Troubleshooting Azure Virtual Networks

Common Issues and Solutions

This guide provides common troubleshooting steps for issues encountered with Azure Virtual Networks (VNets). We cover connectivity problems, performance bottlenecks, and configuration errors.

1. Connectivity Issues

If resources within your VNet or connected VNets cannot communicate, consider the following:

  • Network Security Groups (NSGs): Ensure NSGs attached to subnets or network interfaces do not block required traffic. Check both inbound and outbound rules. Use the VNet Network Watcher for NSG flow logs.
  • Azure Firewall/NVA: If you're using Azure Firewall or a Network Virtual Appliance (NVA), verify its rules and routing configurations. A common mistake is forgetting to update routes to point traffic through the firewall.
  • Route Tables: Confirm that route tables associated with your subnets direct traffic correctly. For VNet peering, ensure that "Allow 'Microsoft.ContainerRegistry'" is enabled if needed.
  • VNet Peering: If using VNet peering, ensure that the peering is correctly configured in both directions and that 'Allow Gateway Transit' and 'Use Remote Gateway' are set appropriately if you're using a VPN gateway in one of the peered VNets.
  • DNS Resolution: Verify that DNS resolution is working as expected. If using custom DNS servers, ensure they are accessible and correctly configured.
Use the IP Flow Verify and Connection Troubleshoot features in Azure Network Watcher to diagnose connectivity from a VM.

2. Performance Problems

Slow network performance can impact application responsiveness. Here are common causes:

  • Bandwidth Limits: VNets and their associated resources (VMs, VPN gateways) have bandwidth limitations. Check the documentation for the specific SKUs you are using.
  • Network Latency: High latency can occur between regions or over VPN connections. Consider deploying resources closer to your users or optimizing your VPN configuration.
  • Network Address Translation (NAT): Excessive NAT translations can cause performance issues.
  • Application-Level Bottlenecks: Sometimes, the network is not the bottleneck. Profile your application to rule out CPU, memory, or disk I/O issues.

3. Configuration Errors

Incorrectly configured VNets can lead to unpredictable behavior.

  • IP Addressing Conflicts: Ensure there are no overlapping IP address spaces between connected VNets, on-premises networks, or peering VNets.
  • Subnet Size: Subnets that are too small can limit the number of resources you can deploy. While you can resize some subnets, it's often easier to plan appropriately from the start.
  • Service Endpoints and Private Endpoints: If you're using these for service access, ensure they are configured correctly for the services and VNets you intend to use.
Always back up your network configuration before making significant changes, especially when dealing with routing or firewall rules.

4. Diagnostic Tools

Azure provides several tools to help you diagnose VNet issues:

  • Network Watcher: A comprehensive suite of tools including IP Flow Verify, Connection Troubleshoot, Packet Capture, NSG Flow Logs, and Topology.
  • Azure Monitor: For monitoring VNet metrics and setting up alerts.
  • Resource Health: To check the health of your Azure resources.

Advanced Troubleshooting Scenarios

5. VPN Gateway Connectivity

When your VPN gateway is not connecting or is unstable:

  • Verify your on-premises VPN device configuration against Azure's requirements (IKE version, encryption algorithms, etc.).
  • Check the VPN gateway logs in Azure for specific error messages.
  • Ensure that the 'Azure Virtual Network Gateway' has the correct IP address assigned and is reachable.
  • If using BGP, check route propagation and ensure ASNs are unique.

6. ExpressRoute Issues

7. Load Balancer Problems

Consider leveraging Azure's managed services like Azure Bastion for secure RDP/SSH access, which simplifies connectivity and security management.