Troubleshooting Azure VNet Connectivity Issues

 

Troubleshooting Azure VNet Connectivity Issues

Question:

Asked by Jurgen: "I'm having trouble getting resources in one Azure Virtual Network (VNet) to communicate with resources in another VNet. Both VNets are peered, but I still can't connect. I've checked subnet configurations and Network Security Groups, but I suspect there might be a routing issue or a problem with my peering setup. How can I systematically troubleshoot and resolve this issue?"

Greeting:

Hi Jurgen,

Thanks for reaching out! VNet connectivity issues can be tricky, but with a structured approach, you can pinpoint and resolve them efficiently. In this guide, we’ll explore common misconfigurations, key troubleshooting steps, and tools to debug connectivity. By the end, you’ll have the expertise to confidently resolve these issues and maintain seamless communication between VNets. 🌐✨

Clarifying the Issue:

Connectivity issues between VNets often stem from:

• Misconfigured VNet peering or missing routes in the route table.
• Overly restrictive Network Security Group (NSG) or Azure Firewall rules.
• Overlapping IP address spaces between VNets.

These challenges can be resolved by systematically checking configurations from peering to subnets, security rules, and routing paths.

Why This Matters:

Reliable communication between Azure VNets is critical for:

• Supporting multi-tier application architectures.
• Connecting distributed workloads across environments.
• Ensuring performance and uptime for cloud solutions.

Mastering VNet troubleshooting enables you to maintain resilient, secure, and scalable Azure networks.

Key Terms:

• VNet Peering: Enables connectivity between two Azure VNets.
• Network Security Group (NSG): Filters network traffic to and from Azure resources.
• Subnet: A segment of an Azure VNet where resources are deployed.
• Route Table: Directs outbound traffic from subnets.
• Azure Firewall: Provides centralized network traffic filtering.
• Effective Routes: The final set of routes applied to a resource after evaluating all route tables.

Steps at a Glance:

1. Verify VNet Peering Configuration.
2. Check Subnet IP Address Spaces for Overlaps.
3. Inspect Effective NSG Rules for Inbound/Outbound Traffic.
4. Review Route Tables and Effective Routes.
5. Test Connectivity Using Tools like az network CLI and Network Watcher.
6. Analyze Traffic with Azure Network Watcher Logs (includes diagram).
7. Investigate Azure Firewall and Asymmetric Routing Issues.
8. Use Azure Workbooks for Advanced Network Insights.

Step-by-Step Guide:

1. Verify VNet Peering Configuration

   Ensure peering is properly set up on both sides. Verify these settings:

   • Peering status should be Connected.
   • Allow Gateway Transit must be enabled if a gateway is used.
   • Allow Virtual Network Access should be enabled for traffic between VNets.

   CLI Example:

   Bash

   az network vnet peering show --name PeeringName --resource-group ResourceGroupName --vnet-name VNetName  
   

   Common Issue: Forgetting to enable "Allow Virtual Network Access" during peering setup.

2. Check Subnet IP Address Spaces

   Ensure the IP address spaces of the two VNets do not overlap. Overlapping addresses will prevent traffic from being routed correctly.

   Verify address spaces:

   Bash

   az network vnet list --query "[].{Name:name, AddressSpace:addressSpace.addressPrefixes}" 
   

   Pro Tip: Use a CIDR calculator to plan non-overlapping subnets for future deployments.

3. Inspect NSG Rules

   Review inbound and outbound rules in your Network Security Groups (NSGs) for both VNets. Common issues include missing allow rules for required ports.

   Check NSG rules with:

   Bash

   az network nsg rule list --resource-group ResourceGroupName --nsg-name NSGName  
   

   Make sure rules allow traffic between VNets. Example: Allow all traffic between VNets:

   Bash

   az network nsg rule create --nsg-name NSGName --resource-group ResourceGroupName \
   --name AllowVNetTraffic --priority 100 --direction Inbound --access Allow --protocol '*' --source-address-prefix VNetAddressSpace \
   --destination-address-prefix VNetAddressSpace --source-port-range '*' --destination-port-range '*' 
   

4. Review Route Tables and Effective Routes

   Ensure proper routes are set up to direct traffic between VNets.

   Check effective routes for a VM:

   Bash

   az network nic show-effective-route-table --resource-group ResourceGroupName --name NICName  
   

   If routes are missing, update the route table:

   Bash

   az network route-table route create --resource-group ResourceGroupName --route-table-name RouteTableName --name RouteToPeeredVNet \
   --address-prefix VNetAddressSpace --next-hop-type VirtualNetworkPeering 

5. Test Connectivity Locally

   Use tools like ping or traceroute to test connectivity between resources.

   Example for traceroute:

   Bash

   traceroute destination-ip  
   

   Common Issue: ICMP traffic is blocked in NSG rules, preventing ping from working. Ensure ICMP is allowed for testing.

6. Analyze Traffic with Azure Network Watcher Logs

   Enable and analyze Network Watcher to inspect packet flows and detect blocked traffic:

   Bash

   az network watcher flow-log configure --nsg-name NSGName --resource-group ResourceGroupName --enabled true --storage-account StorageAccountName  
   

   Query logs for blocked traffic:

   AzureDiagnostics 
   | where OperationName == "NetworkSecurityGroupFlowEvent" 
   | where FlowAction == "Deny"  
   

   Network Traffic Flow Diagram:

   ├── VNet 1
   │   └── Subnet 1
   │       └── NSG Rules
   │           └── Route Table
   │               └── Peering Connection
   └── VNet 2
       └── Subnet 2
           └── NSG Rules
               └── Route Table
                   └── Peering Connection
   

   This diagram highlights the flow of traffic between VNets and identifies potential bottlenecks like NSG rules, route table misconfigurations, or peering issues. 

7. Investigate Azure Firewall and Asymmetric Routing Issues

   Azure Firewall: If an Azure Firewall is in place, ensure its rules allow traffic between VNets.

   CLI Example to review firewall rules:

   Bash

   az network firewall show --name FirewallName --resource-group ResourceGroupName  
   

   Asymmetric Routing: Traffic routed asymmetrically (e.g., traffic going through a VPN gateway but returning through a peering connection) can cause connectivity issues. Use route tables to ensure symmetric paths.

8. Use Azure Workbooks for Advanced Insights

   Leverage Azure Workbooks for a visual overview of your network setup and traffic flow. Workbooks can provide insights into connection bottlenecks, dropped packets, and misconfigurations.

   Get started: Azure Workbooks Documentation

Closing Thoughts:

By systematically verifying peering configurations, NSG rules, routes, and traffic flow logs, you can resolve connectivity issues between Azure VNets. Tools like Network Watcher and Azure Workbooks provide valuable insights into misconfigurations and help optimize your network setup.

Farewell:

I hope this guide equips you to troubleshoot VNet connectivity issues like a pro, Jurgen! If you need further assistance, don’t hesitate to reach out. Happy networking! 😊

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