Load balancers with cross-zone load balancing enabled, priced on the inter-AZ transfer it causes

Cross-zone routing sends requests over Availability Zone boundaries

Each load balancer node sits in one Availability Zone. With cross-zone load balancing on, a node spreads traffic across the registered targets in every enabled zone; with it off, a node only uses targets in its own zone. AWS’s own example has two targets in zone A and eight in zone B: with cross-zone on, each target takes 10% of requests, and with it off the two targets in zone A take 25% each.

The defaults differ by type. Application Load Balancers always have cross-zone on at the load balancer level, though you can turn it off per target group. Network and Gateway Load Balancers start with it off. For Network Load Balancers, AWS states that EC2 data transfer charges apply when cross-zone load balancing is enabled, because traffic then crosses zones inside the Region.

Checking the setting on your load balancers

List load balancers, then read the attribute on each one and on its target groups. The target group value overrides the load balancer value:

aws elbv2 describe-load-balancers \
  --query 'LoadBalancers[].[LoadBalancerName,Type,LoadBalancerArn]' --output table

aws elbv2 describe-load-balancer-attributes --load-balancer-arn LOAD_BALANCER_ARN \
  --query "Attributes[?Key=='load_balancing.cross_zone.enabled']"

aws elbv2 describe-target-groups --load-balancer-arn LOAD_BALANCER_ARN \
  --query 'TargetGroups[].TargetGroupArn'

aws elbv2 describe-target-group-attributes --target-group-arn TARGET_GROUP_ARN \
  --query "Attributes[?Key=='load_balancing.cross_zone.enabled']"

The two things ZopNight needs before it would fire

1. The load balancer’s cross-zone attribute reads true. If ZopNight could not read the attribute, it treats the setting as unknown and does nothing.
2. A billed dollar amount for the inter-AZ data transfer that this specific load balancer causes.

Why no finding is raised today

The second input does not exist yet. AWS reports regional data transfer as a usage type for the whole account, and the load balancer’s processed-bytes figure counts all traffic, not the part that crossed a zone. Taking a percentage of the load balancer bill would be a guess, so the rule stays silent once it confirms cross-zone is on. ZopNight does not price cross-AZ transfer at the account level either; the DataTransfer-Regional-Bytes usage types in Cost Explorer show it directly.

What the saving will be once per-LB transfer cost exists

saving = billed inter-AZ data transfer attributable to this load balancer
cost after change = 0 for that line item

Turning cross-zone off removes the transfer it causes, so the whole line item is the saving. There is no fixed fraction involved.

Turning cross-zone off without starving a zone

1. Confirm each enabled zone has enough healthy targets to carry its share alone. Uneven target counts are the main reason to keep cross-zone on.
2. For a Network Load Balancer, set the load balancer attribute: aws elbv2 modify-load-balancer-attributes --load-balancer-arn LOAD_BALANCER_ARN --attributes Key=load_balancing.cross_zone.enabled,Value=false
3. For an Application Load Balancer, change it on the target group instead with aws elbv2 modify-target-group-attributes and the same key.
4. Watch target health and p95 latency per zone for a day before touching the next one.