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Masaki Okuda
Masaki Okuda

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[Research][AWS]Investigating Transit Gateway Routes with VPC Reachability Analyzer

Introduction

Thank you for reading this article! I'm a mob engineer who previously wrote about Transit Gateway configurations. After successfully completing an AWS hands-on lab involving Transit Gateway, I wanted to share my findings about analyzing Transit Gateway communication paths using VPC Reachability Analyzer.

Background

This article builds upon the AWS Official Hands-on Lab for Transit Gateway configurations. I'll be investigating the network reachability between different VPC configurations through the Transit Gateway setup.

https://catalog.us-east-1.prod.workshops.aws/workshops/e0d1c19d-c80b-4695-a3fc-5c4a25132f47/ja-JP/2-hands-on/2-5-lab5

Current Architecture Overview

The current setup consists of:

  • Boundary VPC with EC2 instances
  • Private VPC1 with EC2 instances
  • Private VPC2 with Transit Gateway attachments
  • Private VPC3 with Transit Gateway attachments

All connected through a Transit Gateway infrastructure.

Investigation Scope

I wanted to analyze the following connectivity scenarios:

  1. Boundary VPC EC2 ↔ Private VPC1 EC2
  2. Boundary VPC EC2 ↔ Private VPC2 TGW Attachment
  3. Boundary VPC EC2 ↔ Private VPC3 TGW Attachment

Current Ping Connectivity Status

Before diving into the VPC Reachability Analyzer results, I verified the current ping connectivity status between the instances to establish a baseline.

VPC Reachability Analyzer Results

Test 1: Boundary VPC EC2 ↔ Private VPC1 EC2

Analysis Configuration:

  • Source: EC2 instance in Boundary VPC
  • Destination: EC2 instance in Private VPC1

Result: Unreachable

The analysis showed that the path was not reachable between these two instances.

Test 2: Boundary VPC EC2 ↔ Private VPC2 TGW Attachment

Analysis Configuration:

  • Source: EC2 instance in Boundary VPC
  • Destination: Transit Gateway attachment in Private VPC2

Result: Unreachable

Similar to the previous test, this configuration also resulted in an unreachable status.

Test 3: Boundary VPC EC2 ↔ Private VPC3 TGW Attachment

Analysis Configuration:

  • Source: EC2 instance in Boundary VPC
  • Destination: Transit Gateway attachment in Private VPC3

Result: Unreachable

Analysis and Insights

Common Error Pattern

All three connectivity tests resulted in the same error: TGW_RTB_NO_ROUTE_TO_TGW_ATTACHMENT

This error indicates that the Transit Gateway route table doesn't have the necessary routes configured to reach the target attachments.

Root Cause Analysis

The primary reason for these connectivity issues appears to be related to the hands-on lab configuration. Specifically:

  • Missing Route Table Entries: The Transit Gateway route tables lack the necessary routes to direct traffic to the target VPC attachments
  • Configuration Gap: Without proper routing configuration, VPC Reachability Analyzer correctly identifies these paths as unreachable

Troubleshooting Resources

For teams encountering similar issues, AWS provides comprehensive documentation about VPC Reachability Analyzer error codes and their meanings. This documentation is invaluable for troubleshooting network connectivity issues in complex Transit Gateway environments.

https://docs.aws.amazon.com/vpc/latest/reachability/explanation-codes.html

Leveraging Amazon Q for Faster Resolution

To accelerate the troubleshooting process, I recommend using Amazon Q to:

  • Quickly identify the root cause of routing issues
  • Get specific guidance on Transit Gateway route table configurations
  • Understand the relationship between different AWS networking components

This AI-powered approach can significantly reduce the time needed to diagnose and resolve network connectivity problems.

Future Investigations

While this analysis focused on VPC Reachability Analyzer, I'm planning to explore Network Access Analyzer in future articles to provide even deeper insights into network communication patterns and security configurations.

Key Takeaways

  1. VPC Reachability Analyzer is an excellent tool for validating network paths in Transit Gateway environments
  2. Route table configuration is critical for Transit Gateway connectivity
  3. Systematic analysis helps identify configuration gaps before they impact production workloads
  4. AI-powered troubleshooting (like Amazon Q) can accelerate problem resolution

Conclusion

This investigation demonstrated how VPC Reachability Analyzer can effectively identify routing issues in Transit Gateway configurations. While the current setup showed connectivity problems due to missing route table entries, this analysis provides a clear path forward for resolving these issues.

The combination of traditional network analysis tools and modern AI-powered assistance creates a powerful troubleshooting workflow for complex AWS networking scenarios.


Thank you for reading this article! Feel free to share your experiences with Transit Gateway routing and VPC Reachability Analyzer in the comments below.

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