# Suboptimal Cross-AZ Routing to NAT Gateway

Canonical: https://www.pointfive.co/efficiency-hub/inefficiencies/suboptimal-cross-az-routing-to-nat-gateway

[Cloud Efficiency Hub](https://www.pointfive.co/efficiency-hub) 

# Suboptimal Cross-AZ Routing to NAT Gateway

## 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

- [AWS NAT Gateway Pricing](https://aws.amazon.com/vpc/pricing/)

- [Best Practices for NAT Gateways](https://docs.aws.amazon.com/vpc/latest/userguide/vpc-nat-gateway.html)

[Submit Feedback](https://hub.pointfive.co/inefficiencies/suboptimal-cross-az-routing-to-nat-gateway)

## At a glance

Reference

CER-0128

Cloud provider

AWS

Service

AWS NAT Gateway

Category

[Networking](https://www.pointfive.co/efficiency-hub/service-category/networking)

Inefficiency type

Inefficient Configuration

## Contributed by

- ### Mike Graff

---
Source: the public page above. Product screenshots and illustrative interfaces are examples, not live customer data.