Troubleshoot and optimize AWS cross-AZ traffic

Tal Shladovsky
March 21, 2023
min. read
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  • AWS regions and availability zones provide a way to design and deploy highly available, performant, and compliant applications and services while optimizing costs.
  • AWS regions and availability zones provide cost optimization opportunities by allowing you to choose the most cost-effective region and availability zones for your applications and services.
  • Data transfer within the same Availability Zone is free.
    * Data transferred between Amazon EC2, Amazon RDS, Amazon Redshift, Amazon ElastiCache instances, and Elastic Network Interfaces in the same Availability         Zone is at no cost.
    * Data transferred "in" to and "out" from Amazon Classic and Application Elastic Load Balancers using private IP addresses, between EC2 instances and the load    balancer in the same AWS VPC is also at no cost.
  • Data transferred "in" to and "out" across Availability Zones in the same AWS Region is charged at $0.01/GB in each direction.
  • Lightlytics can help you optimize your cross-communications network traffic, allowing you to achieve a high-performing, scalable, and cost-effective AWS architecture that meets the needs of your business.

AWS Regions & Availability Zones Overview

AWS operates in multiple geographical regions, each of which is a separate geographic area that contains one or more availability zones.

An AWS region is a physical location around the world where AWS hosts one of its multiple data centers. Currently, AWS operates out of 25 regions globally, including the United States, Europe, Asia, South America, and Australia. Each region is completely independent of the others and is designed to be isolated from failures in other regions.

Each AWS region is divided into multiple availability zones, which are isolated data centers within a region. An availability zone (AZ) is a separate data center that contains its own power, networking, and connectivity resources. Each availability zone is designed to be independent of the others, with its own power and network connectivity, to minimize the risk of a single point of failure.

The availability zones within a region are connected by low-latency links, which enable customers to build highly available and fault-tolerant applications that span multiple availability zones. By deploying resources across multiple availability zones, you can achieve high levels of availability, resiliency and scalability.

Regions and availability zones (AZs) provide geographical isolation for cloud workloads, and they both use mirroring to enhance redundancy and availability. This ensures that workloads remain available even if one availability zone (AZ) fails, or in case of workloads running in multiple cloud regions.
However, regions and availability zones (AZs) differ in their impact on your cloud environment and associated costs.

Using multiple regions or availability zones (AZs) typically results in a higher overall cloud computing bill due to the cost of hosting redundant workloads and data transfer fees when moving data between regions.

Keeping all workloads in the same region makes it easier to predict and optimize costs since most AWS services are priced on a per-Region basis. The cost of a particular service remains the same as long as it's hosted in a given region, regardless of which availability zone is used within that region.

On the other hand, using multiple regions makes it more challenging to predict costs since prices can vary between regions for the same service. For example, the cost of running an EC2 instance in one region may differ from the cost of running the same instance type in another region.

Availability zone structure within a multi-region AWS topology

When to use AWS regions vs AWS availability zones?

Regions and availability zones serve different purposes and are used in different scenarios.
Use regions to choose the most suitable geographic location for your resources based on factors like latency, compliance, and availability. The main use cases for multiple AWS regions are for disaster recovery and to serve users located in discrete locations.
Use availability zones to ensure high availability, increased workload redundancy, and fault tolerance within a region.
Managing AZs is simpler both in terms of cost and administration while providing the same level of redundancy as deploying across multiple regions.

In general, it is recommended to use both regions and availability zones to achieve the highest level of availability and reliability for your applications and services.

What is AWS cross-AZ traffic?

AWS cross-AZ traffic refers to the traffic that flows between resources deployed in different availability zones within the same AWS region.
For example, if you have an application that is deployed across multiple availability zones for high availability and scalability, the traffic between the resources in different availability zones will be considered cross-AZ traffic. Cross-AZ traffic can include data replication, application traffic, and network traffic.
Optimizing cross-AZ traffic is important because it can affect the performance, latency, and cost of your application. By minimizing cross-AZ traffic and ensuring that it flows efficiently within the AWS network, you can improve the performance and reliability of your application while reducing costs associated with data transfer.

AWS does not charge for data transfer between resources within the same Availability Zone (AZ) or for data transfer within the same region. This means that if you have resources, such as EC2 instances, RDS instances, or S3 buckets, deployed in the same AZ, you will not be charged for data transfer between them.

AWS pricing for data transfer is based on the direction of the transfer (inbound or outbound), the type of service, and the regions involved in the transfer. In general, inbound data transfer to AWS services is free, while outbound data transfer incurs charges.

For data transfer between different regions or across different AWS services, AWS charges vary based on the amount of data transferred and the regions involved. You can find more details on AWS pricing for data transfer in the AWS pricing documentation.

It's important to estimate your data transfer costs carefully when planning your AWS architecture to avoid unexpected charges on your AWS bill. Additionally, AWS provides native tools and services to help you monitor and optimize your data transfer costs, such as AWS Trusted Advisor and AWS Cost Explorer.

For additional information on data transfer costs please review Amazon EC2 On-Demand Pricing page

How to troubleshoot and optimize AWS cross-AZ traffic?

When it comes to troubleshooting and optimizing AWS cross-AZ traffic, there are several steps you can take to ensure that your network is running smoothly and efficiently. Here are some best practices:

  1. Deploy resources in the same region: By deploying resources in the same region, you can reduce the amount of cross-AZ traffic. Resources in different regions require data to travel across the public internet, which can increase latency and costs.
  1. Use VPC Peering: VPC peering allows you to connect two VPCs together, which can reduce the amount of cross-AZ traffic. This is because traffic between peered VPCs stays within the AWS network and doesn't traverse the internet.
  1. Use Direct Connect: Direct Connect provides a dedicated network connection between your data center and AWS, which can reduce the amount of cross-AZ traffic by keeping it within the AWS network.
  1. Use Elastic Load Balancing: Elastic Load Balancing distributes traffic evenly across instances in multiple AZs, which can help optimize traffic and reduce congestion.
  1. Monitor Network Traffic: Use CloudWatch and VPC Flow Logs to monitor your network traffic and identify any anomalies or areas that need optimization. You can also use third-party tools for network monitoring and analysis.
  1. Optimize Your Applications: Optimize your applications to minimize cross-AZ traffic by caching frequently accessed data, reducing the number of requests between instances, and using asynchronous communication.
  1. Use Content Delivery Networks: Content Delivery Networks (CDNs), such as CloudFront, can help reduce the amount of cross-AZ traffic by caching frequently accessed data and serving it from edge locations.
  1. Use Auto Scaling: Auto scaling can help you scale your infrastructure horizontally and across multiple AZs to handle changes in traffic demand, which can help optimize your network performance.

By following these best practices, you can troubleshoot and optimize your AWS cross AZ traffic, reduce latency, and improve the overall performance of your applications.

In what cases should I consider moving to a single AZ instead of using cross-AZ traffic?

In general, it is recommended to use multiple availability zones (AZs) when deploying applications in AWS to improve availability, fault tolerance, and resilience. However, there may be situations where using a single AZ could be a better option. Here are some scenarios where using a single AZ may be appropriate:

  1. Cost Restrictions: Deploying resources across multiple AZs incurs additional costs for data transfer and storage replication. If you have budget constraints and your application does not require high availability, you may consider using a single AZ to reduce costs.
  1. Low-risk applications: If your application is not mission-critical and can tolerate some downtime, using a single AZ may be appropriate. For example, if you are deploying a development or test environment, using a single AZ may be sufficient.
  1. Low-latency applications: In some cases, deploying resources in a single AZ can result in lower latency and faster response times. For example, if you are running a high-performance computing application that requires low-latency communication between nodes, deploying in a single AZ may be more appropriate.
  1. Simplified management: Managing resources across multiple AZs can be more complex than managing resources in a single AZ. If you have limited IT resources and expertise, using a single AZ may simplify management and reduce the risk of configuration errors.

It's important to note that deploying resources in a single AZ comes with trade-offs, particularly in terms of availability and resilience. If your application requires high availability, fault tolerance, and resilience, deploying resources across multiple AZs is generally recommended.

The New & Easy Way with Lightlytics

With Lightlytics Cost, you can easily and fully understand your current cloud costs and trends across your cloud environments, accounts and availability zones.
Lightlytics CloudTwin technology helps you to get a complete picture of your cloud costs, with the complete context of your real-time configuration, traffic flow and even event logs.

Below, you can see a complete view of your monthly costs grouped by Region or Availability Zones including the total cost, trend and indirect cost.
Note: Indirect cost refers to the SUM of the direct cost of all the child resources or associated resources.
For example, for an EC2 instance, the indirect costs include the cost of the attached EBS Volumes.
For an EKS cluster, the indirect cost is the SUM of the costs of the associated EC2 Instances, their EBS Volumes and the associated Load balancers.

Once you get a detailed view of your regions and AZ’s costs, you can further investigate it using Lightlytics Network Traffic Activity logs enriches near real-time VPC flow logs providing you with network traffic information that includes details such as source and destination IP addresses, port numbers, protocol, number of bytes and packets, and the flow's status (accepted or rejected). The Network Traffic logs provide a full picture of your Cross Communications network traffic, including Cross-Region, Cross-AZ, Cross-VPC, and Cross-Account network traffic.

The below image shows cross-AZ network traffic in an account integrated with Lightlytics:

The below image shows an aggregated cross-AZ communication between an Application Load Balancer (ALB) and an EC2 instance

With the View Path option, you can review your selected network topology and have an unmatched visibility of your cloud resources and network components, letting you also quickly analyze and troubleshoot configurations and resource usage in your cloud environment

Now that you have all the information about your cross-AZ network traffic, the resources involved, and their configurations, you can take the appropriate steps to optimize your cross-AZ traffic.


To summarize, AWS Regions are geographical areas where AWS operates multiple data centers, while Availability Zones are isolated data centers within a region. By deploying resources across multiple availability zones within a region, customers can build highly available and fault-tolerant systems that are capable of providing continuous service even in the event of disruptions. Optimizing cross-AZ traffic is essential for improving the performance, reliability, and cost-effectiveness of your AWS applications. By implementing best practices, you can improve the efficiency of data transfer across different AZs, regions, and services, while minimizing latency, downtime, and costs. Additionally, monitoring and optimizing your data transfer costs using Lightlytics Cost and Network Traffic Activity logs can provide a full picture of your Cross Communications network traffic. By following these guidelines, you can achieve a high-performing, scalable, and cost-effective AWS architecture that meets the needs of your business.

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