs have unequal capacity.

When Disabled

• Each Load Balancer node sends traffic only to targets in its own AZ.

• This means the load is distributed per AZ, not globally, which can cause imbalance if the number of instances differs.

• Example:
• Same setup: 2 instances in AZ1, 8 in AZ2.
• 100 requests enter:
• 50 requests go to AZ1 → 25 requests per instance (higher load).
• 50 requests go to AZ2 → 6.25 requests per instance (lower load).
• This results in overloaded instances in some AZs and underutilized instances in others.

When Enabled

• Each LB node distributes traffic evenly across all targets in all AZs

• Ensures uniform traffic distribution regardless of how many instances are in each AZ

• Example: 100 requests across 10 instances in 2 AZs → each instance gets 10 requests

When Disabled

• Each LB node sends traffic only to targets in its own AZ

• Can cause uneven load if instance counts differ per AZ

• Example: 50 requests to AZ1 (2 instances → 25 each), 50 requests to AZ2 (8 instances → 6.25 each)

Service Behavior

Application Load Balancer (ALB)

• Enabled by default

• Can be disabled per Target Group

• No inter-AZ data transfer charges

Network Load Balancer (NLB) / Gateway Load Balancer (GWLB)

• Disabled by default

• Enabling incurs inter-AZ data transfer charges

Classic Load Balancer (CLB)

• Disabled by default

• No inter-AZ data transfer charges when enabled