Suboptimal Cross-AZ Routing to NAT Gateway

Mike Graff

Service Category

Networking

Cloud Provider

AWS

Service Name

AWS NAT Gateway

Inefficiency Type

Inefficient Configuration

Explanation

NAT Gateways are designed to serve private subnets within the same Availability Zone. When subnets in one AZ are configured to route traffic through a NAT Gateway in a different AZ, the traffic crosses AZ boundaries and incurs inter-AZ data transfer charges in addition to the standard NAT processing fees.

This typically happens when:

* NAT Gateways are deployed in multiple AZs (as recommended for resilience), but * Route tables for all subnets are configured to send traffic to a single NAT Gateway, ignoring AZ placement

In high-throughput environments, this misalignment silently generates excess cost. Ensuring that each subnet routes through the NAT Gateway in its own AZ avoids inter-AZ charges and aligns with AWS architectural best practices.

Relevant Billing Model

NAT Gateway costs include:
A flat hourly charge per gateway
A per-GB charge for all data processed
If traffic crosses Availability Zones (e.g., from a subnet in AZ-A to a NAT Gateway in AZ-B), **inter-AZ transfer charges** apply
These are billed separately from NAT Gateway processing fees and increase total data egress cost

Detection

Identify whether multiple NAT Gateways exist across different Availability Zones
Review route table configurations to determine if subnets route to a NAT Gateway in a different AZ
Assess whether route tables are shared or duplicated across subnets without AZ awareness
Analyze inter-AZ traffic patterns associated with NAT Gateway usage to confirm misrouted flows

Remediation

Update route tables to ensure that each subnet routes outbound traffic through the NAT Gateway in the same AZ
Ensure one NAT Gateway is deployed per Availability Zone for fault tolerance and cost efficiency
Review and revise any infrastructure templates or automation that create non-AZ-aware routing
Incorporate AZ-based routing validation into network hygiene checks

Relevant Documentation

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