Using Azure Firewall to Route a Multi Hub-and-Spoke Topology

In cloud networking, the hub-and-spoke topology is a prevalent architecture where a central hub virtual network (VNet) connects to multiple spoke VNets. This design facilitates efficient management, centralized security, and streamlined connectivity.

When scaling across regions or to accommodate complex organizational structures, implementing a multi hub-and-spoke topology becomes essential. Azure Firewall plays a pivotal role in securing and routing traffic within this architecture.​
In this blog post, we'll explore how to use Azure Firewall to route traffic effectively in a multi hub-and-spoke topology.​
Understanding Multi Hub-and-Spoke TopologyA multi hub-and-spoke topology consists of multiple hub VNets, each serving as a central point for their respective spoke VNets. This structure is beneficial for organizations operating across various regions or requiring segmentation for different business units. Each hub can connect to on-premises networks and other hubs, facilitating global connectivity.​

The advantages of this topology include:​

• Regional Isolation: Each hub can be deployed in a different Azure region, ensuring redundancy and compliance with data residency requirements.​
  
  
• Scalability: Easier to scale by adding new hubs and spokes as the organization grows.​
  
  
• Segmentation: Improved traffic management and security by isolating workloads across different hubs.​

Role of Azure Firewall in Multi Hub-and-Spoke TopologyAzure Firewall is a managed, cloud-based network security service that protects your Azure Virtual Network resources. In a multi hub-and-spoke topology, Azure Firewall can:​

• Secure Traffic: Inspect and filter traffic between spokes, hubs, and on-premises networks.​
  
  
• Centralize Routing: Act as a central routing point, simplifying the management of User-Defined Routes (UDRs).​
  
  
• Enhance Compliance: Ensure that all inter-spoke and inter-hub traffic passes through security checkpoints.​

Implementing Azure Firewall in Multi Hub-and-Spoke Topology1. Deploy Azure Firewall in Each HubTo ensure comprehensive security and routing, deploy an Azure Firewall instance in each hub VNet. This setup allows each hub to manage and inspect traffic locally.​

Steps:

1. Create a Firewall Subnet: In each hub VNet, create a subnet named AzureFirewallSubnet.​
   
   
2. Deploy Azure Firewall: Use the Azure portal, PowerShell, or ARM templates to deploy Azure Firewall into the AzureFirewallSubnet.​
   
3. Configure VNet Peering Establish VNet peering between:​
   

• Spokes and Hubs: Each spoke VNet should be peered with its respective hub VNet.​
  
• Hubs: Peer hub VNets with each other to facilitate inter-hub connectivity.​
  Ensure that Allow forwarded traffic and Allow gateway transit options are enabled during peering to support traffic flow through Azure Firewall.​

3. Define User-Defined Routes (UDRs) Configure UDRs to direct traffic through Azure Firewall:​

• Spoke Subnets: Associate UDRs that route traffic destined for other spokes or external networks to the Azure Firewall's private IP address in the hub.​
  
  
• Firewall Subnets: Azure Firewall automatically learns routes to local spokes, the hub, and on-premises prefixes via the local Virtual Network Gateway. ​
  
  

4. Implement Global VNet Peering For inter-hub connectivity across regions, use global VNet peering. This approach ensures low-latency, high-bandwidth connectivity between hubs.​

Considerations:

• Transitive Routing: By default, VNet peering is non-transitive. Ensure that UDRs are configured to allow traffic to flow between spokes in different hubs via the Azure Firewalls.​
  
  
• Cost Implications: Be aware of data transfer costs associated with global VNet peering.​

5. Test and Monitor After configuration:​

• Validate Connectivity: Ensure that spokes can communicate with each other through the Azure Firewall and that traffic flows as intended.​
• Monitor Traffic: Use Azure Firewall logs and metrics to monitor traffic patterns and detect anomalies.​

Conclusion
Implementing Azure Firewall in a multi hub-and-spoke topology enhances security and simplifies routing across complex Azure environments. By centralizing traffic inspection and management, organizations can achieve scalable and secure network architectures tailored to their global operations.​