VPC Reachability Analyzer Overview

The VPC Reachability Analyzer is a diagnostic tool within AWS designed to help troubleshoot network connectivity issues between two endpoints within your Virtual Private Cloud (VPC). This tool significantly simplifies the process of identifying connectivity problems by analyzing the network configuration between endpoints, rather than by sending actual packets through the network. Here's a summary of its key aspects:

• Purpose: To diagnose network connectivity issues within a VPC.
• Functionality: It models the network configuration between two endpoints and assesses reachability based on this model.
• Method: Analyzes configuration without sending traffic, providing insights into potential connectivity issues.
• Benefits: Offers a powerful and efficient way to understand and troubleshoot network connectivity problems.

How It Works

Using two EC2 instances (labeled as Instance A and B) as an example, the process involves several steps to determine if Instance A can communicate with Instance B. These steps include traversing:

1. From Instance A to its Elastic Network Interface (ENI)
2. Through the Security Group associated with Instance A
3. Possibly through Network ACLs (Access Control Lists)
4. Back through another Security Group associated with Instance B
5. And finally to the ENI of Instance B

Throughout this journey, the VPC Reachability Analyzer checks each point for proper configuration to determine if the two instances can communicate. If any segment of the configuration is not correctly set up (e.g., a misconfigured security group that blocks traffic), the tool identifies this as the source of the issue and reports that the instances are not reachable due to that specific configuration problem.

Key Advantages

• Diagnostic Precision: Pinpoints the exact location of configuration errors that hinder connectivity (e.g., route tables, security groups).
• Efficiency: Saves time and effort by analyzing configurations directly, without the need to simulate or send actual network traffic.
• Troubleshooting: Ideal for identifying and resolving connectivity issues, ensuring network configurations are implemented as intended.